Python3: check if method is static

Question

Simmilar question (related with Python2: Python: check if method is static)

Lets concider following class definition:

class A:
  def f(self):
    return 'this is f'

  @staticmethod
  def g():
    return 'this is g'

In Python 3 there is no instancemethod anymore, everything is function, so the answer related to Python 2 will not work anymore.

As I told, everything is function, so we can call A.f(0), but of course we cannot call A.f() (argument missmatch). But if we make an instance a=A() and we call a.f() Python passes to the function A.f the self as first argument. Calling a.g() prevents from sending it or captures the self - so there have to be a way to test if this is staticmethod or not.

So can we check in Python3 if a method was declared as static or not?

of course, I'm making my own plugin framework and I want to inspect some interface declarations and I would love to know if something was declared as staticmethod or not :) — Wojciech Danilo
– Wojciech Danilo, Commented Jan 7, 2013 at 0:10

root · Accepted Answer · 2013-01-07 00:08:11Z

10

class A:
  def f(self):
    return 'this is f'

  @staticmethod
  def g():
    return 'this is g'
print(type(A.__dict__['g']))
print(type(A.g))

<class 'staticmethod'>
<class 'function'>

answered Jan 7, 2013 at 0:08

root

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2 Comments

Wojciech Danilo Over a year ago

great! Could you please explain also why A.__dict__['g'] gives something else than A.g?

Benjamin Hodgson Over a year ago

A.__dict__['g'] is different from A.g because functions are descriptors. Function objects are descriptors because they define a __get__ method, which is invoked by magic when the object is accessed using dot notation (like A.f). The descriptor protocol is how (for example) a function gets transformed into a bound method when called on an instance. Going through the __dict__, rather than using dot notation, bypasses the descriptor protocol.

Martijn Pieters · Accepted Answer · 2018-05-20 17:40:30Z

6

For Python 3.2 or newer, use inspect.getattr_static() to retrieve the attribute without invoking the descriptor protocol:

Retrieve attributes without triggering dynamic lookup via the descriptor protocol, __getattr__() or __getattribute__().

Use isinstance(..., staticmethod) on the result:

>>> from inspect import getattr_static
>>> isinstance(getattr_static(A, 'g'), staticmethod)
True

The function can handle both instances and classes, and will scan the full class hierarchy for you:

>>> class B(A): pass
...
>>> isinstance(getattr_static(B, 'g'), staticmethod)  # inherited
True
>>> isinstance(getattr_static(B(), 'g'), staticmethod)  # instance, inherited
True

edited May 20, 2018 at 17:40

answered May 20, 2018 at 12:24

Martijn Pieters

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Comments

eric.frederich · Accepted Answer · 2016-05-10 19:16:59Z

4

I needed this solution and wrote the following based on the answer from @root

def is_method_static(cls, method_name):
    # http://stackoverflow.com/questions/14187973/python3-check-if-method-is-static
    for c in cls.mro():
        if method_name in c.__dict__:
            return isinstance(c.__dict__[method_name], staticmethod)
    raise RuntimeError("Unable to find %s in %s" % (method_name, cls.__name__))

answered May 10, 2016 at 19:16

eric.frederich

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2 Comments

Will S Over a year ago

This will only work for static methods that aren't inherited. You can use inspect.getattr_static instead of directly accessing __dict__ if you want to support inherited static methods.

eric.frederich Over a year ago

Thanks, getattr_static is new in 3.2. My code runs in both Python2 and Python3

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