Not all method descriptors are callable

[larryhastings]

BPO	20309
Nosy	@warsaw, @arigo, @rhettinger, @ncoghlan, @pitrou, @vstinner, @larryhastings, @bitdancer, @ericsnowcurrently, @jdemeyer, @eryksun, @sruggier
Files	curious.py descr_v1.diff: In progress version of the patch (not finished!) descr_v2.diff descr_v3.diff example_use_case.py: Example use case for callable static methods.

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2015-04-14.14:53:36.344>
created_at = <Date 2014-01-20.06:43:37.890>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'Not all method descriptors are callable'
updated_at = <Date 2021-03-31.14:45:13.131>
user = 'https://github.com/larryhastings'

bugs.python.org fields:

activity = <Date 2021-03-31.14:45:13.131>
actor = 'vstinner'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2015-04-14.14:53:36.344>
closer = 'ncoghlan'
components = ['Interpreter Core']
creation = <Date 2014-01-20.06:43:37.890>
creator = 'larry'
dependencies = []
files = ['33559', '34928', '34959', '38942', '47940']
hgrepos = []
issue_num = 20309
keywords = ['patch']
message_count = 16.0
messages = ['208524', '208525', '208531', '208537', '208538', '208544', '216593', '216676', '216769', '240686', '240843', '240894', '240898', '330227', '338473', '389908']
nosy_count = 15.0
nosy_names = ['barry', 'arigo', 'rhettinger', 'ncoghlan', 'pitrou', 'vstinner', 'chrish42', 'larry', 'Arfrever', 'r.david.murray', 'eric.snow', 'jdemeyer', 'eryksun', 'ozialien', 'sruggier']
pr_nums = []
priority = 'low'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20309'
versions = ['Python 3.5']

[larryhastings]

I found something curious while experimenting with types tonight.

Everybody knows, if you access a descriptor in a class through the normal attribute-getting methods, you actually get the result of calling its '__get__' method. If you want to get your hands on the descriptor itself, you can be sneaky and pull it out through the class's __dict__.

I did an experiment tonight, accessing a bunch of methods in a bunch of different ways.

The methods I tried were

• an instance method,
• a class method, and
• a static method.

I accessed them

• as an instance attribute,
• as a class attribute, and
• through the class __dict__.

And for each of the above, I tried

• one user method and
• one built-in method.

That's a total of eighteen methods tried (3x3x2).

For each one, I printed out whether or not it was callable. I found the results startling: fifteen were, but three (!) were not.

All three that weren't were accessed through the class dict. They were:

• Class method, from builtin class ( dict.__dict__['fromkeys'] )
• Static method, from user class
• Static method, from builtin class ( str.__dict__['maketrans'] )

I find it strange that the static method descriptors aren't callable.
I find it *even stranger* that *one* of the class method descriptors isn't callable, but the other one is.

I guess a foolish consistency is the hobgoblin of my small mind, but... shouldn't all eighteen of these objects be callable?

Attached is the test harness I wrote.

(p.s. I attached you guys as nosy because I thought you might be interested.)

[larryhastings]

I should add, I see the same results with current trunk and with my handy Python 3 (currently 3.3.1rc1, huh guess I'm behind).

[arigo]

Instances of 'staticmethod' or 'classmethod' are not callable. I'm unsure why not, but it's good enough, as you need to go through various unexpected hops in order to try to call one.

'dict.fromkeys' is not a classmethod, as seen by "dict.__dict__['fromkeys']". It's an internal object usually never seen by Python code. I believe this is only because implementing a regular classmethod in C would be rather messy.

[ncoghlan]

I believe it's just a matter of pattern of use - applying staticmethod
outside a class (or retrieving the descriptor directly from the dict,
bypassing the descriptor protocol), so nobody every noticed.

Ditto for the C wrapper vs the Python wrapper for a classmethod - it's
really rare to access those without going through the descriptor
machinery, so it's likely just an accident of implementation that one
of them isn't callable.

I don't see any specific reason for them to be non-callable - I
suspect it's just a case of never adding the code to make it happen.

[ncoghlan]

Oh, so that's how that happens :)

(the title reverted because I replied via email from a part of the thread before Larry changed the issue name)

[bitdancer]

See also bpo-19072 for previous report of the classmethod problem.

[chrish42]

Work in progress for fixing this bug. (See descr_v1.diff) I converted the "curious.py" file into additional testcases. I started writing the functions for the tp_call slots for class and static methods.

To do: add tests to make sure that the code works for more than what's accepted by function_call(), then switch to using PyObject_Call() (which is the right function to use here, thanks to ncoghlan for catching that). Will finish this in the next few days.

[eryksun]

classmethod_descriptor instances such as vars(dict)['fromkeys'] are callable. The first argument has to be a subclass of __objclass__:

    >>> vars(dict)['fromkeys'].__objclass__
    <class 'dict'>

Calling the descriptor creates a bound built-in method; slices the args to remove the class; and calls it with the args and kwds.

    >>> vars(dict)['fromkeys'](dict, 'abc')
    {'a': None, 'b': None, 'c': None}

source: classmethoddescr_call
http://hg.python.org/cpython/file/04f714765c13/Objects/descrobject.c#l256

While the classmethod and staticmethod types that are defined in funcobject.c aren't callable, they do expose a __func__ member.

[chrish42]

Here is the (first?) complete version of the patch. All tests pass. Note that I had to remove a test that was checking that the object returned from staticmethod was not callable.

Let me know if I should add more tests, or if there are any other problems with the patch. Thanks!