PEP 3155 – Qualified name for classes and functions
- Antoine Pitrou <solipsis at pitrou.net>
- Standards Track
- Python-Dev message
Python’s introspection facilities have long had poor support for nested classes. Given a class object, it is impossible to know whether it was defined inside another class or at module top-level; and, if the former, it is also impossible to know in which class it was defined. While use of nested classes is often considered poor style, the only reason for them to have second class introspection support is a lousy pun.
Python 3 adds insult to injury by dropping what was formerly known as unbound methods. In Python 2, given the following definition:
class C: def f(): pass
you can then walk up from the
C.f object to its defining class:
>>> C.f.im_class <class '__main__.C'>
This possibility is gone in Python 3:
>>> C.f.im_class Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'function' object has no attribute 'im_class' >>> dir(C.f) ['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
This limits again the introspection capabilities available to the user. It can produce actual issues when porting software to Python 3, for example Twisted Core where the issue of introspecting method objects came up several times. It also limits pickling support .
This PEP proposes the addition of a
__qualname__ attribute to
functions and classes. For top-level functions and classes, the
__qualname__ attribute is equal to the
__name__ attribute. For
nested classes, methods, and nested functions, the
attribute contains a dotted path leading to the object from the module
top-level. A function’s local namespace is represented in that dotted
path by a component named
The repr() and str() of functions and classes is modified to use
__qualname__ rather than
Example with nested classes
>>> class C: ... def f(): pass ... class D: ... def g(): pass ... >>> C.__qualname__ 'C' >>> C.f.__qualname__ 'C.f' >>> C.D.__qualname__ 'C.D' >>> C.D.g.__qualname__ 'C.D.g'
Example with nested functions
>>> def f(): ... def g(): pass ... return g ... >>> f.__qualname__ 'f' >>> f().__qualname__ 'f.<locals>.g'
With nested functions (and classes defined inside functions), the
dotted path will not be walkable programmatically as a function’s
namespace is not available from the outside. It will still be more
helpful to the human reader than the bare
__name__ attribute, the
__qualname__ attribute is computed
statically and it will not automatically follow rebinding.
Excluding the module name
__qualname__ doesn’t include the module name. This
makes it independent of module aliasing and rebinding, and also allows to
compute it at compile time.
Reviving unbound methods
Reviving unbound methods would only solve a fraction of the problems this PEP solves, at a higher price (an additional object type and an additional indirection, rather than an additional attribute).
“Qualified name” is the best approximation, as a short phrase, of what the
additional attribute is about. It is not a “full name” or “fully qualified
name” since it (deliberately) does not include the module name. Calling
it a “path” would risk confusion with filesystem paths and the
The first proposal for the attribute name was to call it
many people (who are not aware of previous use of such jargon in e.g. the
XML specification ) found it obscure and non-obvious, which is why the
slightly less short and more explicit
__qualname__ was finally chosen.
- “pickle should support methods”: http://bugs.python.org/issue9276
- “QName” entry in Wikipedia: http://en.wikipedia.org/wiki/QName
This document has been placed in the public domain.
Last modified: 2021-02-09 16:54:26 GMT