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<div class="section" id="session-three-sequences-iteration-and-string-formatting">
<h1>Session Three: Sequences, Iteration and String Formatting<a class="headerlink" href="#session-three-sequences-iteration-and-string-formatting" title="Permalink to this headline">¶</a></h1>
<div class="section" id="review-questions">
<h2>Review/Questions<a class="headerlink" href="#review-questions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="review-of-previous-session">
<h3>Review of Previous Session<a class="headerlink" href="#review-of-previous-session" title="Permalink to this headline">¶</a></h3>
<ul class="build simple">
<li>Functions</li>
<li>Booleans</li>
<li>Modules</li>
</ul>
</div>
<div class="section" id="homework-review">
<h3>Homework Review<a class="headerlink" href="#homework-review" title="Permalink to this headline">¶</a></h3>
<p class="center large">Any questions that are nagging?</p>
</div>
</div>
<div class="section" id="sequences">
<h2>Sequences<a class="headerlink" href="#sequences" title="Permalink to this headline">¶</a></h2>
<p class="center large">Ordered collections of objects</p>
<div class="section" id="what-is-a-sequence">
<h3>What is a Sequence?<a class="headerlink" href="#what-is-a-sequence" title="Permalink to this headline">¶</a></h3>
<p>Remember Duck Typing? A <em>sequence</em> can be considered as anything that supports
<em>at least</em> these operations:</p>
<ul class="build simple">
<li>Indexing</li>
<li>Slicing</li>
<li>Membership</li>
<li>Concatenation</li>
<li>Length</li>
<li>Iteration</li>
</ul>
</div>
<div class="section" id="sequence-types">
<h3>Sequence Types<a class="headerlink" href="#sequence-types" title="Permalink to this headline">¶</a></h3>
<p>There are seven builtin types in Python that are <em>sequences</em>:</p>
<ul class="simple">
<li>strings</li>
<li>Unicode strings</li>
<li>lists</li>
<li>tuples</li>
<li>bytearrays</li>
<li>buffers</li>
<li>array.arrays</li>
<li>xrange objects (almost)</li>
</ul>
<p>For this class, you won’t see much beyond the string types, lists, tuples – the rest are pretty special purpose.</p>
<p>But what we say today applies to all sequences (with minor caveats)</p>
</div>
<div class="section" id="indexing">
<h3>Indexing<a class="headerlink" href="#indexing" title="Permalink to this headline">¶</a></h3>
<p>Items in a sequence may be looked up by <em>index</em> using the subscription
operator: <tt class="docutils literal"><span class="pre">[]</span></tt></p>
<p>Indexing in Python always starts at zero.</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [98]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"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">u'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">u'i'</span>
</pre></div>
</div>
<p>You can use negative indexes to count from the end:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [105]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"this is a string"</span>
<span class="gp">In [106]: </span><span class="n">s</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="gh">Out[106]: </span><span class="go">u'g'</span>
<span class="gp">In [107]: </span><span class="n">s</span><span class="p">[</span><span class="o">-</span><span class="mi">6</span><span class="p">]</span>
<span class="gh">Out[107]: </span><span class="go">u's'</span>
</pre></div>
</div>
<p>Indexing beyond the end of a sequence causes an IndexError:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [4]: </span><span class="n">s</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</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 [5]: </span><span class="n">s</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">IndexError</span><span class="g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-5-42efaba84d8b></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="mi">4</span><span class="p">]</span>
<span class="ne">IndexError</span>: list index out of range
</pre></div>
</div>
</div>
<div class="section" id="slicing">
<h3>Slicing<a class="headerlink" href="#slicing" title="Permalink to this headline">¶</a></h3>
<p>Slicing a sequence creates a new sequence with a range of objects from the
original sequence.</p>
<p>It also uses the subscription operator (<tt class="docutils literal"><span class="pre">[]</span></tt>), but with a twist.</p>
<p><tt class="docutils literal"><span class="pre">sequence[start:finish]</span></tt> returns all sequence[i] for which start <= i < finish:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [121]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"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">u'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">u'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">u'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">u'bunch'</span>
</pre></div>
</div>
<p>Think of the indexes as pointing to the spaces between the items:</p>
<div class="highlight-python"><div class="highlight"><pre> a b u n c h o f
| | | | | | | | | |
0 1 2 3 4 5 6 7 8 9
</pre></div>
</div>
<p>You do not have to provide both <tt class="docutils literal"><span class="pre">start</span></tt> and <tt class="docutils literal"><span class="pre">finish</span></tt>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [6]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"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">u'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">u'ch of words'</span>
</pre></div>
</div>
<p>Either <tt class="docutils literal"><span class="pre">0</span></tt> or <tt class="docutils literal"><span class="pre">len(s)</span></tt> 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"><div class="highlight"><pre><span class="gp">In [4]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u'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">u'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">u'.txt'</span>
</pre></div>
</div>
</div>
<div class="section" id="why-start-from-zero">
<h3>Why start from zero?<a class="headerlink" href="#why-start-from-zero" title="Permalink to this headline">¶</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 zero?</p>
<p>Why is the last item in the slice <strong>not</strong> included?</p>
<p>Because these lead to some nifty properties:</p>
<div class="highlight-python"><div class="highlight"><pre><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>
<p>Slicing takes a third argument, <tt class="docutils literal"><span class="pre">step</span></tt> which controls which items are
returned:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [289]: </span><span class="n">string</span> <span class="o">=</span> <span class="s">u"a fairly long string"</span>
<span class="gp">In [290]: </span><span class="n">string</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[290]: </span><span class="go">u'a fairly long s'</span>
<span class="gp">In [291]: </span><span class="n">string</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[291]: </span><span class="go">u'afil ogs'</span>
<span class="gp">In [292]: </span><span class="n">string</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[292]: </span><span class="go">u'aallg'</span>
<span class="gp">In [293]: </span><span class="n">string</span><span class="p">[::</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="gh">Out[293]: </span><span class="go">u'gnirts gnol ylriaf a'</span>
</pre></div>
</div>
<p>Though they share an operator, slicing and indexing have a few important
differences:</p>
<p>Indexing will always return one object, slicing will return a sequence of
objects.</p>
<p>Indexing past the end of a sequence will raise an error, slicing will not:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [129]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">"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>(demo)</p>
</div>
<div class="section" id="membership">
<h3>Membership<a class="headerlink" href="#membership" title="Permalink to this headline">¶</a></h3>
<p>All sequences support the <tt class="docutils literal"><span class="pre">in</span></tt> and <tt class="docutils literal"><span class="pre">not</span> <span class="pre">in</span></tt> membership operators:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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 <tt class="docutils literal"><span class="pre">substring</span></tt> operations in
other languages:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [20]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"This is a long string"</span>
<span class="gp">In [21]: </span><span class="s">u"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"><div class="highlight"><pre><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>
</div>
<div class="section" id="concatenation">
<h3>Concatenation<a class="headerlink" href="#concatenation" title="Permalink to this headline">¶</a></h3>
<p>Using <tt class="docutils literal"><span class="pre">+</span></tt> or <tt class="docutils literal"><span class="pre">*</span></tt> on sequences will <em>concatenate</em> them:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [25]: </span><span class="n">s1</span> <span class="o">=</span> <span class="s">u"left"</span>
<span class="gp">In [26]: </span><span class="n">s2</span> <span class="o">=</span> <span class="s">u"right"</span>
<span class="gp">In [27]: </span><span class="n">s1</span> <span class="o">+</span> <span class="n">s2</span>
<span class="gh">Out[27]: </span><span class="go">u'leftright'</span>
<span class="gp">In [28]: </span><span class="p">(</span><span class="n">s1</span> <span class="o">+</span> <span class="n">s2</span><span class="p">)</span> <span class="o">*</span> <span class="mi">3</span>
<span class="gh">Out[28]: </span><span class="go">u'leftrightleftrightleftright'</span>
</pre></div>
</div>
<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"><div class="highlight"><pre><span class="k">def</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"><div class="highlight"><pre><span class="k">def</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>
</div>
<div class="section" id="length">
<h3>Length<a class="headerlink" href="#length" title="Permalink to this headline">¶</a></h3>
<p>All sequences have a length. You can get it with the <tt class="docutils literal"><span class="pre">len</span></tt> builtin:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [36]: </span><span class="n">s</span> <span class="o">=</span> <span class="s">u"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, Python sequences are zero-indexed, so the last index in a sequence is
<tt class="docutils literal"><span class="pre">len(s)</span> <span class="pre">-</span> <span class="pre">1</span></tt>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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-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>Even better: use <tt class="docutils literal"><span class="pre">s[-1]</span></tt></p>
</div>
<div class="section" id="miscellaneous">
<h3>Miscellaneous<a class="headerlink" href="#miscellaneous" title="Permalink to this headline">¶</a></h3>
<p>There are a more operations supported by all sequences</p>
<p>All sequences also support the <tt class="docutils literal"><span class="pre">min</span></tt> and <tt class="docutils literal"><span class="pre">max</span></tt> builtins:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [42]: </span><span class="n">all_letters</span> <span class="o">=</span> <span class="s">u"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">u'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">u'z'</span>
</pre></div>
</div>
<p>Why are those the answers you get? (hint: <tt class="docutils literal"><span class="pre">ord(u'a')</span></tt>)</p>
<p>All sequences also support the <tt class="docutils literal"><span class="pre">index</span></tt> method, which returns the index of the
first occurence of an item in the sequence:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">u'd'</span><span class="p">)</span>
<span class="gh">Out[46]: </span><span class="go">21</span>
</pre></div>
</div>
<p>This causes a <tt class="docutils literal"><span class="pre">ValueError</span></tt> if the item is not in the sequence:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">u'A'</span><span class="p">)</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">ValueError</span><span class="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="s">u'A'</span><span class="p">)</span>
<span class="ne">ValueError</span>: substring not found
</pre></div>
</div>
<p>A sequence can also be queried for the number of times a particular item
appears:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">u'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="s">u'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"><div class="highlight"><pre><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="s">u'A'</span><span class="p">)</span>
<span class="gh">Out[54]: </span><span class="go">0</span>
</pre></div>
</div>
</div>
<div class="section" id="iteration">
<h3>Iteration<a class="headerlink" href="#iteration" title="Permalink to this headline">¶</a></h3>
<p class="center large">More on this in a while.</p>
</div>
</div>
<div class="section" id="lists-tuples">
<h2>Lists, Tuples...<a class="headerlink" href="#lists-tuples" title="Permalink to this headline">¶</a></h2>
<p class="center large">The <em>other</em> sequence types.</p>
<div class="section" id="lists">
<h3>Lists<a class="headerlink" href="#lists" title="Permalink to this headline">¶</a></h3>
<p>Lists can be constructed using list Literals (<tt class="docutils literal"><span class="pre">[]</span></tt>):</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">'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 <tt class="docutils literal"><span class="pre">list</span></tt> type object as a constructor:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">'abc'</span><span class="p">)</span>
<span class="gh">Out[8]: </span><span class="go">['a', 'b', 'c']</span>
</pre></div>
</div>
<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"><div class="highlight"><pre><span class="gp">In [9]: </span><span class="n">name</span> <span class="o">=</span> <span class="s">u'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">u'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">u'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>
</div>
<div class="section" id="tuples">
<h3>Tuples<a class="headerlink" href="#tuples" title="Permalink to this headline">¶</a></h3>
<p>Tuples can be constructed using tuple literals (<tt class="docutils literal"><span class="pre">()</span></tt>):</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">'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>
<p>Tuples don’t NEED parentheses...</p>
<div class="highlight-ipython"><div class="highlight"><pre><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"><div class="highlight"><pre><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="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>You can also use the <tt class="docutils literal"><span class="pre">tuple</span></tt> type object to convert any sequence into a
tuple:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="s">'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>
<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"><div class="highlight"><pre><span class="gp">In [23]: </span><span class="n">name</span> <span class="o">=</span> <span class="s">u'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="k">print</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="gp"> ....:</span>
<span class="go">False False True</span>
</pre></div>
</div>
<p class="center large">So Why Have Both?</p>
</div>
</div>
<div class="section" id="mutability">
<h2>Mutability<a class="headerlink" href="#mutability" title="Permalink to this headline">¶</a></h2>
<a class="reference internal image-reference" href="_images/transmogrifier.jpg"><img alt="Presto change-o" src="_images/transmogrifier.jpg" style="width: 35%;" /></a>
<p class="credit">image from flickr by <a class="reference external" href="https://www.flickr.com/photos/illuminaut/3595530403">illuminaut</a>, (CC by-nc-sa)</p>
<div class="section" id="mutability-in-python">
<h3>Mutability in Python<a class="headerlink" href="#mutability-in-python" title="Permalink to this headline">¶</a></h3>
<p>All objects in Python fall into one of two camps:</p>
<ul class="simple">
<li>Mutable</li>
<li>Immutable</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>
<table border="1" class="docutils">
<colgroup>
<col width="56%" />
<col width="44%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Immutable</th>
<th class="head">Mutable</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>Unicode</td>
<td>List</td>
</tr>
<tr class="row-odd"><td>String</td>
<td> </td>
</tr>
<tr class="row-even"><td>Integer</td>
<td> </td>
</tr>
<tr class="row-odd"><td>Float</td>
<td> </td>
</tr>
<tr class="row-even"><td>Tuple</td>
<td> </td>
</tr>
</tbody>
</table>
<p>Try this out:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [28]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">u'spam'</span><span class="p">,</span> <span class="s">u'eggs'</span><span class="p">,</span> <span class="s">u'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">[u'spam', u'eggs', u'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="s">u'raspberries'</span>
<span class="gp">In [31]: </span><span class="n">food</span>
<span class="gh">Out[31]: </span><span class="go">[u'spam', u'raspberries', u'ham']</span>
</pre></div>
</div>
<p>And repeat the exercise with a Tuple:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [32]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">(</span><span class="s">u'spam'</span><span class="p">,</span> <span class="s">u'eggs'</span><span class="p">,</span> <span class="s">u'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">(u'spam', u'eggs', u'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="s">u'raspberries'</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span><span class="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="s">u'raspberries'</span>
<span class="ne">TypeError</span>: 'tuple' object does not support item assignment
</pre></div>
</div>
<p>This property means you need to be aware of what you are doing with your lists:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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="bp">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>
<p>Our <tt class="docutils literal"><span class="pre">altered</span></tt> list has been updated:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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 <tt class="docutils literal"><span class="pre">original</span></tt> list:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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>Easy container setup, or deadly trap?</p>
<p>(note: you can nest lists to make a 2D-ish array)</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [13]: </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 [14]: </span><span class="n">bins</span>
<span class="gh">Out[14]: </span><span class="go">[[], [], [], [], []]</span>
<span class="gp">In [15]: </span><span class="n">words</span> <span class="o">=</span> <span class="p">[</span><span class="s">u'one'</span><span class="p">,</span> <span class="s">u'three'</span><span class="p">,</span> <span class="s">u'rough'</span><span class="p">,</span> <span class="s">u'sad'</span><span class="p">,</span> <span class="s">u'goof'</span><span class="p">]</span>
<span class="gp">In [16]: </span><span class="k">for</span> <span class="n">word</span> <span class="ow">in</span> <span class="n">words</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">bins</span><span class="p">[</span><span class="nb">len</span><span class="p">(</span><span class="n">word</span><span class="p">)</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">word</span><span class="p">)</span>
<span class="gp"> ....:</span>
</pre></div>
</div>
<p>So, what is going to be in <tt class="docutils literal"><span class="pre">bins</span></tt> now?</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [65]: </span><span class="n">bins</span>
<span class="gh">Out[65]:</span>
<span class="go">[[u'one', u'three', u'rough', u'sad', u'goof'],</span>
<span class="go"> [u'one', u'three', u'rough', u'sad', u'goof'],</span>
<span class="go"> [u'one', u'three', u'rough', u'sad', u'goof'],</span>
<span class="go"> [u'one', u'three', u'rough', u'sad', u'goof'],</span>
<span class="go"> [u'one', u'three', u'rough', u'sad', u'goof']]</span>
</pre></div>
</div>
<p>We multiplied a sequence containing a single <em>mutable</em> object.</p>
<p>We got a list containing five pointers to a single <em>mutable</em> object.</p>
<p>Watch out especially for passing mutable objects as default values for function parameters:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [71]: </span><span class="k">def</span> <span class="nf">accumulator</span><span class="p">(</span><span class="n">count</span><span class="p">,</span> <span class="nb">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="nb">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="nb">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>
</div>
</div>
<div class="section" id="mutable-sequence-methods">
<h2>Mutable Sequence Methods<a class="headerlink" href="#mutable-sequence-methods" title="Permalink to this headline">¶</a></h2>
<p class="left">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 the list.</p>
<p>You can find all these in the Standard Library Documentation:</p>
<p><a class="reference external" href="http://www.python.org/2/library/stdtypes.html#mutable-sequence-types">http://www.python.org/2/library/stdtypes.html#mutable-sequence-types</a></p>
<div class="section" id="assignment">
<h3>Assignment<a class="headerlink" href="#assignment" title="Permalink to this headline">¶</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"><div class="highlight"><pre><span class="gp">In [100]: </span><span class="nb">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="nb">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="nb">list</span>
<span class="gh">Out[102]: </span><span class="go">[1, 2, 10]</span>
</pre></div>
</div>
</div>
<div class="section" id="growing-the-list">
<h3>Growing the List<a class="headerlink" href="#growing-the-list" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">.append()</span></tt>, <tt class="docutils literal"><span class="pre">.insert()</span></tt>, <tt class="docutils literal"><span class="pre">.extend()</span></tt></p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [74]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">u'spam'</span><span class="p">,</span> <span class="s">u'eggs'</span><span class="p">,</span> <span class="s">u'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="s">u'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">[u'spam', u'eggs', u'ham', u'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="s">u'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">[u'beans', u'spam', u'eggs', u'ham', u'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="s">u'bread'</span><span class="p">,</span> <span class="s">u'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">[u'beans', u'spam', u'eggs', u'ham', u'sushi', u'bread', u'water']</span>
</pre></div>
</div>
<p>You can pass any sequence to <tt class="docutils literal"><span class="pre">.extend()</span></tt>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [85]: </span><span class="n">food</span>
<span class="gh">Out[85]: </span><span class="go">[u'beans', u'spam', u'eggs', u'ham', u'sushi', u'bread', u'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="s">u'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">[u'beans', u'spam', u'eggs', u'ham', u'sushi', u'bread', u'water',</span>
<span class="go"> u's', u'p', u'a', u'g', u'h', u'e', u't', u't', u'i']</span>
</pre></div>
</div>
</div>
<div class="section" id="shrinking-the-list">
<h3>Shrinking the List<a class="headerlink" href="#shrinking-the-list" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">.pop()</span></tt>, <tt class="docutils literal"><span class="pre">.remove()</span></tt></p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [203]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">'spam'</span><span class="p">,</span> <span class="s">'eggs'</span><span class="p">,</span> <span class="s">'ham'</span><span class="p">,</span> <span class="s">'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="s">'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 <tt class="docutils literal"><span class="pre">del</span></tt> keyword:</p>
<div class="highlight-ipython"><div class="highlight"><pre><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>
</div>
<div class="section" id="copying-lists">
<h3>Copying Lists<a class="headerlink" href="#copying-lists" title="Permalink to this headline">¶</a></h3>
<p>You can make copies of part of a list using <em>slicing</em>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [227]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">'spam'</span><span class="p">,</span> <span class="s">'eggs'</span><span class="p">,</span> <span class="s">'ham'</span><span class="p">,</span> <span class="s">'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="s">'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"><div class="highlight"><pre><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>
<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"><div class="highlight"><pre><span class="gp">In [249]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">'spam'</span><span class="p">,</span> <span class="p">[</span><span class="s">'eggs'</span><span class="p">,</span> <span class="s">'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>
<p>Consider this common pattern:</p>
<div class="highlight-python"><div class="highlight"><pre><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>
<p>For example:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [121]: </span><span class="nb">list</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 [122]: </span><span class="nb">list</span>
<span class="gh">Out[122]: </span><span class="go">[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]</span>
<span class="gp">In [123]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">list</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">list</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="gp">In [124]: </span><span class="nb">list</span>
<span class="gh">Out[124]: </span><span class="go">[1, 3, 5, 7, 9]</span>
</pre></div>
</div>
<p>Was that what you expected?</p>
<p>Iterate over a copy, and mutate the original:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [126]: </span><span class="nb">list</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 [127]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">list</span><span class="p">[:]:</span>
<span class="gp"> .....: </span> <span class="nb">list</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="gp">In [128]: </span><span class="nb">list</span>
<span class="gh">Out[128]: </span><span class="go">[]</span>
</pre></div>
</div>
<p>Okay, so we’ve done this a bunch already, but let’s state it out loud.</p>
<p>You can iterate over a sequence.</p>
<div class="highlight-python"><div class="highlight"><pre><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>Again, we’ll touch more on this in a short while, but first a few more words
about Lists and Tuples.</p>
</div>
<div class="section" id="miscellaneous-list-methods">
<h3>Miscellaneous List Methods<a class="headerlink" href="#miscellaneous-list-methods" title="Permalink to this headline">¶</a></h3>
<p>These methods change a list in place and are not available on immutable
sequence types.</p>
<p><tt class="docutils literal"><span class="pre">.reverse()</span></tt></p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [129]: </span><span class="n">food</span> <span class="o">=</span> <span class="p">[</span><span class="s">u'spam'</span><span class="p">,</span> <span class="s">u'eggs'</span><span class="p">,</span> <span class="s">u'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">[u'ham', u'eggs', u'spam']</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">.sort()</span></tt></p>
<div class="highlight-ipython"><div class="highlight"><pre><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">[u'eggs', u'ham', u'spam']</span>
</pre></div>
</div>
<p>Because these methods mutate the list in place, they have a return value of
<tt class="docutils literal"><span class="pre">None</span></tt></p>
<p><tt class="docutils literal"><span class="pre">.sort()</span></tt> can take an optional <tt class="docutils literal"><span class="pre">key</span></tt> 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"><div class="highlight"><pre><span class="gp">In [137]: </span><span class="k">def</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">[u'spam', u'eggs', u'ham']</span>
</pre></div>
</div>
</div>
<div class="section" id="list-performance">
<h3>List Performance<a class="headerlink" href="#list-performance" title="Permalink to this headline">¶</a></h3>
<ul class="build simple">
<li>indexing is fast and constant time: O(1)</li>
<li>x in s proportional to n: O(n)</li>
<li>visiting all is proportional to n: O(n)</li>
<li>operating on the end of list is fast and constant time: O(1)<ul>
<li>append(), pop()</li>
</ul>
</li>
<li>operating on the front (or middle) of the list depends on n: O(n)<ul>
<li>pop(0), insert(0, v)</li>
<li>But, reversing is fast. Also, collections.deque</li>
</ul>
</li>
</ul>
<blockquote>
<div><a class="reference external" href="http://wiki.python.org/moin/TimeComplexity">http://wiki.python.org/moin/TimeComplexity</a></div></blockquote>
</div>
<div class="section" id="choosing-lists-or-tuples">
<h3>Choosing Lists or Tuples<a class="headerlink" href="#choosing-lists-or-tuples" title="Permalink to this headline">¶</a></h3>
<p>Here are a few guidelines on when to choose a list or a tuple:</p>
<ul class="simple">
<li>If it needs to mutable: list</li>
<li>If it needs to be immutable: tuple<ul>
<li>(safety when passing to a function)</li>
</ul>
</li>
</ul>
<p>Otherwise ... taste and convention</p>
<p>Lists are Collections (homogeneous):
– contain values of the same type
– simplifies iterating, sorting, etc</p>
<p>tuples are mixed types:
– Group multiple values into one logical thing
– Kind of like simple C structs.</p>
<ul class="build simple">
<li>Do the same operation to each element?<ul>
<li>list</li>
</ul>
</li>
<li>Small collection of values which make a single logical item?<ul>
<li>tuple</li>
</ul>
</li>
<li>To document that these values won’t change?<ul>
<li>tuple</li>
</ul>
</li>
<li>Build it iteratively?<ul>
<li>list</li>
</ul>
</li>
<li>Transform, filter, etc?<ul>
<li>list</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="more-documentation">
<h3>More Documentation<a class="headerlink" href="#more-documentation" title="Permalink to this headline">¶</a></h3>
<p>For more information, read the list docs:</p>
<p><a class="reference external" href="http://docs.python.org/2/library/stdtypes.html#mutable-sequence-types">http://docs.python.org/2/library/stdtypes.html#mutable-sequence-types</a></p>
<p>(actually any mutable sequence....)</p>
</div>
</div>
<div class="section" id="id1">
<h2>Iteration<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<p class="build">Repetition, Repetition, Repetition, Repe...</p>
<div class="section" id="for-loops">
<h3>For Loops<a class="headerlink" href="#for-loops" title="Permalink to this headline">¶</a></h3>
<p>We’ve seen simple iteration over a sequence with <tt class="docutils literal"><span class="pre">for</span> <span class="pre">...</span> <span class="pre">in</span></tt>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [170]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="s">"a string"</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">print</span> <span class="n">x</span>
<span class="gp"> .....:</span>
<span class="go">a</span>
<span class="go">s</span>
<span class="go">t</span>
<span class="go">r</span>
<span class="go">i</span>
<span class="go">n</span>
<span class="go">g</span>
</pre></div>
</div>
<p>Contrast this with other languages, where you must build and use an <tt class="docutils literal"><span class="pre">index</span></tt>:</p>
<div class="highlight-javascript"><div class="highlight"><pre><span class="k">for</span><span class="p">(</span><span class="kd">var</span> <span class="nx">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span> <span class="nx">i</span><span class="o"><</span><span class="nx">arr</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nx">value</span> <span class="o">=</span> <span class="nx">arr</span><span class="p">[</span><span class="nx">i</span><span class="p">];</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">i</span> <span class="o">+</span> <span class="s2">") "</span> <span class="o">+</span> <span class="nx">value</span><span class="p">);</span>
</pre></div>
</div>
<p>If you need an index, though you can use <tt class="docutils literal"><span class="pre">enumerate</span></tt>:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [140]: </span><span class="k">for</span> <span class="n">idx</span><span class="p">,</span> <span class="n">letter</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="s">u'Python'</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">print</span> <span class="n">idx</span><span class="p">,</span> <span class="n">letter</span><span class="p">,</span>
<span class="gp"> .....:</span>
<span class="go">0 P 1 y 2 t 3 h 4 o 5 n</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">range</span></tt> builtin is useful for looping a known number of times:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [171]: </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="gp"> .....: </span> <span class="k">print</span> <span class="n">i</span>
<span class="gp"> .....:</span>
<span class="go">0</span>
<span class="go">1</span>
<span class="go">2</span>
<span class="go">3</span>
<span class="go">4</span>
</pre></div>
</div>
<p>But you don’t really need to do anything at all with <tt class="docutils literal"><span class="pre">i</span></tt></p>
<p>Be alert that a loop does not create a local namespace:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [172]: </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span>
<span class="gp">In [173]: </span><span class="k">for</span> <span class="n">x</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="k">pass</span>
<span class="gp"> .....:</span>
<span class="gp">In [174]: </span><span class="n">x</span>