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<li><a href="../pep-0000/">PEP Index</a> » </li>
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<li>PEP 213</li>
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<section id="pep-content">
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<h1 class="page-title">PEP 213 – Attribute Access Handlers</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Paul Prescod <paul at prescod.net></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Inactive draft that may be taken up again at a later time">Deferred</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">21-Jul-2000</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.1</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></dd>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#introduction">Introduction</a></li>
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<li><a class="reference internal" href="#justification">Justification</a><ul>
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<li><a class="reference internal" href="#scenario-1">Scenario 1</a></li>
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<li><a class="reference internal" href="#scenario-2">Scenario 2</a></li>
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<li><a class="reference internal" href="#scenario-3">Scenario 3</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#current-solution">Current Solution</a></li>
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<li><a class="reference internal" href="#proposed-syntax">Proposed Syntax</a></li>
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<li><a class="reference internal" href="#semantics">Semantics</a></li>
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<li><a class="reference internal" href="#proposed-implementation">Proposed Implementation</a></li>
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<li><a class="reference internal" href="#caveats">Caveats</a></li>
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<li><a class="reference internal" href="#note">Note</a></li>
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</ul>
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</details></section>
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<section id="introduction">
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<h2><a class="toc-backref" href="#introduction" role="doc-backlink">Introduction</a></h2>
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<p>It is possible (and even relatively common) in Python code and
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in extension modules to “trap” when an instance’s client code
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attempts to set an attribute and execute code instead. In other
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words, it is possible to allow users to use attribute assignment/
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retrieval/deletion syntax even though the underlying implementation
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is doing some computation rather than directly modifying a
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binding.</p>
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<p>This PEP describes a feature that makes it easier, more efficient
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and safer to implement these handlers for Python instances.</p>
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</section>
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<section id="justification">
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<h2><a class="toc-backref" href="#justification" role="doc-backlink">Justification</a></h2>
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<section id="scenario-1">
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<h3><a class="toc-backref" href="#scenario-1" role="doc-backlink">Scenario 1</a></h3>
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<p>You have a deployed class that works on an attribute named
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“stdout”. After a while, you think it would be better to
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check that stdout is really an object with a “write” method
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at the moment of assignment. Rather than change to a
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setstdout method (which would be incompatible with deployed
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code) you would rather trap the assignment and check the
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object’s type.</p>
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</section>
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<section id="scenario-2">
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<h3><a class="toc-backref" href="#scenario-2" role="doc-backlink">Scenario 2</a></h3>
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<p>You want to be as compatible as possible with an object
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model that has a concept of attribute assignment. It could
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be the W3C Document Object Model or a particular COM
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interface (e.g. the PowerPoint interface). In that case
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you may well want attributes in the model to show up as
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attributes in the Python interface, even though the
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underlying implementation may not use attributes at all.</p>
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</section>
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<section id="scenario-3">
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<h3><a class="toc-backref" href="#scenario-3" role="doc-backlink">Scenario 3</a></h3>
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<p>A user wants to make an attribute read-only.</p>
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<p>In short, this feature allows programmers to separate the
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interface of their module from the underlying implementation
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for whatever purpose. Again, this is not a new feature but
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merely a new syntax for an existing convention.</p>
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</section>
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</section>
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<section id="current-solution">
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<h2><a class="toc-backref" href="#current-solution" role="doc-backlink">Current Solution</a></h2>
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<p>To make some attributes read-only:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">foo</span><span class="p">:</span>
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<span class="k">def</span> <span class="fm">__setattr__</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="n">val</span> <span class="p">):</span>
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<span class="k">if</span> <span class="n">name</span><span class="o">==</span><span class="s2">"readonlyattr"</span><span class="p">:</span>
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<span class="k">raise</span> <span class="ne">TypeError</span>
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<span class="k">elif</span> <span class="n">name</span><span class="o">==</span><span class="s2">"readonlyattr2"</span><span class="p">:</span>
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<span class="k">raise</span> <span class="ne">TypeError</span>
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<span class="o">...</span>
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<span class="k">else</span><span class="p">:</span>
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<span class="bp">self</span><span class="o">.</span><span class="vm">__dict__</span><span class="p">[</span><span class="s2">"name"</span><span class="p">]</span><span class="o">=</span><span class="n">val</span>
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</pre></div>
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</div>
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<p>This has the following problems:</p>
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<ol class="arabic simple">
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<li>The creator of the method must be intimately aware of whether
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somewhere else in the class hierarchy <code class="docutils literal notranslate"><span class="pre">__setattr__</span></code> has also been
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trapped for any particular purpose. If so, she must specifically
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call that method rather than assigning to the dictionary. There
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are many different reasons to overload <code class="docutils literal notranslate"><span class="pre">__setattr__</span></code> so there is a
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decent potential for clashes. For instance object database
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implementations often overload setattr for an entirely unrelated
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purpose.</li>
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<li>The string-based switch statement forces all attribute handlers
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to be specified in one place in the code. They may then dispatch
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to task-specific methods (for modularity) but this could cause
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performance problems.</li>
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<li>Logic for the setting, getting and deleting must live in
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<code class="docutils literal notranslate"><span class="pre">__getattr__</span></code>, <code class="docutils literal notranslate"><span class="pre">__setattr__</span></code> and <code class="docutils literal notranslate"><span class="pre">__delattr__</span></code>. Once again, this can
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be mitigated through an extra level of method call but this is
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inefficient.</li>
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</ol>
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</section>
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<section id="proposed-syntax">
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<h2><a class="toc-backref" href="#proposed-syntax" role="doc-backlink">Proposed Syntax</a></h2>
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<p>Special methods should declare themselves with declarations of the
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following form:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">x</span><span class="p">:</span>
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<span class="k">def</span> <span class="nf">__attr_XXX__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">op</span><span class="p">,</span> <span class="n">val</span> <span class="p">):</span>
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<span class="k">if</span> <span class="n">op</span><span class="o">==</span><span class="s2">"get"</span><span class="p">:</span>
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<span class="k">return</span> <span class="n">someComputedValue</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">internal</span><span class="p">)</span>
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<span class="k">elif</span> <span class="n">op</span><span class="o">==</span><span class="s2">"set"</span><span class="p">:</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">internal</span><span class="o">=</span><span class="n">someComputedValue</span><span class="p">(</span><span class="n">val</span><span class="p">)</span>
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<span class="k">elif</span> <span class="n">op</span><span class="o">==</span><span class="s2">"del"</span><span class="p">:</span>
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<span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">internal</span>
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</pre></div>
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</div>
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<p>Client code looks like this:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">fooval</span><span class="o">=</span><span class="n">x</span><span class="o">.</span><span class="n">foo</span>
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<span class="n">x</span><span class="o">.</span><span class="n">foo</span><span class="o">=</span><span class="n">fooval</span><span class="o">+</span><span class="mi">5</span>
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<span class="k">del</span> <span class="n">x</span><span class="o">.</span><span class="n">foo</span>
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</pre></div>
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</div>
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</section>
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<section id="semantics">
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<h2><a class="toc-backref" href="#semantics" role="doc-backlink">Semantics</a></h2>
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<p>Attribute references of all three kinds should call the method.
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The op parameter can be “get”/”set”/”del”. Of course this string
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will be interned so the actual checks for the string will be
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very fast.</p>
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<p>It is disallowed to actually have an attribute named XXX in the
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same instance as a method named __attr_XXX__.</p>
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<p>An implementation of __attr_XXX__ takes precedence over an
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implementation of <code class="docutils literal notranslate"><span class="pre">__getattr__</span></code> based on the principle that
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<code class="docutils literal notranslate"><span class="pre">__getattr__</span></code> is supposed to be invoked only after finding an
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appropriate attribute has failed.</p>
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<p>An implementation of __attr_XXX__ takes precedence over an
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implementation of <code class="docutils literal notranslate"><span class="pre">__setattr__</span></code> in order to be consistent. The
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opposite choice seems fairly feasible also, however. The same
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goes for __del_y__.</p>
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</section>
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<section id="proposed-implementation">
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<h2><a class="toc-backref" href="#proposed-implementation" role="doc-backlink">Proposed Implementation</a></h2>
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<p>There is a new object type called an attribute access handler.
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Objects of this type have the following attributes:</p>
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|
|
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">name</span> <span class="p">(</span><span class="n">e</span><span class="o">.</span><span class="n">g</span><span class="o">.</span> <span class="n">XXX</span><span class="p">,</span> <span class="ow">not</span> <span class="n">__attr__XXX__</span><span class="p">)</span>
|
|||
|
|
<span class="n">method</span> <span class="p">(</span><span class="n">pointer</span> <span class="n">to</span> <span class="n">a</span> <span class="n">method</span> <span class="nb">object</span><span class="p">)</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
<p>In PyClass_New, methods of the appropriate form will be detected and
|
|||
|
|
converted into objects (just like unbound method objects). These are
|
|||
|
|
stored in the class <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> under the name XXX. The original method
|
|||
|
|
is stored as an unbound method under its original name.</p>
|
|||
|
|
<p>If there are any attribute access handlers in an instance at all,
|
|||
|
|
a flag is set. Let’s call it “I_have_computed_attributes” for
|
|||
|
|
now. Derived classes inherit the flag from base classes. Instances
|
|||
|
|
inherit the flag from classes.</p>
|
|||
|
|
<p>A get proceeds as usual until just before the object is returned.
|
|||
|
|
In addition to the current check whether the returned object is a
|
|||
|
|
method it would also check whether a returned object is an access
|
|||
|
|
handler. If so, it would invoke the getter method and return
|
|||
|
|
the value. To remove an attribute access handler you could directly
|
|||
|
|
fiddle with the dictionary.</p>
|
|||
|
|
<p>A set proceeds by checking the “I_have_computed_attributes” flag. If
|
|||
|
|
it is not set, everything proceeds as it does today. If it is set
|
|||
|
|
then we must do a dictionary get on the requested object name. If it
|
|||
|
|
returns an attribute access handler then we call the setter function
|
|||
|
|
with the value. If it returns any other object then we discard the
|
|||
|
|
result and continue as we do today. Note that having an attribute
|
|||
|
|
access handler will mildly affect attribute “setting” performance for
|
|||
|
|
all sets on a particular instance, but no more so than today, using
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">__setattr__</span></code>. Gets are more efficient than they are today with
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">__getattr__</span></code>.</p>
|
|||
|
|
<p>The I_have_computed_attributes flag is intended to eliminate the
|
|||
|
|
performance degradation of an extra “get” per “set” for objects not
|
|||
|
|
using this feature. Checking this flag should have minuscule
|
|||
|
|
performance implications for all objects.</p>
|
|||
|
|
<p>The implementation of delete is analogous to the implementation
|
|||
|
|
of set.</p>
|
|||
|
|
</section>
|
|||
|
|
<section id="caveats">
|
|||
|
|
<h2><a class="toc-backref" href="#caveats" role="doc-backlink">Caveats</a></h2>
|
|||
|
|
<ol class="arabic">
|
|||
|
|
<li>You might note that I have not proposed any logic to keep
|
|||
|
|
the I_have_computed_attributes flag up to date as attributes
|
|||
|
|
are added and removed from the instance’s dictionary. This is
|
|||
|
|
consistent with current Python. If you add a <code class="docutils literal notranslate"><span class="pre">__setattr__</span></code> method
|
|||
|
|
to an object after it is in use, that method will not behave as
|
|||
|
|
it would if it were available at “compile” time. The dynamism is
|
|||
|
|
arguably not worth the extra implementation effort. This snippet
|
|||
|
|
demonstrates the current behavior:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">prn</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span><span class="nb">print</span> <span class="n">args</span>
|
|||
|
|
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">a</span><span class="p">:</span>
|
|||
|
|
|
|||
|
|
<span class="go">... __setattr__=prn</span>
|
|||
|
|
<span class="gp">>>> </span><span class="n">a</span><span class="p">()</span><span class="o">.</span><span class="n">foo</span><span class="o">=</span><span class="mi">5</span>
|
|||
|
|
<span class="go">(<__main__.a instance at 882890>, 'foo', 5)</span>
|
|||
|
|
|
|||
|
|
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">b</span><span class="p">:</span> <span class="k">pass</span>
|
|||
|
|
<span class="gp">>>> </span><span class="n">bi</span><span class="o">=</span><span class="n">b</span><span class="p">()</span>
|
|||
|
|
<span class="gp">>>> </span><span class="n">bi</span><span class="o">.</span><span class="fm">__setattr__</span><span class="o">=</span><span class="n">prn</span>
|
|||
|
|
<span class="gp">>>> </span><span class="n">b</span><span class="o">.</span><span class="n">foo</span><span class="o">=</span><span class="mi">5</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
</li>
|
|||
|
|
<li>Assignment to __dict__[“XXX”] can overwrite the attribute
|
|||
|
|
access handler for __attr_XXX__. Typically the access handlers will
|
|||
|
|
store information away in private __XXX variables</li>
|
|||
|
|
<li>An attribute access handler that attempts to call setattr or getattr
|
|||
|
|
on the object itself can cause an infinite loop (as with <code class="docutils literal notranslate"><span class="pre">__getattr__</span></code>)
|
|||
|
|
Once again, the solution is to use a special (typically private)
|
|||
|
|
variable such as __XXX.</li>
|
|||
|
|
</ol>
|
|||
|
|
</section>
|
|||
|
|
<section id="note">
|
|||
|
|
<h2><a class="toc-backref" href="#note" role="doc-backlink">Note</a></h2>
|
|||
|
|
<p>The descriptor mechanism described in <a class="pep reference internal" href="../pep-0252/" title="PEP 252 – Making Types Look More Like Classes">PEP 252</a> is powerful enough
|
|||
|
|
to support this more directly. A ‘getset’ constructor may be
|
|||
|
|
added to the language making this possible:</p>
|
|||
|
|
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
|
|||
|
|
<span class="k">def</span> <span class="nf">get_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="k">def</span> <span class="nf">set_x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">v</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">v</span>
|
|||
|
|
<span class="n">x</span> <span class="o">=</span> <span class="n">getset</span><span class="p">(</span><span class="n">get_x</span><span class="p">,</span> <span class="n">set_x</span><span class="p">)</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
<p>Additional syntactic sugar might be added, or a naming convention
|
|||
|
|
could be recognized.</p>
|
|||
|
|
</section>
|
|||
|
|
</section>
|
|||
|
|
<hr class="docutils" />
|
|||
|
|
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0213.rst">https://github.com/python/peps/blob/main/peps/pep-0213.rst</a></p>
|
|||
|
|
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0213.rst">2023-09-09 17:39:29 GMT</a></p>
|
|||
|
|
|
|||
|
|
</article>
|
|||
|
|
<nav id="pep-sidebar">
|
|||
|
|
<h2>Contents</h2>
|
|||
|
|
<ul>
|
|||
|
|
<li><a class="reference internal" href="#introduction">Introduction</a></li>
|
|||
|
|
<li><a class="reference internal" href="#justification">Justification</a><ul>
|
|||
|
|
<li><a class="reference internal" href="#scenario-1">Scenario 1</a></li>
|
|||
|
|
<li><a class="reference internal" href="#scenario-2">Scenario 2</a></li>
|
|||
|
|
<li><a class="reference internal" href="#scenario-3">Scenario 3</a></li>
|
|||
|
|
</ul>
|
|||
|
|
</li>
|
|||
|
|
<li><a class="reference internal" href="#current-solution">Current Solution</a></li>
|
|||
|
|
<li><a class="reference internal" href="#proposed-syntax">Proposed Syntax</a></li>
|
|||
|
|
<li><a class="reference internal" href="#semantics">Semantics</a></li>
|
|||
|
|
<li><a class="reference internal" href="#proposed-implementation">Proposed Implementation</a></li>
|
|||
|
|
<li><a class="reference internal" href="#caveats">Caveats</a></li>
|
|||
|
|
<li><a class="reference internal" href="#note">Note</a></li>
|
|||
|
|
</ul>
|
|||
|
|
|
|||
|
|
<br>
|
|||
|
|
<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0213.rst">Page Source (GitHub)</a>
|
|||
|
|
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|
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|
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|
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