How to check if a method exists in a class in Python?

Problem:

Consider the following context:

class Call:
    def __call__(self):
        pass

class Foo:
    def __init__(self):
        self.var = Call()

    def method(self):
        pass

foo = Foo()

That is, we have an object foo which is an instance of Foo, having a method method and an attribute var, which is of type Call (which is callable). When checking if a method exists, we expect only method to be returned as valid and var not.

Solution 1:

Spolier alert: do not use! :(

Proposed by Wallace, in his response (adapted):

def method_exists(instance, method):
    return hasattr(instance, method) and callable(instance.method)

Doing the tests for the problem:

>>> print(method_exists(foo, "method"))
True
>>> print(method_exists(foo, "var"))
True

^{see working on Ideone}

I.e. solution failed in the second Test.

Solution 2:

Spolier alert: do not use! :(

Proposed by me, in a version prior to this answer:

def method_exists(instance, method):
    return method in dir(instance) and callable(instance.method)

Doing the tests for the problem:

>>> print(method_exists(foo, "method"))
True
>>> print(method_exists(foo, "var"))
True

^{see working on Ideone}

That is, the solution also failed in the second Test.

Solution 3:

Spolier alert: works! Can use: D

Also proposed by Wallace in his answer (adapted):

from inspect import ismethod

def method_exists(instance, method):
    return hasattr(instance, method) and ismethod(getattr(instance, method))

Doing the tests for the problem:

>>> print(method_exists(foo, "method"))
True
>>> print(method_exists(foo, "var"))
False

^{see working on Ideone}

Therefore passed both Tests.

In Solution 3, the include function getattr for checking if it is a method.

You can use the function called hasattr to know if a certain property exists in the class and then use callable if the property exists to know if it is something "invocable" (in the case the method):

class Test(object):
    def x(self):
        pass



if hasattr(Test, 'x') and callable(Test.x):
    print ("Método 'x' existe")

You can also use the inspect library to check if a particular attribute of a class is a method.

 from inspect import ismethod

 if hasattr(Test, 'x') and ismethod(Test.x):
     print ("Tem 'x'")