Rationale

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 [1].

Proposal

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 __qualname__ 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 <locals>.

The repr() and str() of functions and classes is modified to use __qualname__ rather than __name__.

>>> 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'

>>> def f():
...   def g(): pass
...   return g
...
>>> f.__qualname__
'f'
>>> f().__qualname__
'f.<locals>.g'

Limitations

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__.

As the __name__ attribute, the __qualname__ attribute is computed statically and it will not automatically follow rebinding.

Discussion

Naming choice

“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 __file__ attribute.

The first proposal for the attribute name was to call it __qname__ but many people (who are not aware of previous use of such jargon in e.g. the XML specification [2]) found it obscure and non-obvious, which is why the slightly less short and more explicit __qualname__ was finally chosen.

References

[1]

“pickle should support methods”: http://bugs.python.org/issue9276

[2]

“QName” entry in Wikipedia: http://en.wikipedia.org/wiki/QName

Copyright

This document has been placed in the public domain.