functools.partial objects have no __qualname__ attribute

[cjerdonek]

BPO	34475
Nosy	@vstinner, @cjerdonek, @serhiy-storchaka, @hongweipeng, @tirkarthi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-10-18.21:24:00.758>
created_at = <Date 2018-08-23.14:25:59.091>
labels = ['type-feature', 'library']
title = 'functools.partial objects have no __qualname__ attribute'
updated_at = <Date 2018-10-18.21:29:12.799>
user = 'https://github.com/cjerdonek'

bugs.python.org fields:

activity = <Date 2018-10-18.21:29:12.799>
actor = 'chris.jerdonek'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-18.21:24:00.758>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2018-08-23.14:25:59.091>
creator = 'chris.jerdonek'
dependencies = []
files = []
hgrepos = []
issue_num = 34475
keywords = []
message_count = 11.0
messages = ['323947', '324912', '324913', '324914', '327858', '327862', '327868', '327957', '328002', '328006', '328008']
nosy_count = 5.0
nosy_names = ['vstinner', 'chris.jerdonek', 'serhiy.storchaka', 'hongweipeng', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34475'
versions = ['Python 3.6']

[cjerdonek]

functools.partial objects have no __qualname__ attribute. This means, for example, that code expecting a callable that logs the __qualname__ attribute can break when passed a functools.partial object.

Example:

>>> import functools
>>> int.__qualname__
'int'
>>> p = functools.partial(int)
>>> p.__qualname__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'functools.partial' object has no attribute '__qualname__'

[hongweipeng]

the functools.partial returns an instance not fun or cls.using p.func.__qualname__ may be you want.

[cjerdonek]

Using p.func would name the function passed to functools.partial() rather than the partial object itself.

[tirkarthi]

It seems __repr__ call to partial object has qualname but I think it always returns "partial". Ref :

cpython/Lib/functools.py

Line 276 in 0afada1

qualname = type(self).__qualname__

>>> import functools
>>> int.__qualname__
'int'
>>> p = functools.partial(int)
>>> p.__qualname__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'functools.partial' object has no attribute '__qualname__'
>>> p
functools.partial(<class 'int'>)

Thanks

[hongweipeng]

partial() return an instance not class or function. Why it need __qualname__ attribute?

Ref: https://www.python.org/dev/peps/pep-3155/

[serhiy-storchaka]

I don't see a problem. Not all callables have the __qualname__ attribute. It is not the part of the protocol. The code that expects the __qualname__ attribute should be fixed.

[cjerdonek]

Okay, I thought a partial object was supposed to "look" like a function.

I'm okay with closing this.

[serhiy-storchaka]

partial objects lack many other function attributes: __name__, __module__ (and __qualname__ doesn't make sense without __module__), __annotations__, __get__(), etc. It would be nice to make these types more similar, but attributes shouldn't lie. And I am not sure what partial.__qualname__ can be. It shouldn't be the __qualname__ of the wrapped function, since the partial object differs from it, and is not accessible by same name.

[cjerdonek]

It shouldn't be the __qualname__ of the wrapped function

Yes, I agree with you. I was thinking it should be similar to what it would be for a function defined at the same location.

[vstinner]

functools.partial objects have no __qualname__ attribute, but they don't have a __name__ attribute neither.

$ python3
Python 3.6.6 (default, Jul 19 2018, 14:25:17) 
>>> import functools
>>> func=int
>>> p=functools.partial(func)
>>> p.__name__
AttributeError: 'functools.partial' object has no attribute '__name__'
>>> p.__qualname__
AttributeError: 'functools.partial' object has no attribute '__qualname__'
>>> repr(p)
"functools.partial(<class 'int'>)"

If you want to "inherit" the name of the "wrapped function", you may use: functools.update_wrapper().

I'm not sure that it's correct to inherit the name by default. functools.partial() creates a new function, so if it has a name, for me, it should be different.

I agree to close the issue, it's not a bug.

[cjerdonek]

Sorry, I'm out of practice. I thought I closed this when I marked it rejected.

[JensHeinrich]

Is this still the right approach?
after all even lambda expressions get a __qualname__ now.