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<section id="python-sequences">
<span id="sequences"></span><h1>Python Sequences<a class="headerlink" href="#python-sequences" title="Link to this heading"></a></h1>
<p>Ordered collections of objects</p>
<section id="what-is-a-sequence">
<h2>What is a Sequence?<a class="headerlink" href="#what-is-a-sequence" title="Link to this heading"></a></h2>
<p>A sequence is an ordered collection of objects.</p>
<p>They are analogous to what are often called “arrays” or “lists” in other programming languages.</p>
<p>But in Python, there are number of types that all fit this description, each with special customization. But any object that has the behavior expected of a sequence can be treated the same way in Python:</p>
<p>Remember Duck Typing?</p>
<p>If it looks like a duck and quacks like a duck…</p>
<p>OR: If it looks and acts like a sequence – it <strong>is</strong> a sequence.</p>
<p>Technically, if it satisfies the “Sequence Protocol”, it is a sequence.</p>
<p>Python is all about these protocols – we will see more of them.</p>
<section id="the-sequence-protocol">
<h3>The Sequence Protocol<a class="headerlink" href="#the-sequence-protocol" title="Link to this heading"></a></h3>
<p>A <em>sequence</em> can be considered as anything that supports <em>at least</em> these operations:</p>
<ul class="simple">
<li><p>Indexing</p></li>
<li><p>Slicing</p></li>
<li><p>Membership</p></li>
<li><p>Concatenation</p></li>
<li><p>Length</p></li>
<li><p>Iteration</p></li>
</ul>
<p>I’ll get into all of those as we go along.</p>
</section>
<section id="sequence-types">
<h3>Sequence Types<a class="headerlink" href="#sequence-types" title="Link to this heading"></a></h3>
<p>There are eight built in types in Python that are <em>sequences</em>:</p>
<ul class="simple">
<li><p>string</p></li>
<li><p>list</p></li>
<li><p>tuple</p></li>
<li><p>bytes</p></li>
<li><p>bytearray</p></li>
<li><p>buffer</p></li>
<li><p>array.array</p></li>
<li><p>range object (almost)</p></li>
</ul>
<p>For this lesson, you won’t see much beyond strings, lists, and tuples –
the rest are pretty special purpose.</p>
<p>But what we learn in this lesson applies to all sequences (with minor caveats).</p>
<p>I’ll use lists, strings and tuples in the examples.</p>
<p>So let’s take a look at the key parts of the sequence protocol:</p>
</section>
</section>
<section id="indexing">
<h2>Indexing<a class="headerlink" href="#indexing" title="Link to this heading"></a></h2>
<p>Items in a sequence may be looked up by <em>index</em> using the indexing
operator: <code class="docutils literal notranslate"><span class="pre">[]</span></code></p>
<p>Indexing in Python always starts at zero.</p>
<p>Here is an example with a string – a string is a sequence of characters.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [98]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"this is a string"</span>
<span class="gp">In [99]: </span><span class="n">s</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="gh">Out[99]: </span><span class="go">'t'</span>
<span class="gp">In [100]: </span><span class="n">s</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span>
<span class="gh">Out[100]: </span><span class="go">'i'</span>
</pre></div>
</div>
<p>Note that the first character is indexed with zero – I sometimes call that the “zeroth” item in the sequence.</p>
<p>Zero indexing may seem odd at first (if you are not already a programming geek), but it turns out to make a lot of things easier. More on that later.</p>
<p>You can use negative indexes to count from the end:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [2]: </span><span class="n">a_list</span> <span class="o">=</span> <span class="p">[</span><span class="mi">34</span><span class="p">,</span> <span class="mi">56</span><span class="p">,</span> <span class="mi">19</span><span class="p">,</span> <span class="mi">23</span><span class="p">,</span> <span class="mi">55</span><span class="p">]</span>
<span class="gp">In [3]: </span><span class="n">a_list</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="gh">Out[3]: </span><span class="go">55</span>
<span class="gp">In [4]: </span><span class="n">a_list</span><span class="p">[</span><span class="o">-</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[4]: </span><span class="go">23</span>
<span class="gp">In [5]: </span><span class="n">a_list</span><span class="p">[</span><span class="o">-</span><span class="mi">4</span><span class="p">]</span>
<span class="gh">Out[5]: </span><span class="go">56</span>
</pre></div>
</div>
<p>Indexing beyond the end of a sequence causes an IndexError:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [6]: </span><span class="n">a_list</span>
<span class="gh">Out[6]: </span><span class="go">[34, 56, 19, 23, 55]</span>
<span class="gp">In [7]: </span><span class="n">a_list</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">IndexError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-7-c1f9ac3b6fee></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">a_list</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span>
<span class="ne">IndexError</span>: list index out of range
</pre></div>
</div>
<p>Pretty straight forward so far…</p>
<section id="slicing">
<h3>Slicing<a class="headerlink" href="#slicing" title="Link to this heading"></a></h3>
<p>Slicing is a real “power tool” of Python – it can allow very short code.</p>
<p>Slicing a sequence creates a new sequence with a range of objects from the
original sequence.</p>
<p>It also uses the indexing operator (<code class="docutils literal notranslate"><span class="pre">[]</span></code>), but with a twist.</p>
<p><code class="docutils literal notranslate"><span class="pre">sequence[start:finish]</span></code> returns all <cite>sequence[i]</cite> for which <cite>start <= i < finish</cite></p>
<p>That’s a fancy way to say that it’s all the items from start to finish – including start, but NOT including finish.</p>
<p>This also may be a bit unintuitive – but it’s very practical.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [121]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"a bunch of words"</span>
<span class="gp">In [122]: </span><span class="n">s</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[122]: </span><span class="go">'b'</span>
<span class="gp">In [123]: </span><span class="n">s</span><span class="p">[</span><span class="mi">6</span><span class="p">]</span>
<span class="gh">Out[123]: </span><span class="go">'h'</span>
<span class="gp">In [124]: </span><span class="n">s</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">6</span><span class="p">]</span>
<span class="gh">Out[124]: </span><span class="go">'bunc'</span>
<span class="gp">In [125]: </span><span class="n">s</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">7</span><span class="p">]</span>
<span class="gh">Out[125]: </span><span class="go">'bunch'</span>
</pre></div>
</div>
</section>
<section id="helpful-hint">
<h3>Helpful Hint<a class="headerlink" href="#helpful-hint" title="Link to this heading"></a></h3>
<p>It can really help if you think about slicing this way:</p>
<p>(write this out!)</p>
<p>Think of the indexes as pointing to the spaces between the items:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">a</span> <span class="n">b</span> <span class="n">u</span> <span class="n">n</span> <span class="n">c</span> <span class="n">h</span> <span class="n">o</span> <span class="n">f</span> <span class="n">w</span> <span class="n">o</span> <span class="n">r</span> <span class="n">d</span> <span class="n">s</span>
<span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span> <span class="o">|</span>
<span class="mi">0</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span> <span class="mi">5</span> <span class="mi">6</span> <span class="mi">7</span> <span class="mi">8</span> <span class="mi">9</span> <span class="mi">10</span> <span class="mi">11</span> <span class="mi">12</span> <span class="mi">13</span> <span class="mi">14</span> <span class="mi">15</span>
</pre></div>
</div>
</section>
<section id="id1">
<h3>Slicing<a class="headerlink" href="#id1" title="Link to this heading"></a></h3>
<p>Python has some other slicing shortcuts…</p>
<p>You do not have to provide both <code class="docutils literal notranslate"><span class="pre">start</span></code> and <code class="docutils literal notranslate"><span class="pre">finish</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [6]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"a bunch of words"</span>
<span class="gp">In [7]: </span><span class="n">s</span><span class="p">[:</span><span class="mi">5</span><span class="p">]</span>
<span class="gh">Out[7]: </span><span class="go">'a bun'</span>
<span class="gp">In [8]: </span><span class="n">s</span><span class="p">[</span><span class="mi">5</span><span class="p">:]</span>
<span class="gh">Out[8]: </span><span class="go">'ch of words'</span>
</pre></div>
</div>
<p>Either <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">len(s)</span></code> will be assumed, respectively.</p>
<p>You can combine this with the negative index to get the end of a sequence:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'this_could_be_a_filename.txt'</span>
<span class="gp">In [5]: </span><span class="n">s</span><span class="p">[:</span><span class="o">-</span><span class="mi">4</span><span class="p">]</span>
<span class="gh">Out[5]: </span><span class="go">'this_could_be_a_filename'</span>
<span class="gp">In [6]: </span><span class="n">s</span><span class="p">[</span><span class="o">-</span><span class="mi">4</span><span class="p">:]</span>
<span class="gh">Out[6]: </span><span class="go">'.txt'</span>
</pre></div>
</div>
<p><strong>That</strong> is a real-world example I use all the time.</p>
</section>
<section id="why-start-from-zero">
<h3>Why start from zero?<a class="headerlink" href="#why-start-from-zero" title="Link to this heading"></a></h3>
<p>Python indexing feels ‘weird’ to some folks – particularly those that don’t come with a background in the C family of languages.</p>
<p>Why is the “first” item indexed with <strong>zero</strong>?</p>
<p>Why is the last item in the slice <strong>not</strong> included?</p>
<p><em>Because</em> these lead to some nifty properties:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">[</span><span class="n">a</span><span class="p">:</span><span class="n">b</span><span class="p">])</span> <span class="o">==</span> <span class="n">b</span><span class="o">-</span><span class="n">a</span>
<span class="n">seq</span><span class="p">[:</span><span class="n">b</span><span class="p">]</span> <span class="o">+</span> <span class="n">seq</span><span class="p">[</span><span class="n">b</span><span class="p">:]</span> <span class="o">==</span> <span class="n">seq</span>
<span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">[:</span><span class="n">b</span><span class="p">])</span> <span class="o">==</span> <span class="n">b</span>
<span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">[</span><span class="o">-</span><span class="n">b</span><span class="p">:])</span> <span class="o">==</span> <span class="n">b</span>
</pre></div>
</div>
<p>There are very many fewer “off by one” errors as a result.</p>
</section>
<section id="more-on-slicing">
<h3>More on Slicing<a class="headerlink" href="#more-on-slicing" title="Link to this heading"></a></h3>
<p>Slicing takes a third argument: <code class="docutils literal notranslate"><span class="pre">step</span></code> which controls which items are
returned:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [18]: </span><span class="n">a_tuple</span>
<span class="gh">Out[18]: </span><span class="go">(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)</span>
<span class="gp">In [19]: </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">15</span><span class="p">]</span>
<span class="gh">Out[19]: </span><span class="go">(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)</span>
<span class="gp">In [20]: </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">15</span><span class="p">:</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[20]: </span><span class="go">(0, 2, 4, 6, 8, 10, 12, 14)</span>
<span class="gp">In [21]: </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">15</span><span class="p">:</span><span class="mi">3</span><span class="p">]</span>
<span class="gh">Out[21]: </span><span class="go">(0, 3, 6, 9, 12)</span>
<span class="gp">In [22]: </span><span class="n">a_tuple</span><span class="p">[::</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="gh">Out[22]: </span><span class="go">(19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)</span>
</pre></div>
</div>
<p>Very cool – a negative step reverses the results!</p>
</section>
<section id="slicing-vs-indexing">
<h3>Slicing vs. Indexing<a class="headerlink" href="#slicing-vs-indexing" title="Link to this heading"></a></h3>
<p>Though they share an operator, slicing and indexing have a few important
differences:</p>
<ul class="simple">
<li><p>Indexing will always return one single object (a scalar), whereas slicing will return a sequence of objects.</p></li>
</ul>
<p>So if you start with, say, a list of numbers, indexing will return a single number. Slicing, on the other hand, will return list of numbers – even if that list only has one number in it – or zero!</p>
<p>Note that strings are a bit of an exception – there is no character type in Python – so a single character is a string – a sequence of length-1.</p>
<ul class="simple">
<li><p>Indexing past the end of a sequence will raise an error, slicing will not:</p></li>
</ul>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [129]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"a bunch of words"</span>
<span class="gp">In [130]: </span><span class="n">s</span><span class="p">[</span><span class="mi">17</span><span class="p">]</span>
<span class="go">----> 1 s[17]</span>
<span class="go">IndexError: string index out of range</span>
<span class="gp">In [131]: </span><span class="n">s</span><span class="p">[</span><span class="mi">10</span><span class="p">:</span><span class="mi">20</span><span class="p">]</span>
<span class="gh">Out[131]: </span><span class="go">' words'</span>
<span class="gp">In [132]: </span><span class="n">s</span><span class="p">[</span><span class="mi">20</span><span class="p">:</span><span class="mi">30</span><span class="p">]</span>
<span class="gh">Out[132]: </span><span class="go">''</span>
</pre></div>
</div>
<p>(try it yourself….)</p>
</section>
</section>
<section id="membership">
<h2>Membership<a class="headerlink" href="#membership" title="Link to this heading"></a></h2>
<p>All sequences support the <code class="docutils literal notranslate"><span class="pre">in</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code> membership operators:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [15]: </span><span class="n">s</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span>
<span class="gp">In [16]: </span><span class="mi">5</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gh">Out[16]: </span><span class="go">True</span>
<span class="gp">In [17]: </span><span class="mi">42</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gh">Out[17]: </span><span class="go">False</span>
<span class="gp">In [18]: </span><span class="mi">42</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gh">Out[18]: </span><span class="go">True</span>
</pre></div>
</div>
<p>For strings, the membership operations are like <code class="docutils literal notranslate"><span class="pre">substring</span></code> operations in
other languages:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [20]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"This is a long string"</span>
<span class="gp">In [21]: </span><span class="s2">"long"</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gh">Out[21]: </span><span class="go">True</span>
</pre></div>
</div>
<p>This does not work for sub-sequences of other types (can you think of why?):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [22]: </span><span class="n">s</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
<span class="gp">In [23]: </span><span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gh">Out[23]: </span><span class="go">False</span>
</pre></div>
</div>
</section>
<section id="concatenation">
<h2>Concatenation<a class="headerlink" href="#concatenation" title="Link to this heading"></a></h2>
<p>Using <code class="docutils literal notranslate"><span class="pre">+</span></code> or <code class="docutils literal notranslate"><span class="pre">*</span></code> on sequences will <em>concatenate</em> them:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [18]: </span><span class="n">l1</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">]</span>
<span class="gp">In [19]: </span><span class="n">l2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span><span class="mi">6</span><span class="p">,</span><span class="mi">7</span><span class="p">,</span><span class="mi">8</span><span class="p">]</span>
<span class="gp">In [20]: </span><span class="n">l1</span> <span class="o">+</span> <span class="n">l2</span>
<span class="gh">Out[20]: </span><span class="go">[1, 2, 3, 4, 5, 6, 7, 8]</span>
<span class="gp">In [21]: </span><span class="p">(</span><span class="n">l1</span><span class="o">+</span><span class="n">l2</span><span class="p">)</span> <span class="o">*</span> <span class="mi">2</span>
<span class="gh">Out[21]: </span><span class="go">[1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8]</span>
</pre></div>
</div>
<section id="multiplying-and-slicing">
<h3>Multiplying and Slicing<a class="headerlink" href="#multiplying-and-slicing" title="Link to this heading"></a></h3>
<p>You can apply this concatenation to slices as well, leading to some nicely
concise code:</p>
<p>from CodingBat: Warmup-1 – front3</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">front3</span><span class="p">(</span><span class="nb">str</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="nb">str</span><span class="p">)</span> <span class="o"><</span> <span class="mi">3</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">str</span><span class="o">+</span><span class="nb">str</span><span class="o">+</span><span class="nb">str</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">str</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span><span class="o">+</span><span class="nb">str</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span><span class="o">+</span><span class="nb">str</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<p>This non-pythonic solution can also be expressed like so:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">front3</span><span class="p">(</span><span class="nb">str</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">str</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span> <span class="o">*</span> <span class="mi">3</span>
</pre></div>
</div>
</section>
</section>
<section id="length">
<h2>Length<a class="headerlink" href="#length" title="Link to this heading"></a></h2>
<p>All sequences have a length. You can get it with the <code class="docutils literal notranslate"><span class="pre">len</span></code> builtin:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [36]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"how long is this, anyway?"</span>
<span class="gp">In [37]: </span><span class="nb">len</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="gh">Out[37]: </span><span class="go">25</span>
</pre></div>
</div>
<p>Remember: Sequences are 0-indexed, so the last index is <code class="docutils literal notranslate"><span class="pre">len(s)-1</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [38]: </span><span class="n">count</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="gp">In [39]: </span><span class="n">s</span><span class="p">[</span><span class="n">count</span><span class="p">]</span>
<span class="gt">------------------------------------------------------------</span>
<span class="ne">IndexError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-39-5a33b9d3e525></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">s</span><span class="p">[</span><span class="n">count</span><span class="p">]</span>
<span class="ne">IndexError</span>: string index out of range
</pre></div>
</div>
<p>Better to use <code class="docutils literal notranslate"><span class="pre">s[-1]</span></code></p>
</section>
<section id="miscellaneous">
<h2>Miscellaneous<a class="headerlink" href="#miscellaneous" title="Link to this heading"></a></h2>
<p>There are a bunch more operations supported by most sequences.
Min and Max
———–</p>
<p>All sequences also support the <code class="docutils literal notranslate"><span class="pre">min</span></code> and <code class="docutils literal notranslate"><span class="pre">max</span></code> builtins:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [42]: </span><span class="n">all_letters</span> <span class="o">=</span> <span class="s2">"thequickbrownfoxjumpedoverthelazydog"</span>
<span class="gp">In [43]: </span><span class="nb">min</span><span class="p">(</span><span class="n">all_letters</span><span class="p">)</span>
<span class="gh">Out[43]: </span><span class="go">'a'</span>
<span class="gp">In [44]: </span><span class="nb">max</span><span class="p">(</span><span class="n">all_letters</span><span class="p">)</span>
<span class="gh">Out[44]: </span><span class="go">'z'</span>
</pre></div>
</div>
<p>Why are those the answers you get? (hint: <code class="docutils literal notranslate"><span class="pre">ord('a')</span></code>)</p>
<p>Of course this works with numbers, too!</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="n">seq</span> <span class="o">=</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">7</span><span class="p">]</span>
<span class="gp">In [2]: </span><span class="nb">min</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span>
<span class="gh">Out[2]: </span><span class="go">2</span>
<span class="gp">In [3]: </span><span class="nb">max</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span>
<span class="gh">Out[3]: </span><span class="go">8</span>
</pre></div>
</div>
<section id="index">
<h3>Index<a class="headerlink" href="#index" title="Link to this heading"></a></h3>
<p>All sequences also support the <code class="docutils literal notranslate"><span class="pre">index</span></code> method, which returns the index of the first occurrence of an item in the sequence:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [46]: </span><span class="n">all_letters</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'d'</span><span class="p">)</span>
<span class="gh">Out[46]: </span><span class="go">21</span>
</pre></div>
</div>
<p>This causes a <code class="docutils literal notranslate"><span class="pre">ValueError</span></code> if the item is not in the sequence:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [47]: </span><span class="n">all_letters</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'A'</span><span class="p">)</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">ValueError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-47-2db728a46f78></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">all_letters</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'A'</span><span class="p">)</span>
<span class="ne">ValueError</span>: substring not found
</pre></div>
</div>
</section>
<section id="count">
<h3>Count<a class="headerlink" href="#count" title="Link to this heading"></a></h3>
<p>A sequence can also be queried for the number of times a particular item
appears:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [52]: </span><span class="n">all_letters</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'o'</span><span class="p">)</span>
<span class="gh">Out[52]: </span><span class="go">4</span>
<span class="gp">In [53]: </span><span class="n">all_letters</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'the'</span><span class="p">)</span>
<span class="gh">Out[53]: </span><span class="go">2</span>
</pre></div>
</div>
<p>This does not raise an error if the item you seek is not present:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [54]: </span><span class="n">all_letters</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'A'</span><span class="p">)</span>
<span class="gh">Out[54]: </span><span class="go">0</span>
</pre></div>
</div>
</section>
</section>
<section id="iteration">
<h2>Iteration<a class="headerlink" href="#iteration" title="Link to this heading"></a></h2>
<p>All sequences are “iterables”.</p>
<p>You can iterate over a sequence with <code class="docutils literal notranslate"><span class="pre">for</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">element</span> <span class="ow">in</span> <span class="n">sequence</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">(</span><span class="n">element</span><span class="p">)</span>
</pre></div>
</div>
<p>Which is what we mean when we say a sequence is an “iterable”.</p>
<p>There are some complexities about that – but more on that in another lesson.</p>
</section>
<section id="lists-tuples">
<h2>Lists, Tuples…<a class="headerlink" href="#lists-tuples" title="Link to this heading"></a></h2>
<p>The <em>primary</em> sequence types.</p>
<section id="lists">
<h3>Lists<a class="headerlink" href="#lists" title="Link to this heading"></a></h3>
<p>Lists can be constructed using list literals (<code class="docutils literal notranslate"><span class="pre">[]</span></code>):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="p">[]</span>
<span class="gh">Out[1]: </span><span class="go">[]</span>
<span class="gp">In [2]: </span><span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span>
<span class="gh">Out[2]: </span><span class="go">[1, 2, 3]</span>
<span class="gp">In [3]: </span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="mf">7.34</span><span class="p">]</span>
<span class="gh">Out[3]: </span><span class="go">[1, 'a', 7.34]</span>
</pre></div>
</div>
<p>Or by using the <code class="docutils literal notranslate"><span class="pre">list</span></code> type object as a constructor:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [6]: </span><span class="nb">list</span><span class="p">()</span>
<span class="gh">Out[6]: </span><span class="go">[]</span>
<span class="gp">In [7]: </span><span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">))</span>
<span class="gh">Out[7]: </span><span class="go">[0, 1, 2, 3]</span>
<span class="gp">In [8]: </span><span class="nb">list</span><span class="p">(</span><span class="s1">'abc'</span><span class="p">)</span>
<span class="gh">Out[8]: </span><span class="go">['a', 'b', 'c']</span>
</pre></div>
</div>
<p>It will take any “iterable” (which means any sequence automatically – remember that all sequences are iterable?)</p>
</section>
<section id="list-elements">
<h3>List Elements<a class="headerlink" href="#list-elements" title="Link to this heading"></a></h3>
<p>The elements contained in a list need not be of a single type.</p>
<p>Lists are <em>heterogenous</em>, <em>ordered</em> collections.</p>
<p>Each element in a list is a value, and can be in multiple lists and have
multiple names (or no name):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [9]: </span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Brian'</span>
<span class="gp">In [10]: </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">name</span><span class="p">]</span>
<span class="gp">In [11]: </span><span class="n">b</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="n">name</span><span class="p">]</span>
<span class="gp">In [12]: </span><span class="n">a</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[12]: </span><span class="go">'Brian'</span>
<span class="gp">In [13]: </span><span class="n">b</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[13]: </span><span class="go">'Brian'</span>
<span class="gp">In [14]: </span><span class="n">a</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="ow">is</span> <span class="n">b</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gh">Out[14]: </span><span class="go">True</span>
</pre></div>
</div>
<p>Notice that even with a “literal” – the elements don’t need to be literals as well – they can be names.</p>
<p>They can even be function calls:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="k">def</span><span class="w"> </span><span class="nf">fun</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="k">return</span> <span class="n">n</span> <span class="o">*</span> <span class="mi">2</span>
<span class="gp"> ...:</span>
<span class="gp">In [5]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'four'</span><span class="p">,</span> <span class="n">fun</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span> <span class="n">fun</span><span class="p">(</span><span class="mi">9</span><span class="p">)]</span>
<span class="gp">In [6]: </span><span class="n">l</span>
<span class="gh">Out[6]: </span><span class="go">[3, 'four', 6, 18]</span>
</pre></div>
</div>
</section>
<section id="tuples">
<h3>Tuples<a class="headerlink" href="#tuples" title="Link to this heading"></a></h3>
<p>Tuples can be constructed using tuple literals (<code class="docutils literal notranslate"><span class="pre">()</span></code>):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [15]: </span><span class="p">()</span>
<span class="gh">Out[15]: </span><span class="go">()</span>
<span class="gp">In [16]: </span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="gh">Out[16]: </span><span class="go">(1, 2)</span>
<span class="gp">In [17]: </span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="mf">7.65</span><span class="p">)</span>
<span class="gh">Out[17]: </span><span class="go">(1, 'a', 7.65)</span>
<span class="gp">In [18]: </span><span class="p">(</span><span class="mi">1</span><span class="p">,)</span>
<span class="gh">Out[18]: </span><span class="go">(1,)</span>
</pre></div>
</div>
</section>
<section id="tuples-and-commas">
<h3>Tuples and Commas…<a class="headerlink" href="#tuples-and-commas" title="Link to this heading"></a></h3>
<p>Tuples don’t NEED parentheses…</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [161]: </span><span class="n">t</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">In [162]: </span><span class="n">t</span>
<span class="gh">Out[162]: </span><span class="go">(1, 2, 3)</span>
<span class="gp">In [163]: </span><span class="n">t</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span>
<span class="gp">In [164]: </span><span class="n">t</span>
<span class="gh">Out[164]: </span><span class="go">(1, 2, 3)</span>
<span class="gp">In [165]: </span><span class="nb">type</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="gh">Out[165]: </span><span class="go">tuple</span>
</pre></div>
</div>
<p>But they <em>do</em> need commas…!</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [156]: </span><span class="n">t</span> <span class="o">=</span> <span class="p">(</span> <span class="mi">3</span> <span class="p">)</span>
<span class="gp">In [157]: </span><span class="nb">type</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="gh">Out[157]: </span><span class="go">int</span>
<span class="gp">In [158]: </span><span class="n">t</span> <span class="o">=</span> <span class="p">(</span> <span class="mi">3</span><span class="p">,</span> <span class="p">)</span>
<span class="gp">In [160]: </span><span class="nb">type</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="gh">Out[160]: </span><span class="go">tuple</span>
</pre></div>
</div>
<p>This is a Python “gotcha” – some folks on my team recently had a weird bug that two of them could not figure out. They were getting a type error – something like:</p>
<p>TypeError: unsupported operand type(s) for /: ‘tuple’ and ‘float’</p>
<p>which made no sense – there were no tuples involved – in this case, the value was being pulled from a list – and it WAS a float. They even put type checking code in there, and it was, indeed, a float.</p>
<p>After poking at the code a bit, I suddenly spotted an extra comma – BINGO! that was it.</p>
<p>The code was more involved, and thus harder to see, but it was pretty much like this:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">16</span><span class="p">]:</span> <span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">17</span><span class="p">]:</span> <span class="n">x</span> <span class="o">=</span> <span class="n">l</span><span class="p">[</span><span class="mi">3</span><span class="p">],</span>
</pre></div>
</div>
<p>then a bit further down, x was used:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">18</span><span class="p">]:</span> <span class="n">y</span> <span class="o">=</span> <span class="n">x</span> <span class="o">/</span> <span class="mf">2.0</span>
<span class="o">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span> <span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">)</span>
<span class="o"><</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">18</span><span class="o">-</span><span class="mi">5289811</span><span class="n">a13ac</span><span class="o">></span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span><span class="p">()</span>
<span class="o">----></span> <span class="mi">1</span> <span class="n">y</span> <span class="o">=</span> <span class="n">x</span> <span class="o">/</span> <span class="mf">2.0</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="n">unsupported</span> <span class="n">operand</span> <span class="nb">type</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="k">for</span> <span class="o">/</span><span class="p">:</span> <span class="s1">'tuple'</span> <span class="ow">and</span> <span class="s1">'float'</span>
</pre></div>
</div>
<p>Would you have seen that?</p>
</section>
<section id="converting-something-to-a-tuple">
<h3>Converting something to a Tuple<a class="headerlink" href="#converting-something-to-a-tuple" title="Link to this heading"></a></h3>
<p>You can also use the <code class="docutils literal notranslate"><span class="pre">tuple</span></code> type object to convert any iterable (sequence) into a tuple:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [20]: </span><span class="nb">tuple</span><span class="p">()</span>
<span class="gh">Out[20]: </span><span class="go">()</span>
<span class="gp">In [21]: </span><span class="nb">tuple</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">))</span>
<span class="gh">Out[21]: </span><span class="go">(0, 1, 2, 3)</span>
<span class="gp">In [22]: </span><span class="nb">tuple</span><span class="p">(</span><span class="s1">'garbanzo'</span><span class="p">)</span>
<span class="gh">Out[22]: </span><span class="go">('g', 'a', 'r', 'b', 'a', 'n', 'z', 'o')</span>
</pre></div>
</div>
</section>
<section id="tuple-elements">
<h3>Tuple Elements<a class="headerlink" href="#tuple-elements" title="Link to this heading"></a></h3>
<p>The elements contained in a tuple need not be of a single type.</p>
<p>Tuples are <em>heterogenous</em>, <em>ordered</em> collections.</p>
<p>Each element in a tuple is a value, and can be in multiple tuples and have
multiple names (or no name):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [23]: </span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Brian'</span>
<span class="gp">In [24]: </span><span class="n">other</span> <span class="o">=</span> <span class="n">name</span>
<span class="gp">In [25]: </span><span class="n">a</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
<span class="gp">In [26]: </span><span class="n">b</span> <span class="o">=</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="n">other</span><span class="p">)</span>
<span class="gp">In [27]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="ow">is</span> <span class="n">b</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="go">False False True</span>
</pre></div>
</div>
<p>Look familiar from lists??</p>
</section>
<section id="lists-vs-tuples">
<h3>Lists vs. Tuples<a class="headerlink" href="#lists-vs-tuples" title="Link to this heading"></a></h3>
<blockquote>
<div><p>So why have both?</p>
</div></blockquote>
</section>
</section>
<section id="mutability">
<h2>Mutability<a class="headerlink" href="#mutability" title="Link to this heading"></a></h2>
<a class="reference internal image-reference" href="../_images/transmogrifier.jpg"><img alt="Presto change-o" class="align-center" src="../_images/transmogrifier.jpg" style="width: 35%;" />
</a>
<p>image from flickr by <a class="reference external" href="https://www.flickr.com/photos/illuminaut/3595530403">illuminaut</a>, (CC by-nc-sa)</p>
</section>
<section id="mutability-in-python">
<h2>Mutability in Python<a class="headerlink" href="#mutability-in-python" title="Link to this heading"></a></h2>
<p>All objects in Python fall into one of two camps:</p>
<ul class="simple">
<li><p>Mutable</p></li>
<li><p>Immutable</p></li>
</ul>
<p>Objects which are mutable may be <em>changed in place</em>.</p>
<p>Objects which are immutable may not be changed.</p>
<p>Ever.</p>
<section id="the-types-we-know">
<h3>The Types We Know<a class="headerlink" href="#the-types-we-know" title="Link to this heading"></a></h3>
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Immutable</p></th>
<th class="head"><p>Mutable</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>String</p></td>
<td><p>List</p></td>
</tr>
<tr class="row-odd"><td><p>Integer</p></td>
<td><p>Dictionary</p></td>
</tr>
<tr class="row-even"><td><p>Float</p></td>
<td></td>
</tr>
<tr class="row-odd"><td><p>Tuple</p></td>
<td></td>
</tr>
</tbody>
</table>
<p>This may make it look like the Mutables are rare – but in fact, most “container types”, and most custom objects are mutable.</p>
<p>Immutable types are the exception</p>
</section>
<section id="lists-are-mutable">
<h3>Lists Are Mutable<a class="headerlink" href="#lists-are-mutable" title="Link to this heading"></a></h3>
<p>Try this out:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [28]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">]</span>
<span class="gp">In [29]: </span><span class="n">food</span>
<span class="gh">Out[29]: </span><span class="go">['spam', 'eggs', 'ham']</span>
<span class="gp">In [30]: </span><span class="n">food</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'raspberries'</span>
<span class="gp">In [31]: </span><span class="n">food</span>
<span class="gh">Out[31]: </span><span class="go">['spam', 'raspberries', 'ham']</span>
</pre></div>
</div>
<p>We repeat the exercise with a Tuple:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [32]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">)</span>
<span class="gp">In [33]: </span><span class="n">food</span>
<span class="gh">Out[33]: </span><span class="go">('spam', 'eggs', 'ham')</span>
<span class="gp">In [34]: </span><span class="n">food</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'raspberries'</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-34-0c3401794933></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">food</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'raspberries'</span>
<span class="ne">TypeError</span>: 'tuple' object does not support item assignment
</pre></div>
</div>
</section>
<section id="watch-out-when-name-binding">
<h3>Watch Out when name binding<a class="headerlink" href="#watch-out-when-name-binding" title="Link to this heading"></a></h3>
<p>This property means you need to be aware of what you are doing with your lists:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [36]: </span><span class="n">original</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="gp">In [37]: </span><span class="n">altered</span> <span class="o">=</span> <span class="n">original</span>
<span class="gp">In [38]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">original</span><span class="p">)):</span>
<span class="gp"> ....: </span> <span class="k">if</span> <span class="kc">True</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">altered</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp"> ....:</span>
</pre></div>
</div>
<p>Perhaps we want to check to see if altered has been updated, as a flag for
whatever condition caused it to be updated.</p>
<p>What is the result of this code?</p>
</section>
<section id="perhaps-not-what-you-expect">
<h3>Perhaps Not What You Expect<a class="headerlink" href="#perhaps-not-what-you-expect" title="Link to this heading"></a></h3>
<p>Our <code class="docutils literal notranslate"><span class="pre">altered</span></code> list has been updated as we’d expect:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [39]: </span><span class="n">altered</span>
<span class="gh">Out[39]: </span><span class="go">[2, 3, 4]</span>
</pre></div>
</div>
<p>But so has the <code class="docutils literal notranslate"><span class="pre">original</span></code> list:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [40]: </span><span class="n">original</span>
<span class="gh">Out[40]: </span><span class="go">[2, 3, 4]</span>
</pre></div>
</div>
<p>Why?</p>
<p>Let’s look at that code again.</p>
<p>What does the line: <code class="docutils literal notranslate"><span class="pre">altered</span> <span class="pre">=</span> <span class="pre">original</span></code> do?</p>
<p>It binds the name: “altered” to the same object that “original” is bound to.</p>
<p>That is, there is only one list, even though it is referred to by two names. So when you mutate (or change) that list from <em>either</em> name, the changes show up when you refer to it by the other name.</p>
</section>
<section id="other-gotchas">
<h3>Other Gotchas<a class="headerlink" href="#other-gotchas" title="Link to this heading"></a></h3>
<p>Easy container setup, or deadly trap?</p>
<p>Say you want something like sort of like a 2D array – one way to do that is to nest lists – make a list of lists.</p>
<p>ONe seemingobvious way to create an empty list of lists would be to use multiplcation of lists – make a list with one list in it, and then multiply it by the number of lists you want:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [12]: </span><span class="n">bins</span> <span class="o">=</span> <span class="p">[</span> <span class="p">[]</span> <span class="p">]</span> <span class="o">*</span> <span class="mi">5</span>
<span class="gp">In [13]: </span><span class="n">bins</span>
<span class="gh">Out[13]: </span><span class="go">[[], [], [], [], []]</span>
</pre></div>
</div>
<p>OK – that worked – you have a list with five empty lists in it. So let’s try using that. This is a very contrived example, but say you have list of words:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [14]: </span><span class="n">words</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'one'</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">,</span> <span class="s1">'rough'</span><span class="p">,</span> <span class="s1">'sad'</span><span class="p">,</span> <span class="s1">'goof'</span><span class="p">]</span>
</pre></div>
</div>
<p>and you want to put one in each of the “inside” lists:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [15]: </span><span class="c1"># loop five times</span>
<span class="go"> ...: for i in range(5):</span>
<span class="go"> ...: # add a word to the corresponding bin</span>
<span class="go"> ...: bins[i].append(words[i])</span>
</pre></div>
</div>
<p>So, what is going to be in <code class="docutils literal notranslate"><span class="pre">bins</span></code> now? Think for a bit first – you added one word to each bin, yes? But are those “sublists” independent?</p>
</section>
<section id="there-is-only-one-bin">
<h3>There is only <strong>One</strong> bin<a class="headerlink" href="#there-is-only-one-bin" title="Link to this heading"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [16]: </span><span class="n">bins</span>
<span class="gh">Out[16]:</span>
<span class="go">[['one', 'three', 'rough', 'sad', 'goof'],</span>
<span class="go"> ['one', 'three', 'rough', 'sad', 'goof'],</span>
<span class="go"> ['one', 'three', 'rough', 'sad', 'goof'],</span>
<span class="go"> ['one', 'three', 'rough', 'sad', 'goof'],</span>
<span class="go"> ['one', 'three', 'rough', 'sad', 'goof']]</span>
</pre></div>
</div>
<p>Whoa! So we don’t have 5 lists – we have five <em>references</em> to the same list. Remember that in Python you can have any number of names “bound” to any object – and any object can be contained in any number of containers, or multiple times in one container.</p>
<p>So when we multiplied a sequence containing a single <em>mutable</em> object. We got a list containing five references to a single <em>mutable</em> object.</p>
<p>Since it’s mutable – you can change it “in place”, and when you change it – the change shows everywhere that list is referenced.</p>
<p>So how to make a list of independent lists? You need to loop and call that code that makes an empty list each time in the loop, something like this:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [21]: </span><span class="n">bins</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp">In [22]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="go"> ...: bins.append([])</span>
<span class="go"> ...:</span>
<span class="gp">In [23]: </span><span class="n">bins</span>
<span class="gh">Out[23]: </span><span class="go">[[], [], [], [], []]</span>
<span class="gp">In [24]: </span><span class="c1"># loop five times</span>
<span class="go"> ...: for i in range(5):</span>
<span class="go"> ...: # add a word to the corresponding bin</span>
<span class="go"> ...: bins[i].append(words[i])</span>
<span class="go"> ...:</span>
<span class="gp">In [25]: </span><span class="n">bins</span>
<span class="gh">Out[25]: </span><span class="go">[['one'], ['three'], ['rough'], ['sad'], ['goof']]</span>
</pre></div>
</div>
</section>
<section id="mutable-default-argument">
<h3>Mutable Default Argument<a class="headerlink" href="#mutable-default-argument" title="Link to this heading"></a></h3>
<p>Watch out especially for passing mutable objects as default values for function parameters:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [71]: </span><span class="k">def</span><span class="w"> </span><span class="nf">accumulator</span><span class="p">(</span><span class="n">count</span><span class="p">,</span> <span class="n">ac_list</span><span class="o">=</span><span class="p">[]):</span>
<span class="gp"> ....: </span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">count</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="n">ac_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="n">ac_list</span>
<span class="gp"> ....:</span>
<span class="gp">In [72]: </span><span class="n">accumulator</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="gh">Out[72]: </span><span class="go">[0, 1, 2, 3, 4]</span>
<span class="gp">In [73]: </span><span class="n">accumulator</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span>
<span class="gh">Out[73]: </span><span class="go">[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 5, 6]</span>
</pre></div>
</div>
<p>What is going on here???</p>
<p>It turns out that that code: <code class="docutils literal notranslate"><span class="pre">ac_list=[]</span></code> is evaluated <em>when the function is defined</em> – <strong>not</strong> when the function is called.</p>
<p>So the name “ac_list” in the local scope of that function always refers to the same list. So every time the function is called, more is added to that same list.</p>
<p>The moral of the story here is:</p>
<p><strong>Do not use mutable objects for default arguments!</strong></p>
<p>It turns out that this early evaluation can be useful – but for now, just remember not to use mutables as default arguments.</p>
<p>By the way –this is how you <em>should</em> write that code:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [21]: </span><span class="k">def</span><span class="w"> </span><span class="nf">accumulator</span><span class="p">(</span><span class="n">count</span><span class="p">,</span> <span class="n">ac_list</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="go"> ...: if ac_list is None:</span>
<span class="go"> ...: ac_list = []</span>
<span class="go"> ...: for i in range(count):</span>
<span class="go"> ...: ac_list.append(i)</span>
<span class="go"> ...: return ac_list</span>
<span class="gp">In [22]: </span><span class="n">accumulator</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="gh">Out[22]: </span><span class="go">[0, 1, 2, 3, 4]</span>
<span class="gp">In [23]: </span><span class="n">accumulator</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span>
<span class="gh">Out[23]: </span><span class="go">[0, 1, 2, 3, 4, 5, 6]</span>
</pre></div>
</div>
<p>This will ensure that a new list will be created if one is not passed-in.</p>
</section>
</section>
<section id="mutable-sequence-methods">
<h2>Mutable Sequence Methods<a class="headerlink" href="#mutable-sequence-methods" title="Link to this heading"></a></h2>
<p>In addition to all the methods supported by sequences we’ve seen above, mutable sequences (the list), have a number of other methods that are used to change it in place.</p>
<p>You can find all these in the Standard Library Documentation:</p>
<p><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#typesseq-mutable">https://docs.python.org/3/library/stdtypes.html#typesseq-mutable</a></p>
<section id="assignment">
<h3>Assignment<a class="headerlink" href="#assignment" title="Link to this heading"></a></h3>
<p>You’ve already seen changing a single element of a list by assignment.</p>
<p>Pretty much the same as “arrays” in most languages:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [100]: </span><span class="n">my_list</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="gp">In [101]: </span><span class="n">my_list</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="mi">10</span>
<span class="gp">In [102]: </span><span class="n">my_list</span>
<span class="gh">Out[102]: </span><span class="go">[1, 2, 10]</span>
</pre></div>
</div>
</section>
<section id="growing-the-list">
<h3>Growing the List<a class="headerlink" href="#growing-the-list" title="Link to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">.append()</span></code>, <code class="docutils literal notranslate"><span class="pre">.insert()</span></code>, <code class="docutils literal notranslate"><span class="pre">.extend()</span></code></p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [74]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">]</span>
<span class="gp">In [75]: </span><span class="n">food</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">'sushi'</span><span class="p">)</span>
<span class="gp">In [76]: </span><span class="n">food</span>
<span class="gh">Out[76]: </span><span class="go">['spam', 'eggs', 'ham', 'sushi']</span>
<span class="gp">In [77]: </span><span class="n">food</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="s1">'beans'</span><span class="p">)</span>
<span class="gp">In [78]: </span><span class="n">food</span>
<span class="gh">Out[78]: </span><span class="go">['beans', 'spam', 'eggs', 'ham', 'sushi']</span>
<span class="gp">In [79]: </span><span class="n">food</span><span class="o">.</span><span class="n">extend</span><span class="p">([</span><span class="s1">'bread'</span><span class="p">,</span> <span class="s1">'water'</span><span class="p">])</span>
<span class="gp">In [80]: </span><span class="n">food</span>
<span class="gh">Out[80]: </span><span class="go">['beans', 'spam', 'eggs', 'ham', 'sushi', 'bread', 'water']</span>
</pre></div>
</div>
</section>
<section id="more-on-extend">
<h3>More on Extend<a class="headerlink" href="#more-on-extend" title="Link to this heading"></a></h3>
<p>You can pass any sequence to <code class="docutils literal notranslate"><span class="pre">.extend()</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [85]: </span><span class="n">food</span>
<span class="gh">Out[85]: </span><span class="go">['beans', 'spam', 'eggs', 'ham', 'sushi', 'bread', 'water']</span>
<span class="gp">In [86]: </span><span class="n">food</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="s1">'spaghetti'</span><span class="p">)</span>
<span class="gp">In [87]: </span><span class="n">food</span>
<span class="gh">Out[87]:</span>
<span class="go">['beans', 'spam', 'eggs', 'ham', 'sushi', 'bread', 'water',</span>
<span class="go"> 's', 'p', 'a', 'g', 'h', 'e', 't', 't', 'i']</span>
</pre></div>
</div>
<p>So be careful – a string is a single object – but also a sequence of characters (technically a sequence of length-1 strings).</p>
</section>
<section id="shrinking-a-list">
<h3>Shrinking a list<a class="headerlink" href="#shrinking-a-list" title="Link to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">.pop()</span></code>, <code class="docutils literal notranslate"><span class="pre">.remove()</span></code></p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [203]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">,</span> <span class="s1">'toast'</span><span class="p">]</span>
<span class="gp">In [204]: </span><span class="n">food</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="gh">Out[204]: </span><span class="go">'toast'</span>
<span class="gp">In [205]: </span><span class="n">food</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="gh">Out[205]: </span><span class="go">'spam'</span>
<span class="gp">In [206]: </span><span class="n">food</span>
<span class="gh">Out[206]: </span><span class="go">['eggs', 'ham']</span>
<span class="gp">In [207]: </span><span class="n">food</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s1">'ham'</span><span class="p">)</span>
<span class="gp">In [208]: </span><span class="n">food</span>
<span class="gh">Out[208]: </span><span class="go">['eggs']</span>
</pre></div>
</div>
<p>You can also delete <em>slices</em> of a list with the <code class="docutils literal notranslate"><span class="pre">del</span></code> keyword:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [92]: </span><span class="n">nums</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="gp">In [93]: </span><span class="n">nums</span>
<span class="gh">Out[93]: </span><span class="go">[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]</span>
<span class="gp">In [94]: </span><span class="k">del</span> <span class="n">nums</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="mi">6</span><span class="p">:</span><span class="mi">2</span><span class="p">]</span>
<span class="gp">In [95]: </span><span class="n">nums</span>
<span class="gh">Out[95]: </span><span class="go">[0, 2, 4, 6, 7, 8, 9]</span>
<span class="gp">In [96]: </span><span class="k">del</span> <span class="n">nums</span><span class="p">[</span><span class="o">-</span><span class="mi">3</span><span class="p">:]</span>
<span class="gp">In [97]: </span><span class="n">nums</span>
<span class="gh">Out[97]: </span><span class="go">[0, 2, 4, 6]</span>
</pre></div>
</div>
</section>
<section id="copying-lists">
<h3>Copying Lists<a class="headerlink" href="#copying-lists" title="Link to this heading"></a></h3>
<p>You can make copies of part of a list using <em>slicing</em>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [227]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">,</span> <span class="s1">'sushi'</span><span class="p">]</span>
<span class="gp">In [228]: </span><span class="n">some_food</span> <span class="o">=</span> <span class="n">food</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="mi">3</span><span class="p">]</span>
<span class="gp">In [229]: </span><span class="n">some_food</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'bacon'</span>
<span class="gp">In [230]: </span><span class="n">food</span>
<span class="gh">Out[230]: </span><span class="go">['spam', 'eggs', 'ham', 'sushi']</span>
<span class="gp">In [231]: </span><span class="n">some_food</span>
<span class="gh">Out[231]: </span><span class="go">['eggs', 'bacon']</span>
</pre></div>
</div>
<p>If you provide <em>no</em> arguments to the slice, it makes a copy of the entire list:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [232]: </span><span class="n">food</span>
<span class="gh">Out[232]: </span><span class="go">['spam', 'eggs', 'ham', 'sushi']</span>
<span class="gp">In [233]: </span><span class="n">food2</span> <span class="o">=</span> <span class="n">food</span><span class="p">[:]</span>
<span class="gp">In [234]: </span><span class="n">food</span> <span class="ow">is</span> <span class="n">food2</span>
<span class="gh">Out[234]: </span><span class="go">False</span>
</pre></div>
</div>
</section>
<section id="shallow-copies">
<h3>Shallow Copies<a class="headerlink" href="#shallow-copies" title="Link to this heading"></a></h3>
<p>The copy of a list made this way is a <em>shallow copy</em>.</p>
<p>The list is itself a new object, but the objects it contains are not.</p>
<p><em>Mutable</em> objects in the list can be mutated in both copies:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [249]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="p">[</span><span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">]]</span>
<span class="gp">In [251]: </span><span class="n">food_copy</span> <span class="o">=</span> <span class="n">food</span><span class="p">[:]</span>
<span class="gp">In [252]: </span><span class="n">food</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="gh">Out[252]: </span><span class="go">'ham'</span>
<span class="gp">In [253]: </span><span class="n">food</span>
<span class="gh">Out[253]: </span><span class="go">['spam', ['eggs']]</span>
<span class="gp">In [256]: </span><span class="n">food</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="gh">Out[256]: </span><span class="go">'spam'</span>
<span class="gp">In [257]: </span><span class="n">food</span>
<span class="gh">Out[257]: </span><span class="go">[['eggs']]</span>
<span class="gp">In [258]: </span><span class="n">food_copy</span>
<span class="gh">Out[258]: </span><span class="go">['spam', ['eggs']]</span>
</pre></div>
</div>
</section>
<section id="copies-can-solve-problems">
<h3>Copies can solve problems<a class="headerlink" href="#copies-can-solve-problems" title="Link to this heading"></a></h3>
<p>Consider this common pattern:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">somelist</span><span class="p">:</span>
<span class="k">if</span> <span class="n">should_be_removed</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="n">somelist</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
<p>This looks benign enough, but changing a list while you are iterating over it can be the cause of some pernicious bugs.</p>
</section>
<section id="the-problem">
<h3>The Problem<a class="headerlink" href="#the-problem" title="Link to this heading"></a></h3>
<p>For example:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [27]: </span><span class="n">l</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
<span class="gp">In [28]: </span><span class="n">l</span>
<span class="gh">Out[28]: </span><span class="go">[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]</span>
<span class="gp">In [29]: </span><span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">l</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">l</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="gp">In [30]: </span><span class="n">l</span>
<span class="gh">Out[30]: </span><span class="go">[1, 3, 5, 7, 9]</span>
</pre></div>
</div>
<p>Was that what you expected?</p>
</section>
<section id="the-solution">
<h3>The Solution<a class="headerlink" href="#the-solution" title="Link to this heading"></a></h3>
<p>Iterate over a copy, and mutate the original:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [33]: </span><span class="n">l</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
<span class="gp">In [34]: </span><span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">l</span><span class="p">[:]:</span>
<span class="gp"> ....: </span> <span class="n">l</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="gp">In [35]: </span><span class="n">l</span>
<span class="gh">Out[35]: </span><span class="go">[]</span>
</pre></div>
</div>
</section>
</section>
<section id="miscellaneous-list-methods">
<h2>Miscellaneous List Methods<a class="headerlink" href="#miscellaneous-list-methods" title="Link to this heading"></a></h2>
<p>These methods change a list in place and are not available on immutable sequence types.</p>
<p><code class="docutils literal notranslate"><span class="pre">.reverse()</span></code></p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [129]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">,</span> <span class="s1">'ham'</span><span class="p">]</span>
<span class="gp">In [130]: </span><span class="n">food</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span>
<span class="gp">In [131]: </span><span class="n">food</span>
<span class="gh">Out[131]: </span><span class="go">['ham', 'eggs', 'spam']</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">.sort()</span></code></p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [132]: </span><span class="n">food</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="gp">In [133]: </span><span class="n">food</span>
<span class="gh">Out[133]: </span><span class="go">['eggs', 'ham', 'spam']</span>
</pre></div>
</div>
<p>Because these methods mutate the list in place, they have a return value of <code class="docutils literal notranslate"><span class="pre">None</span></code></p>
<section id="custom-sorting">
<h3>Custom Sorting<a class="headerlink" href="#custom-sorting" title="Link to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">.sort()</span></code> can take an optional <code class="docutils literal notranslate"><span class="pre">key</span></code> parameter.</p>
<p>It should be a function that takes one parameter (list items one at a time) and returns something that can be used for sorting:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [137]: </span><span class="k">def</span><span class="w"> </span><span class="nf">third_letter</span><span class="p">(</span><span class="n">string</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">return</span> <span class="n">string</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="gp"> .....:</span>
<span class="gp">In [138]: </span><span class="n">food</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="n">key</span><span class="o">=</span><span class="n">third_letter</span><span class="p">)</span>
<span class="gp">In [139]: </span><span class="n">food</span>
<span class="gh">Out[139]: </span><span class="go">['spam', 'eggs', 'ham']</span>
</pre></div>
</div>
<p>You end up with the list sorted by the third letter in each element.</p>
</section>
<section id="list-performance">
<h3>List Performance<a class="headerlink" href="#list-performance" title="Link to this heading"></a></h3>
<ul class="simple">
<li><p>indexing is fast and constant time: O(1)</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">l</span></code> is proportional to n: O(n)</p></li>
<li><p>visiting all is proportional to n: O(n)</p></li>
<li><p>operating on the end of list is fast and constant time: O(1)</p>
<ul>
<li><p>append(), pop()</p></li>
</ul>
</li>
<li><p>operating on the front (or middle) of the list depends on n: O(n)</p>
<ul>