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<ul>
<li><a class="reference internal" href="#">weakref – Garbage-collectable references to objects</a><ul>
<li><a class="reference internal" href="#references">References</a></li>
<li><a class="reference internal" href="#reference-callbacks">Reference Callbacks</a></li>
<li><a class="reference internal" href="#proxies">Proxies</a></li>
<li><a class="reference internal" href="#cyclic-references">Cyclic References</a></li>
<li><a class="reference internal" href="#caching-objects">Caching Objects</a></li>
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<div class="section" id="module-weakref">
<span id="weakref-garbage-collectable-references-to-objects"></span><h1>weakref – Garbage-collectable references to objects<a class="headerlink" href="#module-weakref" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">Refer to an “expensive” object, but allow it to be garbage collected if there are no other non-weak references.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">Since 2.1</td>
</tr>
</tbody>
</table>
<p>The <a class="reference internal" href="#module-weakref" title="weakref: Refer to an "expensive" object, but allow it to be garbage collected if there are no other non-weak references."><tt class="xref py py-mod docutils literal"><span class="pre">weakref</span></tt></a> module supports weak references to objects. A
normal reference increments the reference count on the object and
prevents it from being garbage collected. This is not always
desirable, either when a circular reference might be present or when
building a cache of objects that should be deleted when memory is
needed.</p>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<p>Weak references to your objects are managed through the <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt>
class. To retrieve the original object, call the reference object.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">weakref</span>
<span class="k">class</span> <span class="nc">ExpensiveObject</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'(Deleting </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">ExpensiveObject</span><span class="p">()</span>
<span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'obj:'</span><span class="p">,</span> <span class="n">obj</span>
<span class="k">print</span> <span class="s">'ref:'</span><span class="p">,</span> <span class="n">r</span>
<span class="k">print</span> <span class="s">'r():'</span><span class="p">,</span> <span class="n">r</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'deleting obj'</span>
<span class="k">del</span> <span class="n">obj</span>
<span class="k">print</span> <span class="s">'r():'</span><span class="p">,</span> <span class="n">r</span><span class="p">()</span>
</pre></div>
</div>
<p>In this case, since <tt class="docutils literal"><span class="pre">obj</span></tt> is deleted before the second call to the
reference, the <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> returns <tt class="xref docutils literal"><span class="pre">None</span></tt>.</p>
<div class="highlight-python"><pre>$ python weakref_ref.py
obj: <__main__.ExpensiveObject object at 0x100464790>
ref: <weakref at 0x100458ba8; to 'ExpensiveObject' at 0x100464790>
r(): <__main__.ExpensiveObject object at 0x100464790>
deleting obj
(Deleting <__main__.ExpensiveObject object at 0x100464790>)
r(): None</pre>
</div>
</div>
<div class="section" id="reference-callbacks">
<h2>Reference Callbacks<a class="headerlink" href="#reference-callbacks" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> constructor takes an optional second argument that
should be a callback function to invoke when the referenced object is
deleted.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">weakref</span>
<span class="k">class</span> <span class="nc">ExpensiveObject</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'(Deleting </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span>
<span class="k">def</span> <span class="nf">callback</span><span class="p">(</span><span class="n">reference</span><span class="p">):</span>
<span class="sd">"""Invoked when referenced object is deleted"""</span>
<span class="k">print</span> <span class="s">'callback('</span><span class="p">,</span> <span class="n">reference</span><span class="p">,</span> <span class="s">')'</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">ExpensiveObject</span><span class="p">()</span>
<span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">callback</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'obj:'</span><span class="p">,</span> <span class="n">obj</span>
<span class="k">print</span> <span class="s">'ref:'</span><span class="p">,</span> <span class="n">r</span>
<span class="k">print</span> <span class="s">'r():'</span><span class="p">,</span> <span class="n">r</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'deleting obj'</span>
<span class="k">del</span> <span class="n">obj</span>
<span class="k">print</span> <span class="s">'r():'</span><span class="p">,</span> <span class="n">r</span><span class="p">()</span>
</pre></div>
</div>
<p>The callback receives the reference object as an argument, after the
reference is “dead” and no longer refers to the original object. This
lets you remove the weak reference object from a cache, for example.</p>
<div class="highlight-python"><pre>$ python weakref_ref_callback.py
obj: <__main__.ExpensiveObject object at 0x100462990>
ref: <weakref at 0x100458c00; to 'ExpensiveObject' at 0x100462990>
r(): <__main__.ExpensiveObject object at 0x100462990>
deleting obj
callback( <weakref at 0x100458c00; dead> )
(Deleting <__main__.ExpensiveObject object at 0x100462990>)
r(): None</pre>
</div>
</div>
<div class="section" id="proxies">
<h2>Proxies<a class="headerlink" href="#proxies" title="Permalink to this headline">¶</a></h2>
<p>Instead of using <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> directly, it can be more convenient to
use a proxy. Proxies can be used as though they were the original
object, so you do not need to call the <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> first to access
the object.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">weakref</span>
<span class="k">class</span> <span class="nc">ExpensiveObject</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">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'(Deleting </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">ExpensiveObject</span><span class="p">(</span><span class="s">'My Object'</span><span class="p">)</span>
<span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">proxy</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'via obj:'</span><span class="p">,</span> <span class="n">obj</span><span class="o">.</span><span class="n">name</span>
<span class="k">print</span> <span class="s">'via ref:'</span><span class="p">,</span> <span class="n">r</span><span class="p">()</span><span class="o">.</span><span class="n">name</span>
<span class="k">print</span> <span class="s">'via proxy:'</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">name</span>
<span class="k">del</span> <span class="n">obj</span>
<span class="k">print</span> <span class="s">'via proxy:'</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">name</span>
</pre></div>
</div>
<p>If the proxy is access after the referent object is removed, a
<tt class="xref py py-class docutils literal"><span class="pre">ReferenceError</span></tt> exception is raised.</p>
<div class="highlight-python"><pre>$ python weakref_proxy.py
via obj: My Object
via ref: My Object
via proxy: My Object
(Deleting <__main__.ExpensiveObject object at 0x100462810>)
via proxy:
Traceback (most recent call last):
File "weakref_proxy.py", line 26, in <module>
print 'via proxy:', p.name
ReferenceError: weakly-referenced object no longer exists</pre>
</div>
</div>
<div class="section" id="cyclic-references">
<h2>Cyclic References<a class="headerlink" href="#cyclic-references" title="Permalink to this headline">¶</a></h2>
<p>One use for weak references is to allow cyclic references without preventing
garbage collection. This example illustrates the difference between using
regular objects and proxies when a graph includes a cycle.</p>
<p>First, we need a <tt class="xref py py-class docutils literal"><span class="pre">Graph</span></tt> class that accepts any object given to
it as the “next” node in the sequence. For the sake of brevity, this
<tt class="xref py py-class docutils literal"><span class="pre">Graph</span></tt> supports a single outgoing reference from each node,
which results in boring graphs but makes it easy to create cycles. The
function <tt class="xref py py-func docutils literal"><span class="pre">demo()</span></tt> is a utility function to exercise the graph
class by creating a cycle and then removing various references.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">gc</span>
<span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="kn">import</span> <span class="nn">weakref</span>
<span class="k">class</span> <span class="nc">Graph</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">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">other</span> <span class="o">=</span> <span class="bp">None</span>
<span class="k">def</span> <span class="nf">set_next</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'</span><span class="si">%s</span><span class="s">.set_next(</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="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">other</span><span class="p">,</span> <span class="nb">type</span><span class="p">(</span><span class="n">other</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">other</span> <span class="o">=</span> <span class="n">other</span>
<span class="k">def</span> <span class="nf">all_nodes</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s">"Generate the nodes in the graph sequence."</span>
<span class="k">yield</span> <span class="bp">self</span>
<span class="n">n</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">other</span>
<span class="k">while</span> <span class="n">n</span> <span class="ow">and</span> <span class="n">n</span><span class="o">.</span><span class="n">name</span> <span class="o">!=</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">n</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">n</span><span class="o">.</span><span class="n">other</span>
<span class="k">if</span> <span class="n">n</span> <span class="ow">is</span> <span class="bp">self</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">n</span>
<span class="k">return</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'->'</span><span class="o">.</span><span class="n">join</span><span class="p">([</span><span class="n">n</span><span class="o">.</span><span class="n">name</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">all_nodes</span><span class="p">()])</span>
<span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</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="bp">self</span><span class="o">.</span><span class="n">__class__</span><span class="o">.</span><span class="n">__name__</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'(Deleting </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">WeakGraph</span><span class="p">(</span><span class="n">Graph</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">set_next</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">if</span> <span class="n">other</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="c"># See if we should replace the reference</span>
<span class="c"># to other with a weakref.</span>
<span class="k">if</span> <span class="bp">self</span> <span class="ow">in</span> <span class="n">other</span><span class="o">.</span><span class="n">all_nodes</span><span class="p">():</span>
<span class="n">other</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">proxy</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="nb">super</span><span class="p">(</span><span class="n">WeakGraph</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">def</span> <span class="nf">collect_and_show_garbage</span><span class="p">():</span>
<span class="s">"Show what garbage is present."</span>
<span class="k">print</span> <span class="s">'Collecting...'</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">gc</span><span class="o">.</span><span class="n">collect</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Unreachable objects:'</span><span class="p">,</span> <span class="n">n</span>
<span class="k">print</span> <span class="s">'Garbage:'</span><span class="p">,</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">gc</span><span class="o">.</span><span class="n">garbage</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">demo</span><span class="p">(</span><span class="n">graph_factory</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Set up graph:'</span>
<span class="n">one</span> <span class="o">=</span> <span class="n">graph_factory</span><span class="p">(</span><span class="s">'one'</span><span class="p">)</span>
<span class="n">two</span> <span class="o">=</span> <span class="n">graph_factory</span><span class="p">(</span><span class="s">'two'</span><span class="p">)</span>
<span class="n">three</span> <span class="o">=</span> <span class="n">graph_factory</span><span class="p">(</span><span class="s">'three'</span><span class="p">)</span>
<span class="n">one</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="n">two</span><span class="p">)</span>
<span class="n">two</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="n">three</span><span class="p">)</span>
<span class="n">three</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="n">one</span><span class="p">)</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Graphs:'</span>
<span class="k">print</span> <span class="nb">str</span><span class="p">(</span><span class="n">one</span><span class="p">)</span>
<span class="k">print</span> <span class="nb">str</span><span class="p">(</span><span class="n">two</span><span class="p">)</span>
<span class="k">print</span> <span class="nb">str</span><span class="p">(</span><span class="n">three</span><span class="p">)</span>
<span class="n">collect_and_show_garbage</span><span class="p">()</span>
<span class="k">print</span>
<span class="n">three</span> <span class="o">=</span> <span class="bp">None</span>
<span class="n">two</span> <span class="o">=</span> <span class="bp">None</span>
<span class="k">print</span> <span class="s">'After 2 references removed:'</span>
<span class="k">print</span> <span class="nb">str</span><span class="p">(</span><span class="n">one</span><span class="p">)</span>
<span class="n">collect_and_show_garbage</span><span class="p">()</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Removing last reference:'</span>
<span class="n">one</span> <span class="o">=</span> <span class="bp">None</span>
<span class="n">collect_and_show_garbage</span><span class="p">()</span>
</pre></div>
</div>
<p>Now we can set up a test program using the <a class="reference internal" href="../gc/index.html#module-gc" title="gc: Garbage Collector"><tt class="xref py py-mod docutils literal"><span class="pre">gc</span></tt></a> module to help us
debug the leak. The <tt class="docutils literal"><span class="pre">DEBUG_LEAK</span></tt> flag causes <a class="reference internal" href="../gc/index.html#module-gc" title="gc: Garbage Collector"><tt class="xref py py-mod docutils literal"><span class="pre">gc</span></tt></a> to print
information about objects that cannot be seen other than through the
reference the garbage collector has to them.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">gc</span>
<span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="kn">import</span> <span class="nn">weakref</span>
<span class="kn">from</span> <span class="nn">weakref_graph</span> <span class="kn">import</span> <span class="n">Graph</span><span class="p">,</span> <span class="n">demo</span><span class="p">,</span> <span class="n">collect_and_show_garbage</span>
<span class="n">gc</span><span class="o">.</span><span class="n">set_debug</span><span class="p">(</span><span class="n">gc</span><span class="o">.</span><span class="n">DEBUG_LEAK</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Setting up the cycle'</span>
<span class="k">print</span>
<span class="n">demo</span><span class="p">(</span><span class="n">Graph</span><span class="p">)</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Breaking the cycle and cleaning up garbage'</span>
<span class="k">print</span>
<span class="n">gc</span><span class="o">.</span><span class="n">garbage</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span>
<span class="k">while</span> <span class="n">gc</span><span class="o">.</span><span class="n">garbage</span><span class="p">:</span>
<span class="k">del</span> <span class="n">gc</span><span class="o">.</span><span class="n">garbage</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="k">print</span>
<span class="n">collect_and_show_garbage</span><span class="p">()</span>
</pre></div>
</div>
<p>Even after deleting the local references to the <tt class="xref py py-class docutils literal"><span class="pre">Graph</span></tt>
instances in <tt class="xref py py-func docutils literal"><span class="pre">demo()</span></tt>, the graphs all show up in the garbage
list and cannot be collected. The dictionaries in the garbage list
hold the attributes of the <tt class="xref py py-class docutils literal"><span class="pre">Graph</span></tt> instances. We can forcibly
delete the graphs, since we know what they are:</p>
<div class="highlight-python"><pre>$ python -u weakref_cycle.py
Setting up the cycle
Set up graph:
one.set_next(two (<class 'weakref_graph.Graph'>))
two.set_next(three (<class 'weakref_graph.Graph'>))
three.set_next(one->two->three (<class 'weakref_graph.Graph'>))
Graphs:
one->two->three->one
two->three->one->two
three->one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]
After 2 references removed:
one->two->three->one
Collecting...
Unreachable objects: 0
Garbage:[]
Removing last reference:
Collecting...
gc: uncollectable <Graph 0x100477590>
gc: uncollectable <Graph 0x1004775d0>
gc: uncollectable <Graph 0x100477610>
gc: uncollectable <dict 0x10035fb00>
gc: uncollectable <dict 0x1003612e0>
gc: uncollectable <dict 0x10035c210>
Unreachable objects: 6
Garbage:[Graph(one),
Graph(two),
Graph(three),
{'name': 'one', 'other': Graph(two)},
{'name': 'two', 'other': Graph(three)},
{'name': 'three', 'other': Graph(one)}]
Breaking the cycle and cleaning up garbage
one.set_next(None (<type 'NoneType'>))
(Deleting two)
two.set_next(None (<type 'NoneType'>))
(Deleting three)
three.set_next(None (<type 'NoneType'>))
(Deleting one)
one.set_next(None (<type 'NoneType'>))
Collecting...
Unreachable objects: 0
Garbage:[]</pre>
</div>
<p>And now let’s define a more intelligent <tt class="xref py py-class docutils literal"><span class="pre">WeakGraph</span></tt> class that
knows not to create cycles using regular references, but to use a
<tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> when a cycle is detected.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">gc</span>
<span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="kn">import</span> <span class="nn">weakref</span>
<span class="kn">from</span> <span class="nn">weakref_graph</span> <span class="kn">import</span> <span class="n">Graph</span><span class="p">,</span> <span class="n">demo</span>
<span class="k">class</span> <span class="nc">WeakGraph</span><span class="p">(</span><span class="n">Graph</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">set_next</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">if</span> <span class="n">other</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="c"># See if we should replace the reference</span>
<span class="c"># to other with a weakref.</span>
<span class="k">if</span> <span class="bp">self</span> <span class="ow">in</span> <span class="n">other</span><span class="o">.</span><span class="n">all_nodes</span><span class="p">():</span>
<span class="n">other</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">proxy</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="nb">super</span><span class="p">(</span><span class="n">WeakGraph</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">set_next</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="k">return</span>
<span class="n">demo</span><span class="p">(</span><span class="n">WeakGraph</span><span class="p">)</span>
</pre></div>
</div>
<p>Since the <tt class="xref py py-class docutils literal"><span class="pre">WeakGraph</span></tt> instances use proxies to refer to objects
that have already been seen, as <tt class="xref py py-func docutils literal"><span class="pre">demo()</span></tt> removes all local
references to the objects, the cycle is broken and the garbage
collector can delete the objects for us.</p>
<div class="highlight-python"><pre>$ python weakref_weakgraph.py
Set up graph:
one.set_next(two (<class '__main__.WeakGraph'>))
two.set_next(three (<class '__main__.WeakGraph'>))
three.set_next(one->two->three (<type 'weakproxy'>))
Graphs:
one->two->three
two->three->one->two
three->one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]
After 2 references removed:
one->two->three
Collecting...
Unreachable objects: 0
Garbage:[]
Removing last reference:
(Deleting one)
one.set_next(None (<type 'NoneType'>))
(Deleting two)
two.set_next(None (<type 'NoneType'>))
(Deleting three)
three.set_next(None (<type 'NoneType'>))
Collecting...
Unreachable objects: 0
Garbage:[]</pre>
</div>
</div>
<div class="section" id="caching-objects">
<h2>Caching Objects<a class="headerlink" href="#caching-objects" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="xref py py-class docutils literal"><span class="pre">ref</span></tt> and <tt class="xref py py-class docutils literal"><span class="pre">proxy</span></tt> classes are considered “low
level”. While they are useful for maintaining weak references to
individual objects and allowing cycles to be garbage collected, if you
need to create a cache of several objects the
<tt class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></tt> and <tt class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></tt> provide a
more appropriate API.</p>
<p>As you might expect, the <tt class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></tt> uses weak
references to the values it holds, allowing them to be garbage
collected when other code is not actually using them.</p>
<p>To illustrate the difference between memory handling with a regular
dictionary and <tt class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></tt>, let’s go experiment with
explicitly calling the garbage collector again:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">gc</span>
<span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="kn">import</span> <span class="nn">weakref</span>
<span class="n">gc</span><span class="o">.</span><span class="n">set_debug</span><span class="p">(</span><span class="n">gc</span><span class="o">.</span><span class="n">DEBUG_LEAK</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">ExpensiveObject</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">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'ExpensiveObject(</span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span>
<span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'(Deleting </span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="bp">self</span>
<span class="k">def</span> <span class="nf">demo</span><span class="p">(</span><span class="n">cache_factory</span><span class="p">):</span>
<span class="c"># hold objects so any weak references </span>
<span class="c"># are not removed immediately</span>
<span class="n">all_refs</span> <span class="o">=</span> <span class="p">{}</span>
<span class="c"># the cache using the factory we're given</span>
<span class="k">print</span> <span class="s">'CACHE TYPE:'</span><span class="p">,</span> <span class="n">cache_factory</span>
<span class="n">cache</span> <span class="o">=</span> <span class="n">cache_factory</span><span class="p">()</span>
<span class="k">for</span> <span class="n">name</span> <span class="ow">in</span> <span class="p">[</span> <span class="s">'one'</span><span class="p">,</span> <span class="s">'two'</span><span class="p">,</span> <span class="s">'three'</span> <span class="p">]:</span>
<span class="n">o</span> <span class="o">=</span> <span class="n">ExpensiveObject</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="n">cache</span><span class="p">[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">o</span>
<span class="n">all_refs</span><span class="p">[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">o</span>
<span class="k">del</span> <span class="n">o</span> <span class="c"># decref</span>
<span class="k">print</span> <span class="s">'all_refs ='</span><span class="p">,</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">all_refs</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Before, cache contains:'</span><span class="p">,</span> <span class="n">cache</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="k">for</span> <span class="n">name</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">cache</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="k">print</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">name</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="k">del</span> <span class="n">value</span> <span class="c"># decref</span>
<span class="c"># Remove all references to our objects except the cache</span>
<span class="k">print</span> <span class="s">'Cleanup:'</span>
<span class="k">del</span> <span class="n">all_refs</span>
<span class="n">gc</span><span class="o">.</span><span class="n">collect</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'After, cache contains:'</span><span class="p">,</span> <span class="n">cache</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="k">for</span> <span class="n">name</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">cache</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="k">print</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">name</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'demo returning'</span>
<span class="k">return</span>
<span class="n">demo</span><span class="p">(</span><span class="nb">dict</span><span class="p">)</span>
<span class="k">print</span>
<span class="n">demo</span><span class="p">(</span><span class="n">weakref</span><span class="o">.</span><span class="n">WeakValueDictionary</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice that any loop variables that refer to the values we are caching must be
cleared explicitly to decrement the reference count on the object. Otherwise
the garbage collector would not remove the objects and they would remain in
the cache. Similarly, the all_refs variable is used to hold references to
prevent them from being garbage collected prematurely.</p>
<div class="highlight-python"><pre>$ python weakref_valuedict.py
CACHE TYPE: <type 'dict'>
all_refs ={'one': ExpensiveObject(one),
'three': ExpensiveObject(three),
'two': ExpensiveObject(two)}
Before, cache contains: ['three', 'two', 'one']
three = ExpensiveObject(three)
two = ExpensiveObject(two)
one = ExpensiveObject(one)
Cleanup:
After, cache contains: ['three', 'two', 'one']
three = ExpensiveObject(three)
two = ExpensiveObject(two)
one = ExpensiveObject(one)
demo returning
(Deleting ExpensiveObject(three))
(Deleting ExpensiveObject(two))
(Deleting ExpensiveObject(one))
CACHE TYPE: weakref.WeakValueDictionary
all_refs ={'one': ExpensiveObject(one),
'three': ExpensiveObject(three),
'two': ExpensiveObject(two)}
Before, cache contains: ['three', 'two', 'one']
three = ExpensiveObject(three)
two = ExpensiveObject(two)
one = ExpensiveObject(one)
Cleanup:
(Deleting ExpensiveObject(three))
(Deleting ExpensiveObject(two))
(Deleting ExpensiveObject(one))
After, cache contains: []
demo returning</pre>
</div>
<p>The WeakKeyDictionary works similarly but uses weak references for the keys
instead of the values in the dictionary.</p>
<p>The library documentation for weakref contains this warning:</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Caution: Because a WeakValueDictionary is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a WeakValueDictionary because actions performed by
the program during iteration may cause items in the dictionary to vanish
“by magic” (as a side effect of garbage collection).</p>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/lib/module-weakref.html">weakref</a></dt>
<dd>Standard library documentation for this module.</dd>
<dt><a class="reference internal" href="../gc/index.html#module-gc" title="gc: Garbage Collector"><tt class="xref py py-mod docutils literal"><span class="pre">gc</span></tt></a></dt>
<dd>The gc module is the interface to the interpreter’s garbage collector.</dd>
</dl>
</div>
</div>
</div>
</div>
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