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<div class="section" id="session-eight-generators-iterators-decorators-and-context-managers">
<h1>Session Eight: Generators, Iterators, Decorators, and Context Managers<a class="headerlink" href="#session-eight-generators-iterators-decorators-and-context-managers" title="Permalink to this headline">¶</a></h1>
<p class="large centered">The tools of Pythonicity</p>
<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-class">
<h3>Review of Previous Class<a class="headerlink" href="#review-of-previous-class" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li>Advanced OO Concepts<ul>
<li>Properties</li>
<li>Special Methods</li>
</ul>
</li>
<li>Testing with pytest</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>
<ul class="simple">
<li>Circle Class</li>
<li>Writing Tests using the <tt class="docutils literal"><span class="pre">pytest</span></tt> module</li>
</ul>
</div>
</div>
<div class="section" id="decorators">
<h2>Decorators<a class="headerlink" href="#decorators" title="Permalink to this headline">¶</a></h2>
<p><strong>A Short Digression</strong></p>
<div class="left build container">
<p>Functions are things that generate values based on input (arguments).</p>
<p>In Python, functions are first-class objects.</p>
<p>This means that you can bind symbols to them, pass them around, just like
other objects.</p>
<p>Because of this fact, you can write functions that take functions as
arguments and/or return functions as values:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">substitute</span><span class="p">(</span><span class="n">a_function</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">new_function</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">return</span> <span class="s">"I'm not that other function"</span>
<span class="k">return</span> <span class="n">new_function</span>
</pre></div>
</div>
</div>
<div class="section" id="a-definition">
<h3>A Definition<a class="headerlink" href="#a-definition" title="Permalink to this headline">¶</a></h3>
<p>There are many things you can do with a simple pattern like this one. So many,
that we give it a special name:</p>
<p class="centered"><strong>Decorator</strong></p>
<div class="build container">
<p>A decorator is a function that takes a function as an argument and
returns a function as a return value.</p>
<p>That’s nice and all, but why is that useful?</p>
</div>
</div>
<div class="section" id="an-example">
<h3>An Example<a class="headerlink" href="#an-example" title="Permalink to this headline">¶</a></h3>
<p>Imagine you are trying to debug a module with a number of functions like this
one:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
</pre></div>
</div>
<div class="build container">
<p>You want to see when each function is called, with what arguments and with what
result. So you rewrite each function as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">add</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="k">print</span> <span class="s">"Function 'add' called with args: </span><span class="si">%r</span><span class="s">"</span> <span class="o">%</span> <span class="nb">locals</span><span class="p">()</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="k">print</span> <span class="s">"</span><span class="se">\t</span><span class="s">Result --> </span><span class="si">%r</span><span class="s">"</span> <span class="o">%</span> <span class="n">result</span>
<span class="k">return</span> <span class="n">result</span>
</pre></div>
</div>
</div>
<p>That’s not particularly nice, especially if you have lots of functions in your
module.</p>
<p>Now imagine we defined the following, more generic <em>decorator</em>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">logged_func</span><span class="p">(</span><span class="n">func</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">logged</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"Function </span><span class="si">%r</span><span class="s"> called"</span> <span class="o">%</span> <span class="n">func</span><span class="o">.</span><span class="n">__name__</span>
<span class="k">if</span> <span class="n">args</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"</span><span class="se">\t</span><span class="s">with args: </span><span class="si">%r</span><span class="s">"</span> <span class="o">%</span> <span class="n">args</span>
<span class="k">if</span> <span class="n">kwargs</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"</span><span class="se">\t</span><span class="s">with kwargs: </span><span class="si">%r</span><span class="s">"</span> <span class="o">%</span> <span class="n">kwargs</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">print</span> <span class="s">"</span><span class="se">\t</span><span class="s"> Result --> </span><span class="si">%r</span><span class="s">"</span> <span class="o">%</span> <span class="n">result</span>
<span class="k">return</span> <span class="n">result</span>
<span class="k">return</span> <span class="n">logged</span>
</pre></div>
</div>
<p>We could then make logging versions of our module functions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">logging_add</span> <span class="o">=</span> <span class="n">logged_func</span><span class="p">(</span><span class="n">add</span><span class="p">)</span>
</pre></div>
</div>
<p>Then, where we want to see the results, we can use the logged version:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [37]: </span><span class="n">logging_add</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="go">Function 'add' called</span>
<span class="go"> with args: (3, 4)</span>
<span class="go"> Result --> 7</span>
<span class="gh">Out[37]: </span><span class="go">7</span>
</pre></div>
</div>
<div class="build container">
<p>This is nice, but we have to call the new function wherever we originally
had the old one.</p>
<p>It’d be nicer if we could just call the old function and have it log.</p>
</div>
<p>Remembering that you can easily rebind symbols in Python using <em>assignment
statements</em> leads you to this form:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">logged_func</span><span class="p">(</span><span class="n">func</span><span class="p">):</span>
<span class="c"># implemented above</span>
<span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="n">add</span> <span class="o">=</span> <span class="n">logged_func</span><span class="p">(</span><span class="n">add</span><span class="p">)</span>
</pre></div>
</div>
<div class="build container">
<p>And now you can simply use the code you’ve already written and calls to
<tt class="docutils literal"><span class="pre">add</span></tt> will be logged:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [41]: </span><span class="n">add</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="go">Function 'add' called</span>
<span class="go"> with args: (3, 4)</span>
<span class="go"> Result --> 7</span>
<span class="gh">Out[41]: </span><span class="go">7</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="syntax">
<h3>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h3>
<p>Rebinding the name of a function to the result of calling a decorator on that
function is called <strong>decoration</strong>.</p>
<p>Because this is so common, Python provides a special operator to perform it
more <em>declaratively</em>: the <tt class="docutils literal"><span class="pre">@</span></tt> operator:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># this is the imperative version:</span>
<span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="n">add</span> <span class="o">=</span> <span class="n">logged_func</span><span class="p">(</span><span class="n">add</span><span class="p">)</span>
<span class="c"># and this declarative form is exactly equal:</span>
<span class="nd">@logged_func</span>
<span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
</pre></div>
</div>
<div class="build container">
The declarative form (called a decorator expression) is far more common,
but both have the identical result, and can be used interchangeably.</div>
</div>
<div class="section" id="callables">
<h3>Callables<a class="headerlink" href="#callables" title="Permalink to this headline">¶</a></h3>
<p>Our original definition of a <em>decorator</em> was nice and simple, but a tiny bit
incomplete.</p>
<p>In reality, decorators can be used with anything that is <em>callable</em>.</p>
<p>In python a <em>callable</em> is a function, a method on a class, or even a class that
implements the <tt class="docutils literal"><span class="pre">__call__</span></tt> special method.</p>
<p>So in fact the definition should be updated as follows:</p>
<p class="centered">A decorator is a callable that takes a callable as an argument and
returns a callable as a return value.</p>
</div>
<div class="section" id="id1">
<h3>An Example<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
<p>Consider a decorator that would save the results of calling an expensive
function with given arguments:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Memoize</span><span class="p">:</span>
<span class="sd">"""</span>
<span class="sd">memoize decorator from avinash.vora</span>
<span class="sd">http://avinashv.net/2008/04/python-decorators-syntactic-sugar/</span>
<span class="sd">"""</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">function</span><span class="p">):</span> <span class="c"># runs when memoize class is called</span>
<span class="bp">self</span><span class="o">.</span><span class="n">function</span> <span class="o">=</span> <span class="n">function</span>
<span class="bp">self</span><span class="o">.</span><span class="n">memoized</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="c"># runs when memoize instance is called</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">memoized</span><span class="p">[</span><span class="n">args</span><span class="p">]</span>
<span class="k">except</span> <span class="ne">KeyError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">memoized</span><span class="p">[</span><span class="n">args</span><span class="p">]</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">function</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">memoized</span><span class="p">[</span><span class="n">args</span><span class="p">]</span>
</pre></div>
</div>
<p>Let’s try that out with a potentially expensive function:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [56]: </span><span class="nd">@Memoize</span>
<span class="gp"> ....: </span><span class="k">def</span> <span class="nf">sum2x</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">(</span><span class="n">n</span><span class="p">))</span>
<span class="gp"> ....:</span>
<span class="gp">In [57]: </span><span class="n">sum2x</span><span class="p">(</span><span class="mi">10000000</span><span class="p">)</span>
<span class="gh">Out[57]: </span><span class="go">99999990000000</span>
<span class="gp">In [58]: </span><span class="n">sum2x</span><span class="p">(</span><span class="mi">10000000</span><span class="p">)</span>
<span class="gh">Out[58]: </span><span class="go">99999990000000</span>
</pre></div>
</div>
<p>It’s nice to see that in action, but what if we want to know <em>exactly</em> how much
difference it made?</p>
</div>
<div class="section" id="nested-decorators">
<h3>Nested Decorators<a class="headerlink" href="#nested-decorators" title="Permalink to this headline">¶</a></h3>
<p>You can stack decorator expressions. The result is like calling each decorator
in order, from bottom to top:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@decorator_two</span>
<span class="nd">@decorator_one</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="k">pass</span>
<span class="c"># is exactly equal to:</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="k">pass</span>
<span class="n">func</span> <span class="o">=</span> <span class="n">decorator_two</span><span class="p">(</span><span class="n">decorator_one</span><span class="p">(</span><span class="n">func</span><span class="p">))</span>
</pre></div>
</div>
<p>Let’s define another decorator that will time how long a given call takes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">timed_func</span><span class="p">(</span><span class="n">func</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">timed</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">start</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="n">elapsed</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="o">-</span> <span class="n">start</span>
<span class="k">print</span> <span class="s">"time expired: </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="n">elapsed</span>
<span class="k">return</span> <span class="n">result</span>
<span class="k">return</span> <span class="n">timed</span>
</pre></div>
</div>
<p>And now we can use this new decorator stacked along with our memoizing
decorator:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [71]: </span><span class="nd">@timed_func</span>
<span class="gp"> ....: </span><span class="nd">@Memoize</span>
<span class="gp"> ....: </span><span class="k">def</span> <span class="nf">sum2x</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">(</span><span class="n">n</span><span class="p">))</span>
<span class="gp">In [72]: </span><span class="n">sum2x</span><span class="p">(</span><span class="mi">10000000</span><span class="p">)</span>
<span class="go">time expired: 0.997071027756</span>
<span class="gh">Out[72]: </span><span class="go">99999990000000</span>
<span class="gp">In [73]: </span><span class="n">sum2x</span><span class="p">(</span><span class="mi">10000000</span><span class="p">)</span>
<span class="go">time expired: 4.05311584473e-06</span>
<span class="gh">Out[73]: </span><span class="go">99999990000000</span>
</pre></div>
</div>
</div>
<div class="section" id="examples-from-the-standard-library">
<h3>Examples from the Standard Library<a class="headerlink" href="#examples-from-the-standard-library" title="Permalink to this headline">¶</a></h3>
<p>It’s going to be a lot more common for you to use pre-defined decorators than
for you to be writing your own.</p>
<p>Let’s see a few that might help you with work you’ve been doing recently.</p>
<p>For example, a <tt class="docutils literal"><span class="pre">staticmethod()</span></tt> can be implemented with a decorator
expression:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># the way we saw last week:</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="n">add</span> <span class="o">=</span> <span class="nb">staticmethod</span><span class="p">(</span><span class="n">add</span><span class="p">)</span>
<span class="c"># and the decorator form</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@staticmethod</span>
<span class="k">def</span> <span class="nf">add</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="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">classmethod()</span></tt> builtin can do the same thing:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># in imperative style:</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">from_iterable</span><span class="p">(</span><span class="n">cls</span><span class="p">,</span> <span class="n">seq</span><span class="p">):</span>
<span class="c"># method body</span>
<span class="n">from_iterable</span> <span class="o">=</span> <span class="nb">classmethod</span><span class="p">(</span><span class="n">from_iterable</span><span class="p">)</span>
<span class="c"># and in declarative style</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">from_iterable</span><span class="p">(</span><span class="n">cls</span><span class="p">,</span> <span class="n">seq</span><span class="p">):</span>
<span class="c"># method body</span>
</pre></div>
</div>
<p>Perhaps most commonly, you’ll see the <tt class="docutils literal"><span class="pre">property()</span></tt> builtin used this way.</p>
<p>Last week we saw this code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="bp">None</span>
<span class="k">def</span> <span class="nf">getx</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="k">def</span> <span class="nf">setx</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="nf">delx</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="n">x</span> <span class="o">=</span> <span class="nb">property</span><span class="p">(</span><span class="n">getx</span><span class="p">,</span> <span class="n">setx</span><span class="p">,</span> <span class="n">delx</span><span class="p">,</span>
<span class="s">"I'm the 'x' property."</span><span class="p">)</span>
</pre></div>
</div>
<p>Used in a decorator statement, it looks like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="bp">None</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="nd">@x.setter</span>
<span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="n">value</span>
<span class="nd">@x.deleter</span>
<span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
</pre></div>
</div>
<p>Note that in this case, the decorator object returned by the property decorator
itself implements additional decorators as attributes on the returned method
object.</p>
</div>
</div>
<div class="section" id="iterators-and-generators">
<h2>Iterators and Generators<a class="headerlink" href="#iterators-and-generators" title="Permalink to this headline">¶</a></h2>
<div class="section" id="iterators">
<h3>Iterators<a class="headerlink" href="#iterators" title="Permalink to this headline">¶</a></h3>
<p>Iterators are one of the main reasons Python code is so readable:</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">just_about_anything</span><span class="p">:</span>
<span class="n">do_stuff</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
<p>It does not have to be a “sequence”: list, tuple, etc.</p>
<p>Rather: you can loop through anything that satisfies the “iterator protocol”</p>
<p><a class="reference external" href="http://docs.python.org/library/stdtypes.html#iterator-types">http://docs.python.org/library/stdtypes.html#iterator-types</a></p>
</div>
<div class="section" id="the-iterator-protocol">
<h3>The Iterator Protocol<a class="headerlink" href="#the-iterator-protocol" title="Permalink to this headline">¶</a></h3>
<p>An iterator must have the following methods:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">an_iterator</span><span class="o">.</span><span class="n">__iter__</span><span class="p">()</span>
</pre></div>
</div>
<p>Returns the iterator object itself. This is required to allow both containers
and iterators to be used with the <tt class="docutils literal"><span class="pre">for</span></tt> and <tt class="docutils literal"><span class="pre">in</span></tt> statements.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">an_iterator</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
</pre></div>
</div>
<p>Returns the next item from the container. If there are no further items,
raises the <tt class="docutils literal"><span class="pre">StopIteration</span></tt> exception.</p>
</div>
<div class="section" id="list-as-an-iterator">
<h3>List as an Iterator:<a class="headerlink" href="#list-as-an-iterator" title="Permalink to this headline">¶</a></h3>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [10]: </span><span class="n">a_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 [11]: </span><span class="n">list_iter</span> <span class="o">=</span> <span class="n">a_list</span><span class="o">.</span><span class="n">__iter__</span><span class="p">()</span>
<span class="gp">In [12]: </span><span class="n">list_iter</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gh">Out[12]: </span><span class="go">1</span>
<span class="gp">In [13]: </span><span class="n">list_iter</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gh">Out[13]: </span><span class="go">2</span>
<span class="gp">In [14]: </span><span class="n">list_iter</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gh">Out[14]: </span><span class="go">3</span>
<span class="gp">In [15]: </span><span class="n">list_iter</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gt">--------------------------------------------------</span>
<span class="ne">StopIteration</span><span class="g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-15-1a7db9b70878></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">list_iter</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="ne">StopIteration</span>:
</pre></div>
</div>
</div>
<div class="section" id="making-an-iterator">
<h3>Making an Iterator<a class="headerlink" href="#making-an-iterator" title="Permalink to this headline">¶</a></h3>
<p>A simple version of <tt class="docutils literal"><span class="pre">xrange()</span></tt></p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">IterateMe_1</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">stop</span><span class="o">=</span><span class="mi">5</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">=</span> <span class="mi">0</span>
<span class="bp">self</span><span class="o">.</span><span class="n">stop</span> <span class="o">=</span> <span class="n">stop</span>
<span class="k">def</span> <span class="nf">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">def</span> <span class="nf">next</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o"><</span> <span class="bp">self</span><span class="o">.</span><span class="n">stop</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">current</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">StopIteration</span>
</pre></div>
</div>
<p>(demo: <tt class="docutils literal"><span class="pre">code/iterator_1.py</span></tt>)</p>
</div>
<div class="section" id="iter">
<h3><tt class="docutils literal"><span class="pre">iter()</span></tt><a class="headerlink" href="#iter" title="Permalink to this headline">¶</a></h3>
<p>How doyou get the iterator object (the thing with the next() method) from an “iterable”?</p>
<p>The <tt class="docutils literal"><span class="pre">iter()</span></tt> function:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [20]: </span><span class="nb">iter</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="gh">Out[20]: </span><span class="go"><listiterator at 0x101e01350></span>
<span class="gp">In [21]: </span><span class="nb">iter</span><span class="p">(</span><span class="s">"a string"</span><span class="p">)</span>
<span class="gh">Out[21]: </span><span class="go"><iterator at 0x101e01090></span>
<span class="gp">In [22]: </span><span class="nb">iter</span><span class="p">(</span> <span class="p">(</span><span class="s">'a'</span><span class="p">,</span> <span class="s">'tuple'</span><span class="p">)</span> <span class="p">)</span>
<span class="gh">Out[22]: </span><span class="go"><tupleiterator at 0x101e01710></span>
</pre></div>
</div>
<p>for an arbitrary object, <tt class="docutils literal"><span class="pre">iter()</span></tt> calls the <tt class="docutils literal"><span class="pre">__iter__</span></tt> method. But it knows about some object (<tt class="docutils literal"><span class="pre">str</span></tt>, for instance) that don’t have a <tt class="docutils literal"><span class="pre">__iter__</span></tt> method.</p>
</div>
<div class="section" id="what-does-for-do">
<h3>What does <tt class="docutils literal"><span class="pre">for</span></tt> do?<a class="headerlink" href="#what-does-for-do" title="Permalink to this headline">¶</a></h3>
<p>Now that we know the iterator protocol, we can write something like a for loop:</p>
<p>(<tt class="docutils literal"><span class="pre">code/session08/my_for.py</span></tt>)</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">my_for</span><span class="p">(</span><span class="n">an_iterable</span><span class="p">,</span> <span class="n">func</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Emulation of a for loop.</span>
<span class="sd"> func() will be called with each item in an_iterable</span>
<span class="sd"> """</span>
<span class="c"># equiv of "for i in l:"</span>
<span class="n">iterator</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">an_iterable</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">i</span> <span class="o">=</span> <span class="n">iterator</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">StopIteration</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">func</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="itertools">
<h3>Itertools<a class="headerlink" href="#itertools" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">itertools</span></tt> is a collection of utilities that make it easy to
build an iterator that iterates over sequences in various common ways</p>
<p><a class="reference external" href="http://docs.python.org/library/itertools.html">http://docs.python.org/library/itertools.html</a></p>
<p>NOTE:</p>
<p>iterators are not <em>only</em> for <tt class="docutils literal"><span class="pre">for</span></tt></p>
<p>They can be used with anything that expexts an iterator:</p>
<p><tt class="docutils literal"><span class="pre">sum</span></tt>, <tt class="docutils literal"><span class="pre">tuple</span></tt>, <tt class="docutils literal"><span class="pre">sorted</span></tt>, and <tt class="docutils literal"><span class="pre">list</span></tt></p>
<p>For example.</p>
</div>
<div class="section" id="lab-homework">
<h3>LAB / Homework<a class="headerlink" href="#lab-homework" title="Permalink to this headline">¶</a></h3>
<p>In the <tt class="docutils literal"><span class="pre">code/session08</span></tt> dir, you will find: <tt class="docutils literal"><span class="pre">iterator_1.py</span></tt></p>
<ul class="simple">
<li>Extend (<tt class="docutils literal"><span class="pre">iterator_1.py</span></tt> ) to be more like <tt class="docutils literal"><span class="pre">xrange()</span></tt> – add three input parameters: <tt class="docutils literal"><span class="pre">iterator_2(start,</span> <span class="pre">stop,</span> <span class="pre">step=1)</span></tt></li>
<li>See what happens if you break out in the middle of the loop:</li>
</ul>
<div class="highlight-python"><div class="highlight"><pre><span class="n">it</span> <span class="o">=</span> <span class="n">IterateMe_2</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">it</span><span class="p">:</span>
<span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">10</span><span class="p">:</span> <span class="k">break</span>
<span class="k">print</span> <span class="n">i</span>
</pre></div>
</div>
<p>And then pick up again:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">it</span><span class="p">:</span>
<span class="k">print</span> <span class="n">i</span>
</pre></div>
</div>
<ul class="simple">
<li>Does <tt class="docutils literal"><span class="pre">xrange()</span></tt> behave the same?<ul>
<li>make yours match <tt class="docutils literal"><span class="pre">xrange()</span></tt></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="generators">
<h3>Generators<a class="headerlink" href="#generators" title="Permalink to this headline">¶</a></h3>
<p>Generators give you the iterator immediately:</p>
<ul class="simple">
<li>no access to the underlying data ... if it even exists</li>
</ul>
<dl class="docutils">
<dt>Conceptually:</dt>
<dd>Iterators are about various ways to loop over data, generators generate the data on the fly</dd>
<dt>Practically:</dt>
<dd><p class="first">You can use either either way (and a generator is one type of iterator</p>
<p class="last">Generators do some of the book-keeping for you.</p>
</dd>
</dl>
</div>
<div class="section" id="yield">
<h3>yield<a class="headerlink" href="#yield" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">yield</span></tt> is a way to make a quickie generator with a function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">a_generator_function</span><span class="p">(</span><span class="n">params</span><span class="p">):</span>
<span class="n">some_stuff</span>
<span class="k">yield</span> <span class="n">something</span>
</pre></div>
</div>
<p>Generator functions “yield” a value, rather than returning a value.</p>
<p>State is preserved in between yields.</p>
<p>A function with <tt class="docutils literal"><span class="pre">yield</span></tt> in it is a “factory” for a generator</p>
<p>Each time you call it, you get a new generator:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">gen_a</span> <span class="o">=</span> <span class="n">a_generator</span><span class="p">()</span>
<span class="n">gen_b</span> <span class="o">=</span> <span class="n">a_generator</span><span class="p">()</span>
</pre></div>
</div>
<p>Each instance keeps its own state.</p>
<p>Really just a shorthand for an iterator class that does the book keeping for you.</p>
<p>An example: like <tt class="docutils literal"><span class="pre">xrange()</span></tt></p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">y_xrange</span><span class="p">(</span><span class="n">start</span><span class="p">,</span> <span class="n">stop</span><span class="p">,</span> <span class="n">step</span><span class="o">=</span><span class="mi">1</span><span class="p">):</span>
<span class="n">i</span> <span class="o">=</span> <span class="n">start</span>
<span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="n">stop</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">i</span>
<span class="n">i</span> <span class="o">+=</span> <span class="n">step</span>
</pre></div>
</div>
<p>Real World Example from FloatCanvas:</p>
<p><a class="reference external" href="https://github.com/svn2github/wxPython/blob/master/3rdParty/FloatCanvas/floatcanvas/FloatCanvas.py#L100">https://github.com/svn2github/wxPython/blob/master/3rdParty/FloatCanvas/floatcanvas/FloatCanvas.py#L100</a></p>
<p>Note:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [164]: </span><span class="n">gen</span> <span class="o">=</span> <span class="n">y_xrange</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="gp">In [165]: </span><span class="nb">type</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="gh">Out[165]: </span><span class="go">generator</span>
<span class="gp">In [166]: </span><span class="nb">dir</span><span class="p">(</span><span class="n">gen</span><span class="p">)</span>
<span class="gh">Out[166]:</span>
<span class="go">...</span>
<span class="go"> '__iter__',</span>
<span class="go">...</span>
<span class="go"> 'next',</span>
</pre></div>
</div>
<p>So the generator <strong>is</strong> an iterator</p>
<p>A generator function can also be a method in a class</p>
<p>More about iterators and generators:</p>
<p><a class="reference external" href="http://www.learningpython.com/2009/02/23/iterators-iterables-and-generators-oh-my/">http://www.learningpython.com/2009/02/23/iterators-iterables-and-generators-oh-my/</a></p>
<p><tt class="docutils literal"><span class="pre">code/session08/yield_example.py</span></tt></p>
</div>
<div class="section" id="generator-comprehension">
<h3>generator comprehension<a class="headerlink" href="#generator-comprehension" title="Permalink to this headline">¶</a></h3>
<p>yet another way to make a generator:</p>
<div class="highlight-python"><div class="highlight"><pre>>>> [x * 2 for x in [1, 2, 3]]
[2, 4, 6]
>>> (x * 2 for x in [1, 2, 3])
<generator object <genexpr> at 0x10911bf50>
>>> for n in (x * 2 for x in [1, 2, 3]):
... print n
... 2 4 6
</pre></div>
</div>
<p>More interesting if [1, 2, 3] is also a generator</p>
</div>
<div class="section" id="generator-lab-homework">
<h3>Generator LAB / Homework<a class="headerlink" href="#generator-lab-homework" title="Permalink to this headline">¶</a></h3>
<p>Write a few generators:</p>
<ul class="simple">
<li>Sum of integers</li>
<li>Doubler</li>
<li>Fibonacci sequence</li>
<li>Prime numbers</li>
</ul>
<p>(test code in <tt class="docutils literal"><span class="pre">code/session08/test_generator.py</span></tt>)</p>
<p>Descriptions:</p>
<dl class="docutils">
<dt>Sum of the integers:</dt>
<dd><p class="first">keep adding the next integer</p>
<p>0 + 1 + 2 + 3 + 4 + 5 + ...</p>
<p>so the sequence is:</p>
<p class="last">0, 1, 3, 6, 10, 15 .....</p>
</dd>
</dl>
<dl class="docutils">
<dt>Doubler:</dt>
<dd><p class="first">Each value is double the previous value:</p>
<p class="last">1, 2, 4, 8, 16, 32,</p>
</dd>
<dt>Fibonacci sequence:</dt>
<dd><p class="first">The fibonacci sequence as a generator:</p>
<p>f(n) = f(n-1) + f(n-2)</p>
<p class="last">1, 1, 2, 3, 5, 8, 13, 21, 34...</p>
</dd>
<dt>Prime numbers:</dt>
<dd><p class="first">Generate the prime numbers (numbers only divisible by them self and 1):</p>
<p class="last">2, 3, 5, 7, 11, 13, 17, 19, 23...</p>
</dd>
<dt>Others to try:</dt>
<dd>Try x^2, x^3, counting by threes, x^e, counting by minus seven, ...</dd>
</dl>
</div>
</div>
<div class="section" id="context-managers">
<h2>Context Managers<a class="headerlink" href="#context-managers" title="Permalink to this headline">¶</a></h2>
<p><strong>A Short Digression</strong></p>
<div class="left build container">
<p>Repetition in code stinks.</p>
<p>A large source of repetition in code deals with the handling of externals
resources.</p>
<p>As an example, how many times do you think you might type the following
code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">file_handle</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'filename.txt'</span><span class="p">,</span> <span class="s">'r'</span><span class="p">)</span>
<span class="n">file_content</span> <span class="o">=</span> <span class="n">file_handle</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">file_handle</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="c"># do some stuff with the contents</span>
</pre></div>
</div>
<p>What happens if you forget to call <tt class="docutils literal"><span class="pre">.close()</span></tt>?</p>
<p>What happens if reading the file raises an exception?</p>
</div>
<div class="section" id="resource-handling">
<h3>Resource Handling<a class="headerlink" href="#resource-handling" title="Permalink to this headline">¶</a></h3>
<p>Leaving an open file handle laying around is bad enough. What if the resource
is a network connection, or a database cursor?</p>
<p>You can write more robust code for handling your resources:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">file_handle</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'filename.txt'</span><span class="p">,</span> <span class="s">'r'</span><span class="p">)</span>
<span class="n">file_content</span> <span class="o">=</span> <span class="n">file_handle</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">file_handle</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="c"># do something with file_content here</span>
</pre></div>
</div>
<p>But what exceptions do you want to catch? And do you really want to have to
remember all that <strong>every</strong> time you open a resource?</p>
<p>Starting in version 2.5, Python provides a structure for reducing the
repetition needed to handle resources like this.</p>
<p class="centered"><strong>Context Managers</strong></p>
<p>You can encapsulate the setup, error handling and teardown of resources in a
few simple steps.</p>
<p>The key is to use the <tt class="docutils literal"><span class="pre">with</span></tt> statement.</p>
<p>Since the introduction of the <tt class="docutils literal"><span class="pre">with</span></tt> statement in <a class="reference external" href="http://legacy.python.org/dev/peps/pep-0343/">pep343</a>, the above six
lines of defensive code have been replaced with this simple form:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s">'filename'</span><span class="p">,</span> <span class="s">'r'</span><span class="p">)</span> <span class="k">as</span> <span class="n">file_handle</span><span class="p">:</span>
<span class="n">file_content</span> <span class="o">=</span> <span class="n">file_handle</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="c"># do something with file_content</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">open</span></tt> builtin is defined as a <em>context manager</em>.</p>
<p>The resource it returnes (<tt class="docutils literal"><span class="pre">file_handle</span></tt>) is automatically and reliably closed
when the code block ends.</p>
<p>At this point in Python history, many functions you might expect to behave this
way do:</p>
<ul class="build simple">
<li><tt class="docutils literal"><span class="pre">open</span></tt> and <tt class="docutils literal"><span class="pre">codecs.open</span></tt> both work as context managers</li>
<li>networks connections via <tt class="docutils literal"><span class="pre">socket</span></tt> do as well.</li>
<li>most implementations of database wrappers can open connections or cursors as
context managers.</li>
<li>...</li>
</ul>
<p>But what if you are working with a library that doesn’t support this
(<tt class="docutils literal"><span class="pre">urllib</span></tt>)?</p>
<p>There are a couple of ways you can go.</p>
<p>If the resource in questions has a <tt class="docutils literal"><span class="pre">.close()</span></tt> method, then you can simply use
the <tt class="docutils literal"><span class="pre">closing</span></tt> context manager from <tt class="docutils literal"><span class="pre">contextlib</span></tt> to handle the issue:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">urllib</span>
<span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">closing</span>
<span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">urllib</span><span class="o">.</span><span class="n">urlopen</span><span class="p">(</span><span class="s">'http://google.com'</span><span class="p">))</span> <span class="k">as</span> <span class="n">web_connection</span><span class="p">:</span>
<span class="c"># do something with the open resource</span>
<span class="c"># and here, it will be closed automatically</span>
</pre></div>
</div>
<p>But what if the thing doesn’t have a <tt class="docutils literal"><span class="pre">close()</span></tt> method, or you’re creating the thing and it shouldn’t?</p>
<p>You can also define a context manager of your own.</p>
<p>The interface is simple. It must be a class that implements these two <em>special
methods</em>:</p>
<dl class="docutils">
<dt><tt class="docutils literal"><span class="pre">__enter__(self)</span></tt>:</dt>
<dd>Called when the <tt class="docutils literal"><span class="pre">with</span></tt> statement is run, it should return something to work
with in the created context.</dd>
<dt><tt class="docutils literal"><span class="pre">__exit__(self,</span> <span class="pre">e_type,</span> <span class="pre">e_val,</span> <span class="pre">e_traceback)</span></tt>:</dt>
<dd><p class="first">Clean-up that needs to happen is implemented here.</p>
<p>The arguments will be the exception raised in the context.</p>
<p class="last">If the exception will be handled here, return True. If not, return False.</p>
</dd>
</dl>
<p>Let’s see this in action to get a sense of what happens.</p>
</div>
<div class="section" id="id2">
<h3>An Example<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h3>
<p>Consider this code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Context</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="sd">"""from Doug Hellmann, PyMOTW</span>
<span class="sd">http://pymotw.com/2/contextlib/#module-contextlib</span>
<span class="sd">"""</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">handle_error</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'__init__(</span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="n">handle_error</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handle_error</span> <span class="o">=</span> <span class="n">handle_error</span>
<span class="k">def</span> <span class="nf">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'__enter__()'</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">def</span> <span class="nf">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_val</span><span class="p">,</span> <span class="n">exc_tb</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'__exit__(</span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="p">(</span><span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_val</span><span class="p">,</span> <span class="n">exc_tb</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">handle_error</span>
</pre></div>
</div>
<p>This class doesn’t do much of anything, but playing with it can help clarify
the order in which things happen:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [46]: </span><span class="k">with</span> <span class="n">Context</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span> <span class="k">as</span> <span class="n">foo</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="k">print</span> <span class="s">'This is in the context'</span>
<span class="gp"> ....: </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'this is the error message'</span><span class="p">)</span>
<span class="go">__init__(True)</span>
<span class="go">__enter__()</span>
<span class="go">This is in the context</span>
<span class="go">__exit__(<type 'exceptions.RuntimeError'>, this is the error message, <traceback object at 0x1049cca28>)</span>
</pre></div>
</div>
<div class="build container">
Because the exit method returns True, the raised error is ‘handled’.</div>
<p>What if we try with <tt class="docutils literal"><span class="pre">False</span></tt>?</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [47]: </span><span class="k">with</span> <span class="n">Context</span><span class="p">(</span><span class="bp">False</span><span class="p">)</span> <span class="k">as</span> <span class="n">foo</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="k">print</span> <span class="s">'This is in the context'</span>
<span class="gp"> ....: </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'this is the error message'</span><span class="p">)</span>
<span class="go">__init__(False)</span>
<span class="go">__enter__()</span>
<span class="go">This is in the context</span>
<span class="go">__exit__(<type 'exceptions.RuntimeError'>, this is the error message, <traceback object at 0x1049ccb90>)</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">RuntimeError</span><span class="g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-47-de2c0c873dfc></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="g-Whitespace"> </span><span class="mi">1</span> <span class="k">with</span> <span class="n">Context</span><span class="p">(</span><span class="bp">False</span><span class="p">)</span> <span class="k">as</span> <span class="n">foo</span><span class="p">:</span>
<span class="g-Whitespace"> </span><span class="mi">2</span> <span class="k">print</span> <span class="s">'This is in the context'</span>
<span class="ne">----> </span><span class="mi">3</span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'this is the error message'</span><span class="p">)</span>
<span class="g-Whitespace"> </span><span class="mi">4</span>
<span class="ne">RuntimeError</span>: this is the error message
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">contextlib.contextmanager</span></tt> turns generator functions into context managers</p>
<p>Consider this code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">context</span><span class="p">(</span><span class="n">boolean</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"__init__ code here"</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"__enter__ code goes here"</span>
<span class="k">yield</span> <span class="nb">object</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"errors handled here"</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">boolean</span><span class="p">:</span>
<span class="k">raise</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"__exit__ cleanup goes here"</span>
</pre></div>
</div>
<p>The code is similar to the class defined previously.</p>
<p>And using it has similar results. We can handle errors:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [50]: </span><span class="k">with</span> <span class="n">context</span><span class="p">(</span><span class="bp">True</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">print</span> <span class="s">"in the context"</span>
<span class="gp"> ....: </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">"error raised"</span><span class="p">)</span>
<span class="go">__init__ code here</span>
<span class="go">__enter__ code goes here</span>
<span class="go">in the context</span>
<span class="go">errors handled here</span>
<span class="go">__exit__ cleanup goes here</span>
</pre></div>
</div>
<p>Or, we can allow them to propagate:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [51]: </span><span class="k">with</span> <span class="n">context</span><span class="p">(</span><span class="bp">False</span><span class="p">):</span>
<span class="gp"> ....: </span><span class="k">print</span> <span class="s">"in the context"</span>
<span class="gp"> ....: </span><span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">"error raised"</span><span class="p">)</span>
<span class="go">__init__ code here</span>
<span class="go">__enter__ code goes here</span>
<span class="go">in the context</span>
<span class="go">errors handled here</span>
<span class="go">__exit__ cleanup goes here</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">RuntimeError</span><span class="g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-51-641528ffa695></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="g-Whitespace"> </span><span class="mi">1</span> <span class="k">with</span> <span class="n">context</span><span class="p">(</span><span class="bp">False</span><span class="p">):</span>
<span class="g-Whitespace"> </span><span class="mi">2</span> <span class="k">print</span> <span class="s">"in the context"</span>
<span class="ne">----> </span><span class="mi">3</span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">"error raised"</span><span class="p">)</span>
<span class="g-Whitespace"> </span><span class="mi">4</span>
<span class="ne">RuntimeError</span>: error raised
</pre></div>
</div>
</div>
</div>
<div class="section" id="homework">
<h2>Homework<a class="headerlink" href="#homework" title="Permalink to this headline">¶</a></h2>
<p>Python Power</p>
<div class="section" id="assignments">
<h3>Assignments<a class="headerlink" href="#assignments" title="Permalink to this headline">¶</a></h3>
<p>Task 1: Timing Context Manager</p>
<p>Create a context manager that will print to stdout the elapsed time taken to
run all the code inside the context:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [3]: </span><span class="k">with</span> <span class="n">Timer</span><span class="p">()</span> <span class="k">as</span> <span class="n">t</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="mi">100000</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="n">i</span> <span class="o">=</span> <span class="n">i</span> <span class="o">**</span> <span class="mi">20</span>
<span class="gp"> ...:</span>
<span class="go">this code took 0.206805 seconds</span>
</pre></div>
</div>
<p><strong>Extra Credit</strong>: allow the <tt class="docutils literal"><span class="pre">Timer</span></tt> context manager to take a file-like
object as an argument (the default should be sys.stdout). The results of the
timing should be printed to the file-like object.</p>
<p>Task 2: <tt class="docutils literal"><span class="pre">p-wrapper</span></tt> Decorator</p>
<p>Write a simple decorator you can apply to a function that returns a string.
Decorating such a function should result in the original output, wrapped by an
HTML ‘p’ tag:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [4]: </span><span class="nd">@p_wrapper</span>
<span class="gp"> ...: </span><span class="k">def</span> <span class="nf">return_a_string</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="gp"> ...:</span>
<span class="gp">In [5]: </span><span class="n">return_a_string</span><span class="p">(</span><span class="s">"this is a string"</span><span class="p">)</span>
<span class="gh">Out[5]: </span><span class="go">'<p> this is a string </p>'</span>
</pre></div>
</div>
<p>Task 3: Generator Homework (documented above)</p>
<p>Task 4: Iterator Homework (documented above)</p>
</div>
</div>
</div>
</div>
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