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<div class="section" id="context-functions">
<h1>Context Functions<a class="headerlink" href="#context-functions" title="Permalink to this headline">ΒΆ</a></h1>
<p>It should be clear by this point that the ability to use a general context
as the namespace when executing code permits some complex behaviors to be
implemented simply. There are circumstances where it would be useful to
replace the namespace of a function with a context. The <tt class="xref docutils literal"><span class="pre">context_function</span></tt>
module provides some tools for doing this.</p>
<p>This example shows how to set up a simple listening data context that displays
changes within a function’s local namespace as it executes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">enthought.traits.api</span> <span class="kn">import</span> <span class="n">HasTraits</span><span class="p">,</span> <span class="n">on_trait_change</span>
<span class="kn">from</span> <span class="nn">enthought.contexts.api</span> <span class="kn">import</span> <span class="n">DataContext</span><span class="p">,</span> <span class="n">context_function</span>
<span class="k">class</span> <span class="nc">ListeningDataContext</span><span class="p">(</span><span class="n">DataContext</span><span class="p">):</span>
<span class="sd">""" A simple subclass of DataContext which listens for items_modified</span>
<span class="sd"> events and pretty-prints them."""</span>
<span class="nd">@on_trait_change</span><span class="p">(</span><span class="s">'items_modified'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">print_event</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">event</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"Event: items_modified"</span>
<span class="k">for</span> <span class="n">added</span> <span class="ow">in</span> <span class="n">event</span><span class="o">.</span><span class="n">added</span><span class="p">:</span>
<span class="k">print</span> <span class="s">" Added:"</span><span class="p">,</span> <span class="n">added</span><span class="p">,</span> <span class="s">"="</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="p">[</span><span class="n">added</span><span class="p">])</span>
<span class="k">for</span> <span class="n">modified</span> <span class="ow">in</span> <span class="n">event</span><span class="o">.</span><span class="n">modified</span><span class="p">:</span>
<span class="k">print</span> <span class="s">" Modified:"</span><span class="p">,</span> <span class="n">modified</span><span class="p">,</span> <span class="s">"="</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="p">[</span><span class="n">modified</span><span class="p">])</span>
<span class="k">for</span> <span class="n">removed</span> <span class="ow">in</span> <span class="n">event</span><span class="o">.</span><span class="n">removed</span><span class="p">:</span>
<span class="k">print</span> <span class="s">" Removed:"</span><span class="p">,</span> <span class="n">removed</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">t</span><span class="o">=</span><span class="mf">3</span><span class="p">):</span>
<span class="sd">""" A function which fires add, modify and delete events. """</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">x</span><span class="o">+</span><span class="mf">2</span>
<span class="n">y</span> <span class="o">+=</span> <span class="mf">1</span>
<span class="n">z</span> <span class="o">=</span> <span class="s">'12'</span>
<span class="k">del</span> <span class="n">z</span>
<span class="k">return</span> <span class="n">y</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">context_function</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">ListeningDataContext</span><span class="p">)</span>
</pre></div>
</div>
<p>The functionality is also available as a function decorator:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@local_context</span><span class="p">(</span><span class="n">ListeningDataContext</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">t</span><span class="o">=</span><span class="mf">3</span><span class="p">):</span>
<span class="sd">""" A function which will fire add, modify and delete events. """</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">x</span><span class="o">+</span><span class="mf">2</span>
<span class="n">y</span> <span class="o">+=</span> <span class="mf">1</span>
<span class="n">z</span> <span class="o">=</span> <span class="s">'12'</span>
<span class="k">del</span> <span class="n">z</span>
<span class="k">return</span> <span class="n">y</span>
</pre></div>
</div>
<p>The <tt class="xref docutils literal"><span class="pre">context_function()</span></tt> and <tt class="xref docutils literal"><span class="pre">local_context()</span></tt> functions both take an
argument which is a context factory: each invocation of the function results in
a call to the context factory. In most cases it returns a freshly created
context.</p>
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