getattr on class objects

Question

class A:
    def foo(self):
        print "foo()"

getattr(A, foo) # True
A.foo() # error

getattr(A(), foo) # True
A().foo() # prints "foo()"

That being said, here is my problem:

I wish to store test case meta information as attributes of the Test Case class objects themselves, not on instances of them.
I have a list of attribute names to extract, but if there is an instance method of the same name, then getattr(class_obj, attr) will return True, but getattr(class_obj, attr)() raises an Error.

Is there a way to tell getattr not to include attributes of the instantiated class and only of the class object itself?

EDIT: I tried accessing class_obj.__dict__ directly (which I understand is bad practice), but it does not include some attributes like __name__

EDIT: Rephrase of the question. Is there a way to differentiate between methods of the class obj and the methods of an instance of the class?

Answer 1

Is this good enough?

import types
class Test(object):
    @staticmethod
    def foo():
        print 'foo'

    def bar(self):
        print 'bar'

In combination with:

>>>(isinstance(getattr(Test, 'foo'), types.FunctionType),
    isinstance(getattr(Test, 'bar'), types.FunctionType))
True, False

You can also use the inspect module:

>>> inspect.isfunction(Test.foo)
True
>>> inspect.isfunction(Test.bar)
False

With a little additional work you can even distinguish class methods from instance methods and static methods:

import inspect

def get_type(cls, attr):
    try:
        return [a.kind for a in inspect.classify_class_attrs(cls) if a.name == attr][0]
    except IndexError:
        return None

class Test(object):
    @classmethod
    def foo(cls):
        print 'foo'

    def bar(self):
        print 'bar'

    @staticmethod
    def baz():
        print 'baz'

You can use it as:

>>> get_type(Test, 'foo')
'class method'
>>> get_type(Test, 'bar')
'method'
>>> get_type(Test, 'baz')
'static method'
>>> get_type(Test, 'nonexistant')
None

Answer 2

Your results from an incorrect definition of foo, not any underlying semantics of class attributes. By default, a function declared inside a class is an instance method, which must take at least one argument, an instance of the class. Conventionally, it is referred to as self:

class A:
    def foo(self):
        print "foo()"

Normally, you would call such a method like this:

a = A()
a.foo()    # passes the object 'a' implicitly as the value of the parameter 'self'

but this is legal as well

a = A()
A.foo(a)   # pass the object 'a' explicitly as the value of the parameter 'self'

In order to define a function inside a class that doesn't take any such implicit arguments, you need to decorate it with the @staticmethod decorator:

class A:
    @staticmethod
    def foo():
        print "foo()"

Now, you can call foo the way you tried to previously:

>>> A.foo()
foo()

Answer 3

You want something like this:

from inspect import ismethod
from collections import Callable
def can_i_call(func):
    if not isinstance(func, Callable):
        # not a callable at all
        return False

    if not ismethod(func):
        # regular function or class or whatever
        return True

    # func is a method
    return func.im_self is not None

Note: this will only test whether or not an attempt to call will error out because you're calling an unbound method without a self. It doesn't guarantee that func() will succeed, i.e. not fail for any other reason.