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<li class="toctree-l2 current"><a class="current reference internal" href="#">Generators &amp; Comprehension Expressions</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#Introducing-Generators">Introducing Generators</a><ul>
<li class="toctree-l4"><a class="reference internal" href="#The-range-generator">The <code class="docutils literal notranslate"><span class="pre">range</span></code> generator</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#Creating-your-own-generator:-generator-comprehensions">Creating your own generator: generator comprehensions</a><ul>
<li class="toctree-l4"><a class="reference internal" href="#Storing-generators">Storing generators</a></li>
<li class="toctree-l4"><a class="reference internal" href="#Consuming-generators">Consuming generators</a></li>
<li class="toctree-l4"><a class="reference internal" href="#Chaining-comprehensions">Chaining comprehensions</a></li>
<li class="toctree-l4"><a class="reference internal" href="#Using-generator-comprehensions-on-the-fly">Using generator comprehensions on the fly</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#Iterating-over-generators-using-next">Iterating over generators using <code class="docutils literal notranslate"><span class="pre">next</span></code></a><ul>
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<div class="section" id="Generators-&amp;-Comprehension-Expressions">
<h1>Generators &amp; Comprehension Expressions<a class="headerlink" href="#Generators-&-Comprehension-Expressions" title="Permalink to this headline">¶</a></h1>
<div class="admonition warning">
<p class="first fa fa-exclamation-circle"><strong>Note</strong>:</p>
<p class="last">There are reading-comprehension exercises included throughout the text.
These are meant to help you put your reading to practice. Solutions for
the exercises are included at the bottom of this page.</p>
</div>
<div class="section" id="Introducing-Generators">
<h2>Introducing Generators<a class="headerlink" href="#Introducing-Generators" title="Permalink to this headline">¶</a></h2>
<p>Now we introduce an important type of object called a <strong>generator</strong>,
which allows us to generate arbitrarily-many items in a series, without
having to store them all in memory at once.</p>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Definition</strong>:</p>
<p class="last">A <strong>generator</strong> is a special kind of iterator, which stores the
instructions for how to <em>generate</em> each of its members, in order, along
with its current state of iterations. It generates each member, one at a
time, only as it is requested via iteration.</p>
</div>
<p>Recall that that a list readily stores all of its members; you can
access any of its contents via indexing. A generator, on the other hand,
<em>does not store any items</em>. Instead, it stores the instructions for
generating each of its members, and stores its iteration state; this
means that the generator will know if it has generated its second
member, and will thus generate its third member the next time it is
iterated on.</p>
<p>The whole point of this is that you can use a generator to produce a
long sequence of items, without having to store them all in memory.</p>
<div class="section" id="The-range-generator">
<h3>The <code class="docutils literal notranslate"><span class="pre">range</span></code> generator<a class="headerlink" href="#The-range-generator" title="Permalink to this headline">¶</a></h3>
<p>An extremely popular built-in generator is <code class="docutils literal notranslate"><span class="pre">range</span></code>, which, given the
values:</p>
<ul class="simple">
<li>‘start’ (inclusive, default=0)</li>
<li>‘stop’ (exclusive)</li>
<li>‘step’ (default=1)</li>
</ul>
<p>will generate the corresponding sequence of integers (from start to
stop, using the step size) upon iteration. Consider the following
example usages of <code class="docutils literal notranslate"><span class="pre">range</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># start: 2  (included)</span>
<span class="c1">#  stop: 7  (excluded)</span>
<span class="c1">#  step: 1  (default)</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">2</span><span class="p">,</span> <span class="mi">7</span><span class="p">):</span>
    <span class="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="c1"># prints: 2.. 3.. 4.. 5.. 6</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># start:  1  (included)</span>
<span class="c1">#  stop: 10  (excluded)</span>
<span class="c1">#  step:  2</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">1</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">2</span><span class="p">):</span>
    <span class="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="c1"># prints: 1.. 3.. 5.. 7.. 9</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># A very common use case!</span>
<span class="c1"># start:  0  (default, included)</span>
<span class="c1">#  stop:  5  (excluded)</span>
<span class="c1">#  step:  1  (default)</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="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="c1"># prints: 0.. 1.. 2.. 3.. 4</span>
</pre></div>
</div>
<p>Because <code class="docutils literal notranslate"><span class="pre">range</span></code> is a generator, the command <code class="docutils literal notranslate"><span class="pre">range(5)</span></code> will simply
store the instructions needed to produce the sequence of numbers 0-4,
whereas the list <code class="docutils literal notranslate"><span class="pre">[0,</span> <span class="pre">1,</span> <span class="pre">2,</span> <span class="pre">3,</span> <span class="pre">4]</span></code> stores all of these items in memory
at once. For short sequences, this seems to be a rather paltry savings;
this is not the case for long sequences. The following graph compares
the memory consumption used when defining a generator for the sequence
of numbers <span class="math notranslate nohighlight">\(0-N\)</span> using <code class="docutils literal notranslate"><span class="pre">range</span></code>, compared to storing the sequence
in a list:</p>
<div class="figure" id="id1">
<img alt="Memory consumption figure" src="../_images/Mem_Consumption_Generator.png" />
<p class="caption"><span class="caption-text">Memory consumption figure</span></p>
</div>
<p>Given our discussion of generators, it should make sense that the memory
consumed simply by defining <code class="docutils literal notranslate"><span class="pre">range(N)</span></code> is independent of <span class="math notranslate nohighlight">\(N\)</span>,
whereas the memory consumed by the list grows linearly with <span class="math notranslate nohighlight">\(N\)</span>
(for large <span class="math notranslate nohighlight">\(N\)</span>).</p>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p class="last"><code class="docutils literal notranslate"><span class="pre">range</span></code> is a built-in generator, which generates sequences of
integers.</p>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Using ``range``</strong>:</p>
<p class="last">Using <code class="docutils literal notranslate"><span class="pre">range</span></code> in a for-loop, print the numbers 10-1, in sequence.</p>
</div>
</div>
</div>
<div class="section" id="Creating-your-own-generator:-generator-comprehensions">
<h2>Creating your own generator: generator comprehensions<a class="headerlink" href="#Creating-your-own-generator:-generator-comprehensions" title="Permalink to this headline">¶</a></h2>
<p>Python provides a sleek syntax for defining a simple generator in a
single line of code; this expression is known as a <strong>generator
comprehension</strong>. The following syntax is extremely useful and will
appear very frequently in Python code:</p>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Definition</strong>:</p>
<p class="last">The syntax <code class="docutils literal notranslate"><span class="pre">(&lt;expression&gt;</span> <span class="pre">for</span> <span class="pre">&lt;var&gt;</span> <span class="pre">in</span> <span class="pre">&lt;iterable&gt;</span> <span class="pre">[if</span> <span class="pre">&lt;condition&gt;])</span></code>
specifies the general form for a <strong>generator comprehension</strong>. This
produces a generator, whose instructions for generating its members are
provided within the parenthetical statement.</p>
</div>
<p>Written in a long form, the pseudo-code for</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="o">&lt;</span><span class="n">expression</span><span class="o">&gt;</span> <span class="k">for</span> <span class="o">&lt;</span><span class="n">var</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">iterable</span><span class="o">&gt;</span> <span class="k">if</span> <span class="o">&lt;</span><span class="n">condition</span><span class="o">&gt;</span><span class="p">)</span>
</pre></div>
</div>
<p>is:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="o">&lt;</span><span class="n">var</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">iterable</span><span class="o">&gt;</span><span class="p">:</span>
    <span class="k">if</span> <span class="nb">bool</span><span class="p">(</span><span class="o">&lt;</span><span class="n">condition</span><span class="o">&gt;</span><span class="p">):</span>
        <span class="k">yield</span> <span class="o">&lt;</span><span class="n">expression</span><span class="o">&gt;</span>
</pre></div>
</div>
<p>The following expression defines a generator for all the even numbers in
0-99:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># when iterated over, `even_gen` will generate 0.. 2.. 4.. ... 98</span>
<span class="n">even_gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</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">100</span><span class="p">)</span> <span class="k">if</span> <span class="n">i</span><span class="o">%</span><span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">&lt;condition&gt;</span></code> clause in the generator expression is optional.
The generator comprehension</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="o">&lt;</span><span class="n">expression</span><span class="o">&gt;</span> <span class="k">for</span> <span class="o">&lt;</span><span class="n">var</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">iterable</span><span class="o">&gt;</span><span class="p">)</span>
</pre></div>
</div>
<p>corresponds to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="o">&lt;</span><span class="n">var</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">iterable</span><span class="o">&gt;</span><span class="p">:</span>
    <span class="k">yield</span> <span class="o">&lt;</span><span class="n">expression</span><span class="o">&gt;</span>
</pre></div>
</div>
<p>For example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># when iterated over, `example_gen` will generate 0/2.. 9/2.. 21/2.. 32/2</span>
<span class="n">example_gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">/</span><span class="mi">2</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">21</span><span class="p">,</span> <span class="mi">32</span><span class="p">])</span>

<span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">example_gen</span><span class="p">:</span>
    <span class="k">print</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
<span class="c1"># prints: 0.5.. 4.5.. 10.5.. 16.0</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">&lt;expression&gt;</span></code> can be any valid single-line of Python code that
returns an object:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">((</span><span class="n">i</span><span class="p">,</span> <span class="n">i</span><span class="o">**</span><span class="mi">2</span><span class="p">,</span> <span class="n">i</span><span class="o">**</span><span class="mi">3</span><span class="p">)</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">10</span><span class="p">))</span>
<span class="c1"># will generate:</span>
<span class="c1"># (0, 0, 0)</span>
<span class="c1"># (1, 1, 1)</span>
<span class="c1"># (2, 4, 8)</span>
<span class="c1"># (3, 9, 27)</span>
<span class="c1"># (4, 16, 64)</span>
<span class="c1"># (5, 25, 125)</span>
<span class="c1"># (6, 36, 216)</span>
<span class="c1"># (7, 49, 343)</span>
<span class="c1"># (8, 64, 512)</span>
<span class="c1"># (9, 81, 729)</span>
</pre></div>
</div>
<p>This means that <code class="docutils literal notranslate"><span class="pre">&lt;expression&gt;</span></code> can even involve inline if-else
statements!</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">((</span><span class="s2">&quot;apple&quot;</span> <span class="k">if</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="mi">3</span> <span class="k">else</span> <span class="s2">&quot;pie&quot;</span><span class="p">)</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">6</span><span class="p">))</span>
<span class="c1"># will generate:</span>
<span class="c1"># &#39;apple&#39;..</span>
<span class="c1"># &#39;apple&#39;..</span>
<span class="c1"># &#39;apple&#39;..</span>
<span class="c1"># &#39;pie&#39;..</span>
<span class="c1"># &#39;pie&#39;..</span>
<span class="c1"># &#39;pie&#39;</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p class="last">A generator comprehension is a single-line specification for defining a
generator in Python. It is absolutely essential to learn this syntax in
order to write simple and readable code.</p>
</div>
<div class="admonition warning">
<p class="first fa fa-exclamation-circle"><strong>Note</strong>:</p>
<p class="last">Generator comprehensions are <strong>not</strong> the only method for defining
generators in Python. We will briefly introduced a more fully-fledged
syntax for defining generators. One can define a generator similar to
the way one can define a function (which we will encounter soon). <a class="reference external" href="https://docs.python.org/3/tutorial/classes.html#generators">See
this section of the official Python
tutorial</a>
if you are interested in diving deeper into generators.</p>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Writing a Generator Comprehension</strong>:</p>
<p>Using a generator comprehension, define a generator for the series:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span><span class="o">..</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="o">..</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span><span class="o">..</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span><span class="o">..</span> <span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">7</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that (3, 5) is <em>not</em> in the series.</p>
<p class="last">Iterate over the generator and print its contents to verify your
solution.</p>
</div>
<div class="section" id="Storing-generators">
<h3>Storing generators<a class="headerlink" href="#Storing-generators" title="Permalink to this headline">¶</a></h3>
<p>Just like we saw with the <code class="docutils literal notranslate"><span class="pre">range</span></code> generator, defining a generator
using a comprehension does <em>not</em> perform any computations or consume any
memory beyond defining the rules for producing the sequence of data. See
what happens when we try to print this generator:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># will generate 0, 1, 4, 9, 25, ..., 9801</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">100</span><span class="p">))</span>
<span class="o">&gt;&gt;&gt;</span> <span class="k">print</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="o">&lt;</span><span class="n">generator</span> <span class="nb">object</span> <span class="o">&lt;</span><span class="n">genexpr</span><span class="o">&gt;</span> <span class="n">at</span> <span class="mh">0x000001E768FE8A40</span><span class="o">&gt;</span>
</pre></div>
</div>
<p>This output simply indicates that <code class="docutils literal notranslate"><span class="pre">gen</span></code> stores a generator-expression
at the memory address <code class="docutils literal notranslate"><span class="pre">0x000001E768FE8A40</span></code>; this is simply where the
instructions for generating our sequence of squared numbers is stored.
<code class="docutils literal notranslate"><span class="pre">gen</span></code> will not produce any results until we iterate over it. For this
reason, generators cannot be inspected in the same way that lists and
other sequences can be. You <strong>cannot</strong> do the following:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># you **cannot** do the following...</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">100</span><span class="p">))</span>

<span class="c1"># query the length of a generator</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">len</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="nb">object</span> <span class="n">of</span> <span class="nb">type</span> <span class="s1">&#39;generator&#39;</span> <span class="n">has</span> <span class="n">no</span> <span class="nb">len</span><span class="p">()</span>

<span class="c1"># index into a generator</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">gen</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="s1">&#39;generator&#39;</span> <span class="nb">object</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">subscriptable</span>
</pre></div>
</div>
<p>The sole exception to this is the <code class="docutils literal notranslate"><span class="pre">range</span></code> generator, for which all of
these inspections are valid.</p>
</div>
<div class="section" id="Consuming-generators">
<h3>Consuming generators<a class="headerlink" href="#Consuming-generators" title="Permalink to this headline">¶</a></h3>
<p>We can feed this to any function that accepts iterables. For instance,
we can feed <code class="docutils literal notranslate"><span class="pre">gen</span></code> to the built-in <code class="docutils literal notranslate"><span class="pre">sum</span></code> function, which sums the
contents of an iterable:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">100</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">sum</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>  <span class="c1"># computes the sum 0 + 1 + 4 + 9 + 25 + ... + 9801</span>
<span class="go">328350</span>
</pre></div>
</div>
<p>This computes the sum of the sequence of numbers <em>without ever storing
the full sequence of numbers in memory</em>. In fact, only two numbers need
be stored during any given iteration of the sum: the current value of
the sum, and the number being added to it.</p>
<p>What happens if we run this command a second time:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># computes the sum of ... nothing!</span>
<span class="c1"># `gen` has already been consumed!</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">sum</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="mi">0</span>
</pre></div>
</div>
<p>It may be surprising to see that the sum now returns 0. This is because
<strong>a generator is exhausted after it is iterated over in full</strong>. You must
redefine the generator if you want to iterate over it again;
fortunately, defining a generator requires very few resources, so this
is not a point of concern.</p>
<p>You can also check for membership in a generator, but this also consumes
the generator:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># checking for membership consumes a generator until</span>
<span class="c1"># it finds that item (consuming the entire generator</span>
<span class="c1"># if the item is not contained within it)</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</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">1</span><span class="p">,</span> <span class="mi">11</span><span class="p">))</span>
<span class="o">&gt;&gt;&gt;</span> <span class="mi">5</span> <span class="ow">in</span> <span class="n">gen</span>  <span class="c1"># first 5 elements are consumed</span>
<span class="bp">True</span>

<span class="c1"># 1-5 are no longer contained in gen</span>
<span class="c1"># this check consumes the entire generator!</span>
<span class="o">&gt;&gt;&gt;</span> <span class="mi">5</span> <span class="ow">in</span> <span class="n">gen</span>
<span class="bp">False</span>

<span class="o">&gt;&gt;&gt;</span> <span class="nb">sum</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="mi">0</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p class="last">A generator can only be iterated over once, after which it is exhausted
and must be re-defined in order to be iterated over again.</p>
</div>
</div>
<div class="section" id="Chaining-comprehensions">
<h3>Chaining comprehensions<a class="headerlink" href="#Chaining-comprehensions" title="Permalink to this headline">¶</a></h3>
<p>Because generators are iterables, they can be fed into subsequent
generator comprehensions. That is, they can be “chained” together.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># chaining two generator comprehensions</span>

<span class="c1"># generates 400.. 100.. 0.. 100.. 400</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">gen_1</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="p">[</span><span class="o">-</span><span class="mi">20</span><span class="p">,</span> <span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="mi">0</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="c1"># sums the generated numbers, excluding any numbers whose absolute value is greater than 150</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">sum</span><span class="p">(</span><span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">gen_1</span> <span class="k">if</span> <span class="nb">abs</span><span class="p">(</span><span class="n">i</span><span class="p">)</span> <span class="o">&lt;=</span> <span class="mi">150</span><span class="p">)</span>
<span class="mi">200</span>
</pre></div>
</div>
</div>
<div class="section" id="Using-generator-comprehensions-on-the-fly">
<h3>Using generator comprehensions on the fly<a class="headerlink" href="#Using-generator-comprehensions-on-the-fly" title="Permalink to this headline">¶</a></h3>
<p>A feature of Python, that can make your code supremely readable and
intuitive, is that generator comprehensions can be fed <em>directly</em> into
functions that operate on iterables. That is,</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">100</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">sum</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="go">328350</span>
</pre></div>
</div>
<p>can be simplified as:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="nb">sum</span><span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">100</span><span class="p">))</span>
<span class="go">328350</span>
</pre></div>
</div>
<p>If you want your code to compute the finite harmonic series:
<span class="math notranslate nohighlight">\(\sum_{k=1}^{100} \frac{1}{n} = 1 + \frac{1}{2} + ... + \frac{1}{100}\)</span>,
you can simply write:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="nb">sum</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="n">n</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">101</span><span class="p">))</span>
<span class="go">5.187377517639621</span>
</pre></div>
</div>
<p>This convenient syntax works for any function that expects an iterable
as an argument, such as the <code class="docutils literal notranslate"><span class="pre">list</span></code> function and <code class="docutils literal notranslate"><span class="pre">all</span></code> function:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># providing generator expressions as arguments to functions</span>
<span class="c1"># that operate on iterables</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">list</span><span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">10</span><span class="p">))</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">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">25</span><span class="p">,</span> <span class="mi">36</span><span class="p">,</span> <span class="mi">49</span><span class="p">,</span> <span class="mi">64</span><span class="p">,</span> <span class="mi">81</span><span class="p">]</span>

<span class="o">&gt;&gt;&gt;</span> <span class="nb">all</span><span class="p">(</span><span class="n">i</span> <span class="o">&lt;</span> <span class="mi">10</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</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="mi">5</span><span class="p">,</span> <span class="mi">7</span><span class="p">])</span>
<span class="bp">True</span>

<span class="o">&gt;&gt;&gt;</span> <span class="s2">&quot;, &quot;</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">10</span><span class="p">,</span> <span class="mi">200</span><span class="p">,</span> <span class="mi">4000</span><span class="p">,</span> <span class="mi">80000</span><span class="p">])</span>
<span class="s1">&#39;10, 200, 4000, 80000&#39;</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p class="last">A generator comprehension can be specified directly as an argument to a
function, wherever a single iterable is expected as an input to that
function.</p>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Using Generator Comprehensions on the Fly</strong>:</p>
<p class="last">In a single line, compute the sum of all of the odd-numbers in 0-100.</p>
</div>
</div>
</div>
<div class="section" id="Iterating-over-generators-using-next">
<h2>Iterating over generators using <code class="docutils literal notranslate"><span class="pre">next</span></code><a class="headerlink" href="#Iterating-over-generators-using-next" title="Permalink to this headline">¶</a></h2>
<p>The built-in function <code class="docutils literal notranslate"><span class="pre">next</span></code> allows you manually “request” the next
member of a generator, or more generally, any kind of <em>iterator</em>.
Calling <code class="docutils literal notranslate"><span class="pre">next</span></code> on an exhausted iterator will raise a <code class="docutils literal notranslate"><span class="pre">StopIteration</span></code>
signal.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># consuming an iterator using `next`</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">short_gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">/</span><span class="mi">2</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</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="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">short_gen</span><span class="p">)</span>
<span class="mf">0.5</span>

<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">short_gen</span><span class="p">)</span>
<span class="mf">1.0</span>

<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">short_gen</span><span class="p">)</span>
<span class="mf">1.5</span>

<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">short_gen</span><span class="p">)</span>
<span class="ne">StopIteration</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">&lt;</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">5</span><span class="o">-</span><span class="n">ed60a54ccf0b</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span><span class="p">()</span>
<span class="o">----&gt;</span> <span class="mi">1</span> <span class="nb">next</span><span class="p">(</span><span class="n">short_gen</span><span class="p">)</span>

<span class="ne">StopIteration</span><span class="p">:</span>
</pre></div>
</div>
<p>This is a great tool for retrieving content from a generator, or any
iterator, without having to perform a for-loop over it.</p>
<div class="section" id="Iterables-vs.&nbsp;Iterators">
<h3>Iterables vs.&nbsp;Iterators<a class="headerlink" href="#Iterables-vs. Iterators" title="Permalink to this headline">¶</a></h3>
<p>This subsection is <em>not</em> essential to your basic understanding of the
material. I am including it to prevent this text from being misleading
to those who already know quite a bit about Python. <strong>This is a bit
advanced, feel free to skip it…</strong></p>
<p>There is a bit of confusing terminology to be cleared up: an iterable is
not the same thing as an iterator.</p>
<p>An <em>iterator</em> object stores its current state of iteration and “yields”
each of its members in order, on demand via <code class="docutils literal notranslate"><span class="pre">next</span></code>, until it is
exhausted. As we’ve seen, a generator is an example of an iterator. We
now must understand that every iterator is an iterable, but not every
iterable is an iterator.</p>
<p>An <em>iterable</em> is an object that <em>can</em> be iterated over but does not
necessarily have all the machinery of an iterator. For example,
sequences (e.g lists, tuples, and strings) and other containers
(e.g.&nbsp;dictionaries and sets) do not keep track of their own state of
iteration. Thus you cannot call <code class="docutils literal notranslate"><span class="pre">next</span></code> on one of these outright:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># a list is an example of an iterable that is *not*</span>
<span class="c1"># an iterator - you cannot call `next` on it.</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">x</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="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">x</span><span class="p">)</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">&lt;</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">19</span><span class="o">-</span><span class="n">b9d20096048c</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span><span class="p">()</span>
<span class="o">----&gt;</span> <span class="mi">1</span> <span class="nb">next</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="ne">TypeError</span><span class="p">:</span> <span class="s1">&#39;list&#39;</span> <span class="nb">object</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">an</span> <span class="n">iterator</span>
</pre></div>
</div>
<p>In order to iterate over, say, a list you must first pass it to the
built-in <code class="docutils literal notranslate"><span class="pre">iter</span></code> function. This function will return an <em>iterator</em> for
that list, which stores its state of iteration and the instructions to
yield each one of the list’s members:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># any iterable can be fed to `iter`</span>
<span class="c1"># to produce an iterator for that object</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">x</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="o">&gt;&gt;&gt;</span> <span class="n">x_it</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>  <span class="c1"># `x_it` is an iterator</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">x_it</span><span class="p">)</span>
<span class="mi">1</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">x_it</span><span class="p">)</span>
<span class="mi">2</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">next</span><span class="p">(</span><span class="n">x_it</span><span class="p">)</span>
<span class="mi">3</span>
</pre></div>
</div>
<p>In this way, a list is an <em>iterable</em> but not an <em>iterator</em>, which is
also the case for tuples, strings, sets, and dictionaries.</p>
<p>Python actually creates an iterator “behind the scenes”, whenever you
perform a for-loop over an iterable like a list. It feeds that iterable
to <code class="docutils literal notranslate"><span class="pre">iter</span></code>, and then proceeds to call <code class="docutils literal notranslate"><span class="pre">next</span></code> on the resulting
iterator for each of the for-loop’s iterations.</p>
</div>
</div>
<div class="section" id="List-&amp;-Tuple-Comprehensions">
<h2>List &amp; Tuple Comprehensions<a class="headerlink" href="#List-&-Tuple-Comprehensions" title="Permalink to this headline">¶</a></h2>
<p>Using generator comprehensions to initialize lists is so useful that
Python actually reserves a specialized syntax for it, known as the list
comprehension. A <strong>list comprehension</strong> is a syntax for constructing a
list, which exactly mirrors the generator comprehension syntax:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="o">&lt;</span><span class="n">expression</span><span class="o">&gt;</span> <span class="k">for</span> <span class="o">&lt;</span><span class="n">var</span><span class="o">&gt;</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">iterable</span><span class="o">&gt;</span> <span class="p">{</span><span class="k">if</span> <span class="o">&lt;</span><span class="n">condition</span><span class="p">}]</span>
</pre></div>
</div>
<p>For example, if we want to create a list of square-numbers, we can
simply write:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># a simple list comprehension</span>
<span class="o">&gt;&gt;&gt;</span> <span class="p">[</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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">10</span><span class="p">)]</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">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">25</span><span class="p">,</span> <span class="mi">36</span><span class="p">,</span> <span class="mi">49</span><span class="p">,</span> <span class="mi">64</span><span class="p">,</span> <span class="mi">81</span><span class="p">]</span>
</pre></div>
</div>
<p>This produces the exact same result as feeding the <code class="docutils literal notranslate"><span class="pre">list</span></code> function a
generator comprehension. However, using a list comprehension is slightly
more efficient than is feeding the <code class="docutils literal notranslate"><span class="pre">list</span></code> function a generator
comprehension.</p>
<p>Let’s appreciate how economical list comprehensions are. The following
code stores words that contain the letter “o”, in a list:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">words_with_o</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">word_collection</span> <span class="o">=</span> <span class="p">[</span><span class="s1">&#39;Python&#39;</span><span class="p">,</span> <span class="s1">&#39;Like&#39;</span><span class="p">,</span> <span class="s1">&#39;You&#39;</span><span class="p">,</span> <span class="s1">&#39;Mean&#39;</span><span class="p">,</span> <span class="s1">&#39;It&#39;</span><span class="p">]</span>

<span class="k">for</span> <span class="n">word</span> <span class="ow">in</span> <span class="n">word_collection</span><span class="p">:</span>
    <span class="k">if</span> <span class="s2">&quot;o&quot;</span> <span class="ow">in</span> <span class="n">word</span><span class="o">.</span><span class="n">lower</span><span class="p">():</span>
        <span class="n">words_with_o</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">word</span><span class="p">)</span>
</pre></div>
</div>
<p>This can be written in a single line, using a list comprehension:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">word_collection</span> <span class="o">=</span> <span class="p">[</span><span class="s1">&#39;Python&#39;</span><span class="p">,</span> <span class="s1">&#39;Like&#39;</span><span class="p">,</span> <span class="s1">&#39;You&#39;</span><span class="p">,</span> <span class="s1">&#39;Mean&#39;</span><span class="p">,</span> <span class="s1">&#39;It&#39;</span><span class="p">]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">words_with_o</span> <span class="o">=</span> <span class="p">[</span><span class="n">word</span> <span class="k">for</span> <span class="n">word</span> <span class="ow">in</span> <span class="n">word_collection</span> <span class="k">if</span> <span class="s2">&quot;o&quot;</span> <span class="ow">in</span> <span class="n">word</span><span class="o">.</span><span class="n">lower</span><span class="p">()]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">words_with_o</span>
<span class="go">[&#39;Python&#39;, &#39;You&#39;]</span>
</pre></div>
</div>
<p>Tuples can be created using comprehension expressions too, but we must
explicitly invoke the <code class="docutils literal notranslate"><span class="pre">tuple</span></code> constructor since parentheses are
already reserved for defining a generator-comprehension.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># creating a tuple using a comprehension expression</span>
<span class="o">&gt;&gt;&gt;</span> <span class="nb">tuple</span><span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</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="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">16</span><span class="p">)</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p class="last">The comprehensions-statement is an extremely useful syntax for creating
simple and complicated lists and tuples alike.</p>
</div>
<div class="section" id="Nesting-comprehensions">
<h3>Nesting comprehensions<a class="headerlink" href="#Nesting-comprehensions" title="Permalink to this headline">¶</a></h3>
<p>It can be useful to nest comprehension expressions within one another,
although this should be used sparingly.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Nested list comprehensions.</span>
<span class="c1"># This creates a 3x4 &quot;matrix&quot; (list of lists) of zeros.</span>
<span class="o">&gt;&gt;&gt;</span> <span class="p">[[</span><span class="mi">0</span> <span class="k">for</span> <span class="n">col</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)]</span> <span class="k">for</span> <span class="n">row</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="p">[[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">]]</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: List Comprehensions</strong>:</p>
<p class="last">Use a list comprehension to create a list that contains the string
“hello” 100 times.</p>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Fancier List Comprehensions</strong>:</p>
<p>Use the inline <code class="docutils literal notranslate"><span class="pre">if-else</span></code> statement (discussed earlier in this module),
along with a list comprehension, to create the list:</p>
<div class="last highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s1">&#39;hello&#39;</span><span class="p">,</span>
 <span class="s1">&#39;goodbye&#39;</span><span class="p">,</span>
 <span class="s1">&#39;hello&#39;</span><span class="p">,</span>
 <span class="s1">&#39;goodbye&#39;</span><span class="p">,</span>
 <span class="s1">&#39;hello&#39;</span><span class="p">,</span>
 <span class="s1">&#39;goodbye&#39;</span><span class="p">,</span>
 <span class="s1">&#39;hello&#39;</span><span class="p">,</span>
 <span class="s1">&#39;goodbye&#39;</span><span class="p">,</span>
 <span class="s1">&#39;hello&#39;</span><span class="p">,</span>
 <span class="s1">&#39;goodbye&#39;</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Tuple Comprehensions</strong>:</p>
<p class="last">Use a tuple-comprehension to extract comma-separated numbers from a
string, converting them into a tuple of floats. I.e. <code class="docutils literal notranslate"><span class="pre">&quot;3.2,2.4,99.8&quot;</span></code>
should become <code class="docutils literal notranslate"><span class="pre">(3.2,</span> <span class="pre">2.4,</span> <span class="pre">99.8)</span></code>. You will want to use the built-in
string function
<a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#str.split">str.split</a>.</p>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Translating a For-Loop</strong>:</p>
<p>Replicate the functionality of the the following code by writing a list
comprehension.</p>
<div class="last highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># skip all non-lowercased letters (including punctuation)</span>
<span class="c1"># append 1 if lowercase letter is &quot;o&quot;</span>
<span class="c1"># append 0 if lowercase letter is not &quot;o&quot;</span>
<span class="n">out</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="s2">&quot;Hello. How Are You?&quot;</span><span class="p">:</span>
    <span class="k">if</span> <span class="n">i</span><span class="o">.</span><span class="n">islower</span><span class="p">():</span>
        <span class="n">out</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span> <span class="k">if</span> <span class="n">i</span> <span class="ow">is</span> <span class="s2">&quot;o&quot;</span> <span class="k">else</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="admonition note">
<p class="first fa fa-exclamation-circle"><strong>Reading Comprehension: Memory Efficiency</strong>:</p>
<p>Is there any difference in performance between the following
expressions?</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># feeding `sum` a generator comprehension</span>
<span class="nb">sum</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="n">n</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">101</span><span class="p">))</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># feeding `sum` a list comprehension</span>
<span class="nb">sum</span><span class="p">([</span><span class="mi">1</span><span class="o">/</span><span class="n">n</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">101</span><span class="p">)])</span>
</pre></div>
</div>
<p class="last">Is one expression preferable over the other? Why?</p>
</div>
</div>
</div>
<div class="section" id="Links-to-Official-Documentation">
<h2>Links to Official Documentation<a class="headerlink" href="#Links-to-Official-Documentation" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><a class="reference external" href="https://docs.python.org/3/glossary.html#term-generator">Generator
definition</a></li>
<li><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#typesseq-range">range</a></li>
<li><a class="reference external" href="https://docs.python.org/3/tutorial/classes.html#generator-expressions">Generator comprehension
expressions</a></li>
<li><a class="reference external" href="https://docs.python.org/3/glossary.html#term-iterator">Iterator
definition</a></li>
<li><a class="reference external" href="https://docs.python.org/3/library/functions.html#next">next</a></li>
<li><a class="reference external" href="https://docs.python.org/3/library/functions.html#iter">iter</a></li>
<li><a class="reference external" href="https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions">List
comprehensions</a></li>
<li><a class="reference external" href="https://docs.python.org/3/tutorial/datastructures.html#nested-list-comprehensions">Nested list
comprehensions</a></li>
</ul>
</div>
<div class="section" id="Reading-Comprehension-Exercise-Solutions:">
<h2>Reading Comprehension Exercise Solutions:<a class="headerlink" href="#Reading-Comprehension-Exercise-Solutions:" title="Permalink to this headline">¶</a></h2>
<p><strong>Using range: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># start=10, stop=0 (excluded), step-size=-1</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">10</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">):</span>
    <span class="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>Writing a Generator Comprehension: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">((</span><span class="n">n</span><span class="p">,</span> <span class="n">n</span><span class="o">+</span><span class="mi">2</span><span class="p">)</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)</span> <span class="k">if</span> <span class="n">n</span> <span class="o">!=</span> <span class="mi">3</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>Using Generator Comprehensions on the Fly: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nb">sum</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">101</span><span class="p">,</span> <span class="mi">2</span><span class="p">))</span>
</pre></div>
</div>
<p>or</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nb">sum</span><span class="p">(</span><span class="n">i</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">101</span><span class="p">)</span> <span class="k">if</span> <span class="n">i</span><span class="o">%</span><span class="mi">2</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>List Comprehensions: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s2">&quot;hello&quot;</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">100</span><span class="p">)]</span>
</pre></div>
</div>
<p><strong>Fancier List Comprehensions: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="p">[(</span><span class="s2">&quot;hello&quot;</span> <span class="k">if</span> <span class="n">i</span><span class="o">%</span><span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span> <span class="k">else</span> <span class="s2">&quot;goodbye&quot;</span><span class="p">)</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">10</span><span class="p">)]</span>
</pre></div>
</div>
<p><strong>Tuple Comprehension: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">string_of_nums</span> <span class="o">=</span> <span class="s2">&quot;3.2, 2.4, 99.8&quot;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">tuple</span><span class="p">(</span><span class="nb">float</span><span class="p">(</span><span class="n">i</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">string_of_nums</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s2">&quot;,&quot;</span><span class="p">))</span>
<span class="go">(3.2, 2.4, 99.8)</span>
</pre></div>
</div>
<p><strong>Translating a For-Loop: Solution</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">out</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span> <span class="k">if</span> <span class="n">i</span> <span class="ow">is</span> <span class="s2">&quot;o&quot;</span> <span class="k">else</span> <span class="mi">0</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="s2">&quot;Hello. How Are You?&quot;</span> <span class="k">if</span> <span class="n">i</span><span class="o">.</span><span class="n">islower</span><span class="p">()]</span>
</pre></div>
</div>
<p><strong>Memory Efficiency: Solution</strong></p>
<p>It is preferable to use the generator expression
<code class="docutils literal notranslate"><span class="pre">sum(1/n</span> <span class="pre">for</span> <span class="pre">n</span> <span class="pre">in</span> <span class="pre">range(1,</span> <span class="pre">101))</span></code>, rather than the list comprehension
<code class="docutils literal notranslate"><span class="pre">sum([1/n</span> <span class="pre">for</span> <span class="pre">n</span> <span class="pre">in</span> <span class="pre">range(1,</span> <span class="pre">101)])</span></code>. Using a list comprehension
unnecessarily creates a list of the one hundred numbers, in memory,
before feeding the list to <code class="docutils literal notranslate"><span class="pre">sum</span></code>. The generator expression need only
produce a single value at a time, as <code class="docutils literal notranslate"><span class="pre">sum</span></code> iterates over it.</p>
</div>
</div>


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