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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 447 – Add __getdescriptor__ method to metaclass | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0447/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 447 – Add __getdescriptor__ method to metaclass | peps.python.org'> <meta property="og:description" content="Currently object.__getattribute__ and super.__getattribute__ peek in the __dict__ of classes on the MRO for a class when looking for an attribute. 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Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 447</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 447 – Add __getdescriptor__ method to metaclass</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Ronald Oussoren <ronaldoussoren at mac.com></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Inactive draft that may be taken up again at a later time">Deferred</abbr></dd> <dt class="field-odd">Type:</dt> <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> <dt class="field-even">Created:</dt> <dd class="field-even">12-Jun-2013</dd> <dt class="field-odd">Post-History:</dt> <dd class="field-odd">02-Jul-2013, 15-Jul-2013, 29-Jul-2013, 22-Jul-2015</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#pep-status">PEP Status</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#background">Background</a></li> </ul> </li> <li><a class="reference internal" href="#the-superclass-attribute-lookup-hook">The superclass attribute lookup hook</a><ul> <li><a class="reference internal" href="#aside-attribute-resolution-algorithm-in-python">Aside: Attribute resolution algorithm in Python</a></li> <li><a class="reference internal" href="#in-python-code">In Python code</a><ul> <li><a class="reference internal" href="#example-usage">Example usage</a></li> </ul> </li> <li><a class="reference internal" href="#in-c-code">In C code</a></li> <li><a class="reference internal" href="#use-of-this-hook-by-the-interpreter">Use of this hook by the interpreter</a></li> <li><a class="reference internal" href="#other-changes-to-the-implementation">Other changes to the implementation</a></li> </ul> </li> <li><a class="reference internal" href="#impact-of-this-pep-on-introspection">Impact of this PEP on introspection</a></li> <li><a class="reference internal" href="#performance-impact">Performance impact</a><ul> <li><a class="reference internal" href="#micro-benchmarks">Micro benchmarks</a></li> <li><a class="reference internal" href="#pybench">Pybench</a></li> </ul> </li> <li><a class="reference internal" href="#alternative-proposals">Alternative proposals</a><ul> <li><a class="reference internal" href="#getattribute-super"><code class="docutils literal notranslate">__getattribute_super__</code></a></li> <li><a class="reference internal" href="#reuse-tp-getattro">Reuse <code class="docutils literal notranslate">tp_getattro</code></a></li> <li><a class="reference internal" href="#alternative-placement-of-the-new-method">Alternative placement of the new method</a></li> </ul> </li> <li><a class="reference internal" href="#history">History</a></li> <li><a class="reference internal" href="#discussion-threads">Discussion threads</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> Currently <code class="docutils literal notranslate">object.__getattribute__</code> and <code class="docutils literal notranslate">super.__getattribute__</code> peek in the <code class="docutils literal notranslate">__dict__</code> of classes on the MRO for a class when looking for an attribute. This PEP adds an optional <code class="docutils literal notranslate">__getdescriptor__</code> method to a metaclass that replaces this behavior and gives more control over attribute lookup, especially when using a <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super</a> object. That is, the MRO walking loop in <code class="docutils literal notranslate">_PyType_Lookup</code> and <code class="docutils literal notranslate">super.__getattribute__</code> gets changed from: <div class="highlight-default notranslate"><div class="highlight"><pre>def lookup(mro_list, name): for cls in mro_list: if name in cls.__dict__: return cls.__dict__ return NotFound </pre></div> </div> to: <div class="highlight-default notranslate"><div class="highlight"><pre>def lookup(mro_list, name): for cls in mro_list: try: return cls.__getdescriptor__(name) except AttributeError: pass return NotFound </pre></div> </div> The default implementation of <code class="docutils literal notranslate">__getdescriptor__</code> looks in the class dictionary: <div class="highlight-default notranslate"><div class="highlight"><pre>class type: def __getdescriptor__(cls, name): try: return cls.__dict__[name] except KeyError: raise AttributeError(name) from None </pre></div> </div> </section> <section id="pep-status"> <h2><a class="toc-backref" href="#pep-status" role="doc-backlink">PEP Status</a></h2> This PEP is deferred until someone has time to update this PEP and push it forward. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> It is currently not possible to influence how the <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super class</a> looks up attributes (that is, <code class="docutils literal notranslate">super.__getattribute__</code> unconditionally peeks in the class <code class="docutils literal notranslate">__dict__</code>), and that can be problematic for dynamic classes that can grow new methods on demand, for example dynamic proxy classes. The <code class="docutils literal notranslate">__getdescriptor__</code> method makes it possible to dynamically add attributes even when looking them up using the <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super class</a>. The new method affects <code class="docutils literal notranslate">object.__getattribute__</code> (and <a class="reference external" href="http://docs.python.org/3/c-api/object.html#PyObject_GenericGetAttr">PyObject_GenericGetAttr</a>) as well for consistency and to have a single place to implement dynamic attribute resolution for classes. <section id="background"> <h3><a class="toc-backref" href="#background" role="doc-backlink">Background</a></h3> The current behavior of <code class="docutils literal notranslate">super.__getattribute__</code> causes problems for classes that are dynamic proxies for other (non-Python) classes or types, an example of which is <a class="reference external" href="http://pyobjc.sourceforge.net/">PyObjC</a>. PyObjC creates a Python class for every class in the Objective-C runtime, and looks up methods in the Objective-C runtime when they are used. This works fine for normal access, but doesn’t work for access with <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super</a> objects. Because of this PyObjC currently includes a custom <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super</a> that must be used with its classes, as well as completely reimplementing <a class="reference external" href="http://docs.python.org/3/c-api/object.html#PyObject_GenericGetAttr">PyObject_GenericGetAttr</a> for normal attribute access. The API in this PEP makes it possible to remove the custom <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super</a> and simplifies the implementation because the custom lookup behavior can be added in a central location. <div class="admonition note"> Note <a class="reference external" href="http://pyobjc.sourceforge.net/">PyObjC</a> cannot precalculate the contents of the class <code class="docutils literal notranslate">__dict__</code> because Objective-C classes can grow new methods at runtime. Furthermore, Objective-C classes tend to contain a lot of methods while most Python code will only use a small subset of them, this makes precalculating unnecessarily expensive. </div> </section> </section> <section id="the-superclass-attribute-lookup-hook"> <h2><a class="toc-backref" href="#the-superclass-attribute-lookup-hook" role="doc-backlink">The superclass attribute lookup hook</a></h2> Both <code class="docutils literal notranslate">super.__getattribute__</code> and <code class="docutils literal notranslate">object.__getattribute__</code> (or <a class="reference external" href="http://docs.python.org/3/c-api/object.html#PyObject_GenericGetAttr">PyObject_GenericGetAttr</a> and in particular <code class="docutils literal notranslate">_PyType_Lookup</code> in C code) walk an object’s MRO and currently peek in the class’ <code class="docutils literal notranslate">__dict__</code> to look up attributes. With this proposal both lookup methods no longer peek in the class <code class="docutils literal notranslate">__dict__</code> but call the special method <code class="docutils literal notranslate">__getdescriptor__</code>, which is a slot defined on the metaclass. The default implementation of that method looks up the name the class <code class="docutils literal notranslate">__dict__</code>, which means that attribute lookup is unchanged unless a metatype actually defines the new special method. <section id="aside-attribute-resolution-algorithm-in-python"> <h3><a class="toc-backref" href="#aside-attribute-resolution-algorithm-in-python" role="doc-backlink">Aside: Attribute resolution algorithm in Python</a></h3> The attribute resolution process as implemented by <code class="docutils literal notranslate">object.__getattribute__</code> (or <code class="docutils literal notranslate">PyObject_GenericGetAttr</code> in CPython’s implementation) is fairly straightforward, but not entirely so without reading C code. The current CPython implementation of object.__getattribute__ is basically equivalent to the following (pseudo-) Python code (excluding some house keeping and speed tricks): <div class="highlight-default notranslate"><div class="highlight"><pre>def _PyType_Lookup(tp, name): mro = tp.mro() assert isinstance(mro, tuple) for base in mro: assert isinstance(base, type) # PEP 447 will change these lines: try: return base.__dict__[name] except KeyError: pass return None class object: def __getattribute__(self, name): assert isinstance(name, str) tp = type(self) descr = _PyType_Lookup(tp, name) f = None if descr is not None: f = descr.__get__ if f is not None and descr.__set__ is not None: # Data descriptor return f(descr, self, type(self)) dict = self.__dict__ if dict is not None: try: return self.__dict__[name] except KeyError: pass if f is not None: # Non-data descriptor return f(descr, self, type(self)) if descr is not None: # Regular class attribute return descr raise AttributeError(name) class super: def __getattribute__(self, name): assert isinstance(name, unicode) if name != '__class__': starttype = self.__self_type__ mro = startype.mro() try: idx = mro.index(self.__thisclass__) except ValueError: pass else: for base in mro[idx+1:]: # PEP 447 will change these lines: try: descr = base.__dict__[name] except KeyError: continue f = descr.__get__ if f is not None: return f(descr, None if (self.__self__ is self.__self_type__) else self.__self__, starttype) else: return descr return object.__getattribute__(self, name) </pre></div> </div> This PEP should change the dict lookup at the lines starting at “# PEP 447” with a method call to perform the actual lookup, making is possible to affect that lookup both for normal attribute access and access through the <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super proxy</a>. Note that specific classes can already completely override the default behaviour by implementing their own <code class="docutils literal notranslate">__getattribute__</code> slot (with or without calling the super class implementation). </section> <section id="in-python-code"> <h3><a class="toc-backref" href="#in-python-code" role="doc-backlink">In Python code</a></h3> A meta type can define a method <code class="docutils literal notranslate">__getdescriptor__</code> that is called during attribute resolution by both <code class="docutils literal notranslate">super.__getattribute__</code> and <code class="docutils literal notranslate">object.__getattribute</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>class MetaType(type): def __getdescriptor__(cls, name): try: return cls.__dict__[name] except KeyError: raise AttributeError(name) from None </pre></div> </div> The <code class="docutils literal notranslate">__getdescriptor__</code> method has as its arguments a class (which is an instance of the meta type) and the name of the attribute that is looked up. It should return the value of the attribute without invoking descriptors, and should raise <a class="reference external" href="http://docs.python.org/3/library/exceptions.html#AttributeError">AttributeError</a> when the name cannot be found. The <a class="reference external" href="http://docs.python.org/3/library/functions.html#type">type</a> class provides a default implementation for <code class="docutils literal notranslate">__getdescriptor__</code>, that looks up the name in the class dictionary. <section id="example-usage"> <h4><a class="toc-backref" href="#example-usage" role="doc-backlink">Example usage</a></h4> The code below implements a silly metaclass that redirects attribute lookup to uppercase versions of names: <div class="highlight-default notranslate"><div class="highlight"><pre>class UpperCaseAccess (type): def __getdescriptor__(cls, name): try: return cls.__dict__[name.upper()] except KeyError: raise AttributeError(name) from None class SillyObject (metaclass=UpperCaseAccess): def m(self): return 42 def M(self): return "fortytwo" obj = SillyObject() assert obj.m() == "fortytwo" </pre></div> </div> As mentioned earlier in this PEP a more realistic use case of this functionality is a <code class="docutils literal notranslate">__getdescriptor__</code> method that dynamically populates the class <code class="docutils literal notranslate">__dict__</code> based on attribute access, primarily when it is not possible to reliably keep the class dict in sync with its source, for example because the source used to populate <code class="docutils literal notranslate">__dict__</code> is dynamic as well and does not have triggers that can be used to detect changes to that source. An example of that are the class bridges in PyObjC: the class bridge is a Python object (class) that represents an Objective-C class and conceptually has a Python method for every Objective-C method in the Objective-C class. As with Python it is possible to add new methods to an Objective-C class, or replace existing ones, and there are no callbacks that can be used to detect this. </section> </section> <section id="in-c-code"> <h3><a class="toc-backref" href="#in-c-code" role="doc-backlink">In C code</a></h3> A new type flag <code class="docutils literal notranslate">Py_TPFLAGS_GETDESCRIPTOR</code> with value <code class="docutils literal notranslate">(1UL << 11)</code> that indicates that the new slot is present and to be used. A new slot <code class="docutils literal notranslate">tp_getdescriptor</code> is added to the <code class="docutils literal notranslate">PyTypeObject</code> struct, this slot corresponds to the <code class="docutils literal notranslate">__getdescriptor__</code> method on <a class="reference external" href="http://docs.python.org/3/library/functions.html#type">type</a>. The slot has the following prototype: <div class="highlight-default notranslate"><div class="highlight"><pre>PyObject* (*getdescriptorfunc)(PyTypeObject* cls, PyObject* name); </pre></div> </div> This method should lookup name in the namespace of cls, without looking at superclasses, and should not invoke descriptors. The method returns <code class="docutils literal notranslate">NULL</code> without setting an exception when the name cannot be found, and returns a new reference otherwise (not a borrowed reference). Classes with a <code class="docutils literal notranslate">tp_getdescriptor</code> slot must add <code class="docutils literal notranslate">Py_TPFLAGS_GETDESCRIPTOR</code> to <code class="docutils literal notranslate">tp_flags</code> to indicate that new slot must be used. </section> <section id="use-of-this-hook-by-the-interpreter"> <h3><a class="toc-backref" href="#use-of-this-hook-by-the-interpreter" role="doc-backlink">Use of this hook by the interpreter</a></h3> The new method is required for metatypes and as such is defined on <code class="docutils literal notranslate">type_</code>. Both <code class="docutils literal notranslate">super.__getattribute__</code> and <code class="docutils literal notranslate">object.__getattribute__</code>/<a class="reference external" href="http://docs.python.org/3/c-api/object.html#PyObject_GenericGetAttr">PyObject_GenericGetAttr</a> (through <code class="docutils literal notranslate">_PyType_Lookup</code>) use the this <code class="docutils literal notranslate">__getdescriptor__</code> method when walking the MRO. </section> <section id="other-changes-to-the-implementation"> <h3><a class="toc-backref" href="#other-changes-to-the-implementation" role="doc-backlink">Other changes to the implementation</a></h3> The change for <a class="reference external" href="http://docs.python.org/3/c-api/object.html#PyObject_GenericGetAttr">PyObject_GenericGetAttr</a> will be done by changing the private function <code class="docutils literal notranslate">_PyType_Lookup</code>. This currently returns a borrowed reference, but must return a new reference when the <code class="docutils literal notranslate">__getdescriptor__</code> method is present. Because of this <code class="docutils literal notranslate">_PyType_Lookup</code> will be renamed to <code class="docutils literal notranslate">_PyType_LookupName</code>, this will cause compile-time errors for all out-of-tree users of this private API. For the same reason <code class="docutils literal notranslate">_PyType_LookupId</code> is renamed to <code class="docutils literal notranslate">_PyType_LookupId2</code>. A number of other functions in typeobject.c with the same issue do not get an updated name because they are private to that file. The attribute lookup cache in <code class="docutils literal notranslate">Objects/typeobject.c</code> is disabled for classes that have a metaclass that overrides <code class="docutils literal notranslate">__getdescriptor__</code>, because using the cache might not be valid for such classes. </section> </section> <section id="impact-of-this-pep-on-introspection"> <h2><a class="toc-backref" href="#impact-of-this-pep-on-introspection" role="doc-backlink">Impact of this PEP on introspection</a></h2> Use of the method introduced in this PEP can affect introspection of classes with a metaclass that uses a custom <code class="docutils literal notranslate">__getdescriptor__</code> method. This section lists those changes. The items listed below are only affected by custom <code class="docutils literal notranslate">__getdescriptor__</code> methods, the default implementation for <code class="docutils literal notranslate">object</code> won’t cause problems because that still only uses the class <code class="docutils literal notranslate">__dict__</code> and won’t cause visible changes to the visible behaviour of the <code class="docutils literal notranslate">object.__getattribute__</code>. <ul> <li><code class="docutils literal notranslate">dir</code> might not show all attributesAs with a custom <code class="docutils literal notranslate">__getattribute__</code> method <a class="reference external" href="http://docs.python.org/3/library/functions.html#dir">dir()</a> might not see all (instance) attributes when using the <code class="docutils literal notranslate">__getdescriptor__()</code> method to dynamically resolve attributes. The solution for that is quite simple: classes using <code class="docutils literal notranslate">__getdescriptor__</code> should also implement <a class="reference external" href="https://docs.python.org/3/reference/datamodel.html#object.__dir__">__dir__()</a> if they want full support for the builtin <a class="reference external" href="http://docs.python.org/3/library/functions.html#dir">dir()</a> function. </li> <li><code class="docutils literal notranslate">inspect.getattr_static</code> might not show all attributesThe function <code class="docutils literal notranslate">inspect.getattr_static</code> intentionally does not invoke <code class="docutils literal notranslate">__getattribute__</code> and descriptors to avoid invoking user code during introspection with this function. The <code class="docutils literal notranslate">__getdescriptor__</code> method will also be ignored and is another way in which the result of <code class="docutils literal notranslate">inspect.getattr_static</code> can be different from that of <code class="docutils literal notranslate">builtin.getattr</code>. </li> <li><code class="docutils literal notranslate">inspect.getmembers</code> and <code class="docutils literal notranslate">inspect.classify_class_attrs</code>Both of these functions directly access the class __dict__ of classes along the MRO, and hence can be affected by a custom <code class="docutils literal notranslate">__getdescriptor__</code> method. Code with a custom <code class="docutils literal notranslate">__getdescriptor__</code> method that want to play nice with these methods also needs to ensure that the <code class="docutils literal notranslate">__dict__</code> is set up correctly when that is accessed directly by Python code. Note that <code class="docutils literal notranslate">inspect.getmembers</code> is used by <code class="docutils literal notranslate">pydoc</code> and hence this can affect runtime documentation introspection. </li> <li>Direct introspection of the class <code class="docutils literal notranslate">__dict__</code>Any code that directly access the class <code class="docutils literal notranslate">__dict__</code> for introspection can be affected by a custom <code class="docutils literal notranslate">__getdescriptor__</code> method, see the previous item. </li> </ul> </section> <section id="performance-impact"> <h2><a class="toc-backref" href="#performance-impact" role="doc-backlink">Performance impact</a></h2> WARNING: The benchmark results in this section are old, and will be updated when I’ve ported the patch to the current trunk. I don’t expect significant changes to the results in this section. <section id="micro-benchmarks"> <h3><a class="toc-backref" href="#micro-benchmarks" role="doc-backlink">Micro benchmarks</a></h3> <a class="reference external" href="http://bugs.python.org/issue18181">Issue 18181</a> has a micro benchmark as one of its attachments (<a class="reference external" href="http://bugs.python.org/file40013/pep447-micro-bench.py">pep447-micro-bench.py</a>) that specifically tests the speed of attribute lookup, both directly and through super. Note that attribute lookup with deep class hierarchies is significantly slower when using a custom <code class="docutils literal notranslate">__getdescriptor__</code> method. This is because the attribute lookup cache for CPython cannot be used when having this method. </section> <section id="pybench"> <h3><a class="toc-backref" href="#pybench" role="doc-backlink">Pybench</a></h3> The pybench output below compares an implementation of this PEP with the regular source tree, both based on changeset a5681f50bae2, run on an idle machine and Core i7 processor running Centos 6.4. Even though the machine was idle there were clear differences between runs, I’ve seen difference in “minimum time” vary from -0.1% to +1.5%, with similar (but slightly smaller) differences in the “average time” difference. <div class="highlight-default notranslate"><div class="highlight"><pre>------------------------------------------------------------------------------- PYBENCH 2.1 ------------------------------------------------------------------------------- * using CPython 3.4.0a0 (default, Jul 29 2013, 13:01:34) [GCC 4.4.7 20120313 (Red Hat 4.4.7-3)] * disabled garbage collection * system check interval set to maximum: 2147483647 * using timer: time.perf_counter * timer: resolution=1e-09, implementation=clock_gettime(CLOCK_MONOTONIC) ------------------------------------------------------------------------------- Benchmark: pep447.pybench ------------------------------------------------------------------------------- Rounds: 10 Warp: 10 Timer: time.perf_counter Machine Details: Platform ID: Linux-2.6.32-358.114.1.openstack.el6.x86_64-x86_64-with-centos-6.4-Final Processor: x86_64 Python: Implementation: CPython Executable: /tmp/default-pep447/bin/python3 Version: 3.4.0a0 Compiler: GCC 4.4.7 20120313 (Red Hat 4.4.7-3) Bits: 64bit Build: Jul 29 2013 14:09:12 (#default) Unicode: UCS4 ------------------------------------------------------------------------------- Comparing with: default.pybench ------------------------------------------------------------------------------- Rounds: 10 Warp: 10 Timer: time.perf_counter Machine Details: Platform ID: Linux-2.6.32-358.114.1.openstack.el6.x86_64-x86_64-with-centos-6.4-Final Processor: x86_64 Python: Implementation: CPython Executable: /tmp/default/bin/python3 Version: 3.4.0a0 Compiler: GCC 4.4.7 20120313 (Red Hat 4.4.7-3) Bits: 64bit Build: Jul 29 2013 13:01:34 (#default) Unicode: UCS4 Test minimum run-time average run-time this other diff this other diff ------------------------------------------------------------------------------- BuiltinFunctionCalls: 45ms 44ms +1.3% 45ms 44ms +1.3% BuiltinMethodLookup: 26ms 27ms -2.4% 27ms 27ms -2.2% CompareFloats: 33ms 34ms -0.7% 33ms 34ms -1.1% CompareFloatsIntegers: 66ms 67ms -0.9% 66ms 67ms -0.8% CompareIntegers: 51ms 50ms +0.9% 51ms 50ms +0.8% CompareInternedStrings: 34ms 33ms +0.4% 34ms 34ms -0.4% CompareLongs: 29ms 29ms -0.1% 29ms 29ms -0.0% CompareStrings: 43ms 44ms -1.8% 44ms 44ms -1.8% ComplexPythonFunctionCalls: 44ms 42ms +3.9% 44ms 42ms +4.1% ConcatStrings: 33ms 33ms -0.4% 33ms 33ms -1.0% CreateInstances: 47ms 48ms -2.9% 47ms 49ms -3.4% CreateNewInstances: 35ms 36ms -2.5% 36ms 36ms -2.5% CreateStringsWithConcat: 69ms 70ms -0.7% 69ms 70ms -0.9% DictCreation: 52ms 50ms +3.1% 52ms 50ms +3.0% DictWithFloatKeys: 40ms 44ms -10.1% 43ms 45ms -5.8% DictWithIntegerKeys: 32ms 36ms -11.2% 35ms 37ms -4.6% DictWithStringKeys: 29ms 34ms -15.7% 35ms 40ms -11.0% ForLoops: 30ms 29ms +2.2% 30ms 29ms +2.2% IfThenElse: 38ms 41ms -6.7% 38ms 41ms -6.9% ListSlicing: 36ms 36ms -0.7% 36ms 37ms -1.3% NestedForLoops: 43ms 45ms -3.1% 43ms 45ms -3.2% NestedListComprehensions: 39ms 40ms -1.7% 39ms 40ms -2.1% NormalClassAttribute: 86ms 82ms +5.1% 86ms 82ms +5.0% NormalInstanceAttribute: 42ms 42ms +0.3% 42ms 42ms +0.0% PythonFunctionCalls: 39ms 38ms +3.5% 39ms 38ms +2.8% PythonMethodCalls: 51ms 49ms +3.0% 51ms 50ms +2.8% Recursion: 67ms 68ms -1.4% 67ms 68ms -1.4% SecondImport: 41ms 36ms +12.5% 41ms 36ms +12.6% SecondPackageImport: 45ms 40ms +13.1% 45ms 40ms +13.2% SecondSubmoduleImport: 92ms 95ms -2.4% 95ms 98ms -3.6% SimpleComplexArithmetic: 28ms 28ms -0.1% 28ms 28ms -0.2% SimpleDictManipulation: 57ms 57ms -1.0% 57ms 58ms -1.0% SimpleFloatArithmetic: 29ms 28ms +4.7% 29ms 28ms +4.9% SimpleIntFloatArithmetic: 37ms 41ms -8.5% 37ms 41ms -8.7% SimpleIntegerArithmetic: 37ms 41ms -9.4% 37ms 42ms -10.2% SimpleListComprehensions: 33ms 33ms -1.9% 33ms 34ms -2.9% SimpleListManipulation: 28ms 30ms -4.3% 29ms 30ms -4.1% SimpleLongArithmetic: 26ms 26ms +0.5% 26ms 26ms +0.5% SmallLists: 40ms 40ms +0.1% 40ms 40ms +0.1% SmallTuples: 46ms 47ms -2.4% 46ms 48ms -3.0% SpecialClassAttribute: 126ms 120ms +4.7% 126ms 121ms +4.4% SpecialInstanceAttribute: 42ms 42ms +0.6% 42ms 42ms +0.8% StringMappings: 94ms 91ms +3.9% 94ms 91ms +3.8% StringPredicates: 48ms 49ms -1.7% 48ms 49ms -2.1% StringSlicing: 45ms 45ms +1.4% 46ms 45ms +1.5% TryExcept: 23ms 22ms +4.9% 23ms 22ms +4.8% TryFinally: 32ms 32ms -0.1% 32ms 32ms +0.1% TryRaiseExcept: 17ms 17ms +0.9% 17ms 17ms +0.5% TupleSlicing: 49ms 48ms +1.1% 49ms 49ms +1.0% WithFinally: 48ms 47ms +2.3% 48ms 47ms +2.4% WithRaiseExcept: 45ms 44ms +0.8% 45ms 45ms +0.5% ------------------------------------------------------------------------------- Totals: 2284ms 2287ms -0.1% 2306ms 2308ms -0.1% (this=pep447.pybench, other=default.pybench) </pre></div> </div> A run of the benchmark suite (with option “-b 2n3”) also seems to indicate that the performance impact is minimal: <div class="highlight-default notranslate"><div class="highlight"><pre>Report on Linux fangorn.local 2.6.32-358.114.1.openstack.el6.x86_64 #1 SMP Wed Jul 3 02:11:25 EDT 2013 x86_64 x86_64 Total CPU cores: 8 ### call_method_slots ### Min: 0.304120 -> 0.282791: 1.08x faster Avg: 0.304394 -> 0.282906: 1.08x faster Significant (t=2329.92) Stddev: 0.00016 -> 0.00004: 4.1814x smaller ### call_simple ### Min: 0.249268 -> 0.221175: 1.13x faster Avg: 0.249789 -> 0.221387: 1.13x faster Significant (t=2770.11) Stddev: 0.00012 -> 0.00013: 1.1101x larger ### django_v2 ### Min: 0.632590 -> 0.601519: 1.05x faster Avg: 0.635085 -> 0.602653: 1.05x faster Significant (t=321.32) Stddev: 0.00087 -> 0.00051: 1.6933x smaller ### fannkuch ### Min: 1.033181 -> 0.999779: 1.03x faster Avg: 1.036457 -> 1.001840: 1.03x faster Significant (t=260.31) Stddev: 0.00113 -> 0.00070: 1.6112x smaller ### go ### Min: 0.526714 -> 0.544428: 1.03x slower Avg: 0.529649 -> 0.547626: 1.03x slower Significant (t=-93.32) Stddev: 0.00136 -> 0.00136: 1.0028x smaller ### iterative_count ### Min: 0.109748 -> 0.116513: 1.06x slower Avg: 0.109816 -> 0.117202: 1.07x slower Significant (t=-357.08) Stddev: 0.00008 -> 0.00019: 2.3664x larger ### json_dump_v2 ### Min: 2.554462 -> 2.609141: 1.02x slower Avg: 2.564472 -> 2.620013: 1.02x slower Significant (t=-76.93) Stddev: 0.00538 -> 0.00481: 1.1194x smaller ### meteor_contest ### Min: 0.196336 -> 0.191925: 1.02x faster Avg: 0.196878 -> 0.192698: 1.02x faster Significant (t=61.86) Stddev: 0.00053 -> 0.00041: 1.2925x smaller ### nbody ### Min: 0.228039 -> 0.235551: 1.03x slower Avg: 0.228857 -> 0.236052: 1.03x slower Significant (t=-54.15) Stddev: 0.00130 -> 0.00029: 4.4810x smaller ### pathlib ### Min: 0.108501 -> 0.105339: 1.03x faster Avg: 0.109084 -> 0.105619: 1.03x faster Significant (t=311.08) Stddev: 0.00022 -> 0.00011: 1.9314x smaller ### regex_effbot ### Min: 0.057905 -> 0.056447: 1.03x faster Avg: 0.058055 -> 0.056760: 1.02x faster Significant (t=79.22) Stddev: 0.00006 -> 0.00015: 2.7741x larger ### silent_logging ### Min: 0.070810 -> 0.072436: 1.02x slower Avg: 0.070899 -> 0.072609: 1.02x slower Significant (t=-191.59) Stddev: 0.00004 -> 0.00008: 2.2640x larger ### spectral_norm ### Min: 0.290255 -> 0.299286: 1.03x slower Avg: 0.290335 -> 0.299541: 1.03x slower Significant (t=-572.10) Stddev: 0.00005 -> 0.00015: 2.8547x larger ### threaded_count ### Min: 0.107215 -> 0.115206: 1.07x slower Avg: 0.107488 -> 0.115996: 1.08x slower Significant (t=-109.39) Stddev: 0.00016 -> 0.00076: 4.8665x larger The following not significant results are hidden, use -v to show them: call_method, call_method_unknown, chaos, fastpickle, fastunpickle, float, formatted_logging, hexiom2, json_load, normal_startup, nqueens, pidigits, raytrace, regex_compile, regex_v8, richards, simple_logging, startup_nosite, telco, unpack_sequence. </pre></div> </div> </section> </section> <section id="alternative-proposals"> <h2><a class="toc-backref" href="#alternative-proposals" role="doc-backlink">Alternative proposals</a></h2> <section id="getattribute-super"> <h3><a class="toc-backref" href="#getattribute-super" role="doc-backlink"><code class="docutils literal notranslate">__getattribute_super__</code></a></h3> An earlier version of this PEP used the following static method on classes: <div class="highlight-default notranslate"><div class="highlight"><pre>def __getattribute_super__(cls, name, object, owner): pass </pre></div> </div> This method performed name lookup as well as invoking descriptors and was necessarily limited to working only with <code class="docutils literal notranslate">super.__getattribute__</code>. </section> <section id="reuse-tp-getattro"> <h3><a class="toc-backref" href="#reuse-tp-getattro" role="doc-backlink">Reuse <code class="docutils literal notranslate">tp_getattro</code></a></h3> It would be nice to avoid adding a new slot, thus keeping the API simpler and easier to understand. A comment on <a class="reference external" href="http://bugs.python.org/issue18181">Issue 18181</a> asked about reusing the <code class="docutils literal notranslate">tp_getattro</code> slot, that is super could call the <code class="docutils literal notranslate">tp_getattro</code> slot of all methods along the MRO. That won’t work because <code class="docutils literal notranslate">tp_getattro</code> will look in the instance <code class="docutils literal notranslate">__dict__</code> before it tries to resolve attributes using classes in the MRO. This would mean that using <code class="docutils literal notranslate">tp_getattro</code> instead of peeking the class dictionaries changes the semantics of the <a class="reference external" href="http://docs.python.org/3/library/functions.html#super">super class</a>. </section> <section id="alternative-placement-of-the-new-method"> <h3><a class="toc-backref" href="#alternative-placement-of-the-new-method" role="doc-backlink">Alternative placement of the new method</a></h3> This PEP proposes to add <code class="docutils literal notranslate">__getdescriptor__</code> as a method on the metaclass. An alternative would be to add it as a class method on the class itself (similar to how <code class="docutils literal notranslate">__new__</code> is a <a class="reference external" href="http://docs.python.org/3/library/functions.html#staticmethod">staticmethod</a> of the class and not a method of the metaclass). The advantage of using a method on the metaclass is that will give an error when two classes on the MRO have different metaclasses that may have different behaviors for <code class="docutils literal notranslate">__getdescriptor__</code>. With a normal classmethod that problem would pass undetected while it might cause subtle errors when running the code. </section> </section> <section id="history"> <h2><a class="toc-backref" href="#history" role="doc-backlink">History</a></h2> <ul> <li>23-Jul-2015: Added type flag <code class="docutils literal notranslate">Py_TPFLAGS_GETDESCRIPTOR</code> after talking with Guido.The new flag is primarily useful to avoid crashing when loading an extension for an older version of CPython and could have positive speed implications as well. </li> <li>Jul-2014: renamed slot to <code class="docutils literal notranslate">__getdescriptor__</code>, the old name didn’t match the naming style of other slots and was less descriptive.</li> </ul> </section> <section id="discussion-threads"> <h2><a class="toc-backref" href="#discussion-threads" role="doc-backlink">Discussion threads</a></h2> <ul class="simple"> <li>The initial version of the PEP was send with Message-ID <a class="reference external" href="mailto:75030FAC-6918-4E94-95DA-67A88D53E6F5%40mac.com">mailto:75030FAC-6918-4E94-95DA-67A88D53E6F5@mac.com</a></li> <li>Further discussion starting at a message with Message-ID <a class="reference external" href="mailto:5BB87CC4-F31B-4213-AAAC-0C0CE738460C%40mac.com">mailto:5BB87CC4-F31B-4213-AAAC-0C0CE738460C@mac.com</a></li> <li>And more discussion starting at message with Message-ID <a class="reference external" href="mailto:00AA7433-C853-4101-9718-060468EBAC54%40mac.com">mailto:00AA7433-C853-4101-9718-060468EBAC54@mac.com</a></li> </ul> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <ul class="simple"> <li><a class="reference external" href="http://bugs.python.org/issue18181">Issue 18181</a> contains an out of date prototype implementation</li> </ul> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0447.rst">https://github.com/python/peps/blob/main/peps/pep-0447.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0447.rst">2025-02-01 08:55:40 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#pep-status">PEP Status</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#background">Background</a></li> </ul> </li> <li><a class="reference internal" href="#the-superclass-attribute-lookup-hook">The superclass attribute lookup hook</a><ul> <li><a class="reference internal" href="#aside-attribute-resolution-algorithm-in-python">Aside: Attribute resolution algorithm in Python</a></li> <li><a class="reference internal" href="#in-python-code">In Python code</a><ul> <li><a class="reference internal" href="#example-usage">Example usage</a></li> </ul> </li> <li><a class="reference internal" href="#in-c-code">In C code</a></li> <li><a class="reference internal" href="#use-of-this-hook-by-the-interpreter">Use of this hook by the interpreter</a></li> <li><a class="reference internal" href="#other-changes-to-the-implementation">Other changes to the implementation</a></li> </ul> </li> <li><a class="reference internal" href="#impact-of-this-pep-on-introspection">Impact of this PEP on introspection</a></li> <li><a class="reference internal" href="#performance-impact">Performance impact</a><ul> <li><a class="reference internal" href="#micro-benchmarks">Micro benchmarks</a></li> <li><a class="reference internal" href="#pybench">Pybench</a></li> </ul> </li> <li><a class="reference internal" href="#alternative-proposals">Alternative proposals</a><ul> <li><a class="reference internal" href="#getattribute-super"><code class="docutils literal notranslate">__getattribute_super__</code></a></li> <li><a class="reference internal" href="#reuse-tp-getattro">Reuse <code class="docutils literal notranslate">tp_getattro</code></a></li> <li><a class="reference internal" href="#alternative-placement-of-the-new-method">Alternative placement of the new method</a></li> </ul> </li> <li><a class="reference internal" href="#history">History</a></li> <li><a class="reference internal" href="#discussion-threads">Discussion threads</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0447.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>