[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Irit Katriel]

Irit Katriel  added the comment:

See also Issue41294.

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[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Irit Katriel]

Irit Katriel  added the comment:

This is the check that is preventing qualname being a property:

https://github.com/python/cpython/blob/994f90c0772612780361e1dc5fa5223dce22f70a/Objects/typeobject.c#L2853

Note that it is not the same check used for assignment, which is here:

https://github.com/python/cpython/blob/994f90c0772612780361e1dc5fa5223dce22f70a/Objects/typeobject.c#L559


I think some unit tests are missing for properties, they would need to be added.

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nosy: +iritkatriel, vstinner
versions: +Python 3.11 -Python 3.10

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[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Lewis Gaul]

Lewis Gaul  added the comment:

This would also be useful for me - I just hit this same problem. If someone 
could give some guidance on how to make this change I'd be happy to put up a PR.

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[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Irit Katriel]

Change by Irit Katriel :


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versions: +Python 3.10 -Python 3.4

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[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Graham Dumpleton]

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

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[issue19073] Inability to specific __qualname__ as a property on a class instance.

[Antoine Pitrou]

Antoine Pitrou added the comment:

I guess this would be a harmless improvement in any case.

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nosy: +pitrou
stage:  - needs patch
type: behavior - enhancement
versions: +Python 3.4 -Python 3.3

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