Python static method is not always callable

Question

While parsing attributes using __dict__, my @staticmethod is not callable.

Python 2.7.5 (default, Aug 29 2016, 10:12:21)         


        

Answer 1

The reason for this behavior is the descriptor protocol. The C.foo won't return a staticmethod but a normal function while the 'foo' in __dict__ is a staticmethod (and staticmethod is a descriptor).

In short C.foo isn't the same as C.__dict__['foo'] in this case - but rather C.__dict__['foo'].__get__(C) (see also the section in the documentation of the Data model on descriptors):

>>> callable(C.__dict__['foo'].__get__(C))
True
>>> type(C.__dict__['foo'].__get__(C))
function

>>> callable(C.foo)
True
>>> type(C.foo)
function

>>> C.foo is C.__dict__['foo'].__get__(C)
True

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In your case I would check for callables using getattr (which knows about descriptors and how to access them) instead of what is stored as value in the class __dict__:

def bar(self):
    print('Is bar() callable?', callable(C.bar))
    print('Is foo() callable?', callable(C.foo))
    for attribute in C.__dict__.keys():
        if attribute[:2] != '__':
            value = getattr(C, attribute)
            print(attribute, '\t', callable(value), '\t', type(value))

Which prints (on python-3.x):

Is bar() callable? True
Is foo() callable? True
bar      True    <class 'function'>
foo      True    <class 'function'>

The types are different on python-2.x but the result of callable is the same:

Is bar() callable? True
Is foo() callable? True
bar      True    <type 'instancemethod'>
foo      True    <type 'function'>

Answer 2

You can't check if a staticmethod object is callable or not. This was discussed on the tracker in Issue 20309 -- Not all method descriptors are callable and closed as "not a bug".

In short, there's been no rationale for implementing __call__ for staticmethod objects. The built-in callable has no way to know that the staticmethod object is something that essentially "holds" a callable.

Though you could implement it (for staticmethods and classmethods), it would be a maintenance burden that, as previously mentioned, has no real motivating use-cases.

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For your case, you can use getattr(C, name) to perform a look-up for the object named name; this is equivalent to performing C.<name>. getattr, after finding the staticmethod object, will invoke its __get__ to get back the callable it's managing. You can then use callable on that.

A nice primer on descriptors can be found in the docs, take a look at Descriptor HOWTO.