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</style></head><body style="overflow: auto;"><div tabindex="2" class="cell text_cell border-box-sizing ui-widget-content ui-corner-all"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 1px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><h1>Context Managers</h1></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [2]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 65px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span>with open('/etc/hosts') as fp:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">open</span><span class="cm-null">(</span><span class="cm-string">'/etc/hosts'</span><span class="cm-null">) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">fp</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-variable">fp.read</span><span class="cm-null">()</span></pre><pre><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-variable">fp</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>127.0.0.1 localhost
127.0.1.1 precise64
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
<closed file '/etc/hosts', mode 'r' at 0x1882930></pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [5]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 153px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> with open('/etc/hosts') as fp:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">try</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">open</span><span class="cm-null">(</span><span class="cm-string">'/etc/hosts'</span><span class="cm-null">) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">fp</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">raise</span><span class="cm-null"> </span><span class="cm-variable">KeyError</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-variable">fp.read</span><span class="cm-null">()</span></pre><pre><span class="cm-keyword">except</span><span class="cm-null"> </span><span class="cm-variable">KeyError</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'handle keyerror'</span></pre><pre> </pre><pre><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-variable">fp</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>handle keyerror
<closed file '/etc/hosts', mode 'r' at 0x1882930></pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [7]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 47px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span>with open('/etc/hosts') as fp_i, open('/tmp/hosts', 'w') as fp_o:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">open</span><span class="cm-null">(</span><span class="cm-string">'/etc/hosts'</span><span class="cm-null">) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">fp_i</span><span class="cm-null">, </span><span class="cm-variable">open</span><span class="cm-null">(</span><span class="cm-string">'/tmp/hosts'</span><span class="cm-null">, </span><span class="cm-string">'w'</span><span class="cm-null">) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">fp_o</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-variable">fp_o.write</span><span class="cm-null">(</span><span class="cm-variable">fp_i.read</span><span class="cm-null">())</span></pre></div></div></div></div></div></div></div></div></div><div style="display: none;" class="output vbox"></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [8]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 47px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span>with open('/tmp/hosts') as fp:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">open</span><span class="cm-null">(</span><span class="cm-string">'/tmp/hosts'</span><span class="cm-null">) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">fp</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-variable">fp.read</span><span class="cm-null">()</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>127.0.0.1 localhost
127.0.1.1 precise64
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
</pre></div></div></div></div><div tabindex="2" class="cell text_cell border-box-sizing"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 1px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><h2>Context manager protocol</h2></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [16]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 223px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Swallowing %s inside CM' % ex_type</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">class</span><span class="cm-null"> </span><span class="cm-variable">CM</span><span class="cm-null">(</span><span class="cm-builtin">object</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">def</span><span class="cm-null"> </span><span class="cm-variable">__enter__</span><span class="cm-null">(</span><span class="cm-variable">self</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Entering CM'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">return</span><span class="cm-null"> </span><span class="cm-variable">self</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">def</span><span class="cm-null"> </span><span class="cm-variable">__exit__</span><span class="cm-null">(</span><span class="cm-variable">self</span><span class="cm-null">, </span><span class="cm-variable">ex_type</span><span class="cm-null">, </span><span class="cm-variable">ex_val</span><span class="cm-null">, </span><span class="cm-variable">ex_tb</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Exiting CM'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">if</span><span class="cm-null"> </span><span class="cm-variable">ex_type</span><span class="cm-null"> </span><span class="cm-operator">==</span><span class="cm-null"> </span><span class="cm-variable">KeyError</span><span class="cm-null">: </span></pre><pre><span class="cm-null"> </span><span class="cm-comment"># Re-raise same exception</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">return</span><span class="cm-null"> </span><span class="cm-variable">False</span></pre><pre><span class="cm-null"> </span><span class="cm-comment"># Don't re-raise</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Swallowing %s inside CM'</span><span class="cm-null"> </span><span class="cm-operator">%</span><span class="cm-null"> </span><span class="cm-variable">ex_type</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">return</span><span class="cm-null"> </span><span class="cm-variable">True</span></pre></div></div></div></div></div></div></div></div></div><div style="display: none;" class="output vbox"></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [20]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 47px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Inside with statement', cm</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">CM</span><span class="cm-null">() </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">cm</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Inside with statement'</span><span class="cm-null">, </span><span class="cm-variable">cm</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Entering CM
Inside with statement <__main__.CM object at 0x192f8d0>
Exiting CM
Swallowing None inside CM</pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [21]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 118px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Catching KeyError outside CM'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">try</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">CM</span><span class="cm-null">():</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'About to raise KeyError'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">raise</span><span class="cm-null"> </span><span class="cm-variable">KeyError</span></pre><pre><span class="cm-keyword">except</span><span class="cm-null"> </span><span class="cm-variable">KeyError</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Catching KeyError outside CM'</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Entering CM
About to raise KeyError
Exiting CM
Catching KeyError outside CM</pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [22]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 65px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'About to raise ValueError'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">CM</span><span class="cm-null">():</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'About to raise ValueError'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">raise</span><span class="cm-null"> </span><span class="cm-variable">ValueError</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Entering CM
About to raise ValueError
Exiting CM
Swallowing <type 'exceptions.ValueError'> inside CM</pre></div></div></div></div><div tabindex="2" class="cell text_cell border-box-sizing"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 22px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><h3>Exercises</h3>
<ul>
<li>Write a context manager that logs the entry and exit of a block of code (similar to the
decorator before)</li>
<li>Write a context manager that prints out balanced XML nodes. Use the test code below.</li>
</ul>
<p>Test code:</p>
<pre><code class="prettyprint"><span class="kwd">with</span><span class="pln"> node</span><span class="pun">(</span><span class="str">'html'</span><span class="pun">):</span><br><span class="pln"> </span><span class="kwd">with</span><span class="pln"> node</span><span class="pun">(</span><span class="str">'body'</span><span class="pun">):</span><br><span class="pln"> </span><span class="kwd">with</span><span class="pln"> node</span><span class="pun">(</span><span class="str">'h1'</span><span class="pun">):</span><br><span class="pln"> </span><span class="kwd">print</span><span class="pln"> </span><span class="str">'Page Title'</span><br></code></pre>
<p>You should see the following result:</p>
<pre><code class="prettyprint"><span class="tag"><html></span><br><span class="tag"><body></span><br><span class="tag"><h1></span><br><span class="pln">Page Title</span><br><span class="tag"></h1></span><br><span class="tag"></body></span><br><span class="tag"></html></span><br></code></pre></div></div><div tabindex="2" class="cell text_cell border-box-sizing"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 1px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><h2>Contextlib</h2></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [23]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 30px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span>import contextlib</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">import</span><span class="cm-null"> </span><span class="cm-variable">contextlib</span></pre></div></div></div></div></div></div></div></div></div><div style="display: none;" class="output vbox"></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [25]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 153px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Exiting block with exception'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-meta">@contextlib</span><span class="cm-variable">.contextmanager</span></pre><pre><span class="cm-keyword">def</span><span class="cm-null"> </span><span class="cm-variable">so_much_easier</span><span class="cm-null">():</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Entering block'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">try</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">yield</span><span class="cm-null"> </span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Exiting block cleanly'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">except</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Exiting block with exception'</span></pre></div></div></div></div></div></div></div></div></div><div style="display: none;" class="output vbox"></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [26]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 47px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Inside block'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">so_much_easier</span><span class="cm-null">():</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Inside block'</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Entering block
Inside block
Exiting block cleanly</pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [28]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 65px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Raising ValueError'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">so_much_easier</span><span class="cm-null">():</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Raising ValueError'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">raise</span><span class="cm-null"> </span><span class="cm-variable">ValueError</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Entering block
Raising ValueError
Exiting block with exception</pre></div></div></div></div><div tabindex="2" class="cell text_cell border-box-sizing"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 2px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><p><code>contextlib</code> also provides a facility to support the <code>with</code> statement with context manager-like
objects that don't actually support the protocol, but <em>do</em> have a <code>close()</code> method:</p></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [29]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 100px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> print 'Perform some resource acquisition'</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">class</span><span class="cm-null"> </span><span class="cm-variable">MyClass</span><span class="cm-null">(</span><span class="cm-builtin">object</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">def</span><span class="cm-null"> </span><span class="cm-variable">__init__</span><span class="cm-null">(</span><span class="cm-variable">self</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Perform some resource acquisition'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">def</span><span class="cm-null"> </span><span class="cm-variable">close</span><span class="cm-null">(</span><span class="cm-variable">self</span><span class="cm-null">):</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'Close the resource'</span></pre></div></div></div></div></div></div></div></div></div><div style="display: none;" class="output vbox"></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [30]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 47px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span>with contextlib.closing(MyClass()) as myobj:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">contextlib.closing</span><span class="cm-null">(</span><span class="cm-variable">MyClass</span><span class="cm-null">()) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">myobj</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'myobj is'</span><span class="cm-null">, </span><span class="cm-variable">myobj</span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Perform some resource acquisition
myobj is <__main__.MyClass object at 0x19c4450>
Close the resource</pre></div></div></div></div><div tabindex="2" class="cell border-box-sizing code_cell vbox"><div class="input hbox"><div class="prompt input_prompt">In [31]:</div><div class="input_area box-flex1"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px; top: 0px; left: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-ipython"><div style="position: relative; height: 135px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre><span> with contextlib.closing(MyClass()) as myobj:</span></pre></div><div style="position: relative; top: 0px;"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative;" draggable="true"><pre style="top: 0px; left: 0px;" class="CodeMirror-cursor"> </pre><div style=""><pre><span class="cm-keyword">try</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">with</span><span class="cm-null"> </span><span class="cm-variable">contextlib.closing</span><span class="cm-null">(</span><span class="cm-variable">MyClass</span><span class="cm-null">()) </span><span class="cm-keyword">as</span><span class="cm-null"> </span><span class="cm-variable">myobj</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'raising ValueError'</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">raise</span><span class="cm-null"> </span><span class="cm-variable">ValueError</span></pre><pre><span class="cm-keyword">except</span><span class="cm-null">:</span></pre><pre><span class="cm-null"> </span><span class="cm-keyword">print</span><span class="cm-null"> </span><span class="cm-string">'handling exception'</span></pre><pre><span class="cm-null"> </span></pre></div></div></div></div></div></div></div></div></div><div style="display: -moz-box;" class="output vbox"><div class="hbox output_area"><div class="prompt"></div><div class="box_flex1 output_subarea output_text output_stream output_stdout"><pre>Perform some resource acquisition
raising ValueError
Close the resource
handling exception</pre></div></div></div></div><div tabindex="2" class="cell text_cell border-box-sizing"><div style="display: none;" class="text_cell_input"><div class="CodeMirror"><div style="overflow: hidden; position: relative; width: 1px; height: 0px;"><textarea style="position: absolute; width: 2px;" wrap="off" autocorrect="off" autocapitalize="off"></textarea></div><div class="CodeMirror-scroll cm-s-default"><div style="position: relative; height: 3px;"><div style="position: absolute; height: 0; width: 0; overflow: hidden;"><pre>x</pre></div><div style="position: relative"><div style="display: none;" class="CodeMirror-gutter"><div class="CodeMirror-gutter-text"></div></div><div class="CodeMirror-lines"><div style="position: relative" draggable="true"><pre class="CodeMirror-cursor"> </pre><div></div></div></div></div></div></div></div></div><div tabindex="-1" class="text_cell_render rendered_html"><h3>Exercises</h3>
<ul>
<li>Update your context managers from the previous exercise to use the <code>@contextmanager</code> decorator</li>
</ul></div></div><div style="height: 30%;" class="end_space"></div></body></html>