New submission from Graham Dumpleton:
Python 3 introduced __qualname__. This attribute exists on class types and also
instances of certain class types, such as functions. For example:
def f(): pass
print(f.__name__)
print(f.__qualname__)
class Class: pass
print(Class.__name__)
print(Class.__qualname__)
yields:
f
f
Class
Class
An instance of a class however does not have __name__ or __qualname__
attributes. With:
c = Class()
print(c.__name__)
print(c.__qualname__)
yielding:
Traceback (most recent call last):
File "qualnametest.py", line 13, in <module>
print(c.__name__)
AttributeError: 'Class' object has no attribute '__name__'
Traceback (most recent call last):
File "qualnametest.py", line 14, in <module>
print(c.__qualname__)
AttributeError: 'Class' object has no attribute '__qualname__'
For a class, it is possible to override the __name__ attribute using a property.
class Class:
@property
def __name__(self):
return 'override'
c = Class()
print(c.__name__)
With the result being:
override
This is useful in writing object proxies or function wrappers for decorators as
rather than having to copy the __name__ attribute into the wrapper, the lookup
can be deferred until when it is required.
The same though cannot be done for __qualname__. With:
class Class:
@property
def __qualname__(self):
return 'override'
yielding an error when the class definition is being processed:
Traceback (most recent call last):
File "qualnametest.py", line 16, in <module>
class Class:
TypeError: type __qualname__ must be a str, not property
This means the same trick cannot be used in object proxies and function
wrappers and instead __qualname__ must be copied and assigned explicitly as a
string attribute in the __init__() function of the object proxy or function
wrapper.
I can sort of understand a prohibition on __qualname__ being a string attribute
in certain cases, especially if overriding it on a type or instance where
__qualname__ attribute already exists, but I don't understand why a limitation
would be imposed to prevent using a property as a means of generating the value
for a class instance which doesn't otherwise have a __qualname__ attribute.
There is no similar restriction for __name__.
Unless there is a good specific reason for this behaviour, the ability to
override it with a property in cases where the __qualname__ attribute didn't
already exist, would be handy for proxies and wrappers.
----------
components: Interpreter Core
messages: 198275
nosy: grahamd
priority: normal
severity: normal
status: open
title: Inability to specific __qualname__ as a property on a class instance.
type: behavior
versions: Python 3.3
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue19073>
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