Python - function as class attribute becomes a bound method

Question

I noticed that if I define a class attribute equal to a function when I create an instance of that class the attribute becomes a bound method. Can someone explain me the reason of this behaviour?

In [9]: def func():
   ...:     pass
   ...: 

In [10]: class A(object):
   ....:     f = func
   ....:     

In [11]: a = A()

In [12]: a.f
Out[12]: <bound method A.func of <__main__.A object at 0x104add190>>

In [13]: a.f()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-19134f1ad9a8> in <module>()
----> 1 a.f()
    global a.f = <bound method A.func of <__main__.A object at 0x104add190>>

TypeError: func() takes no arguments (1 given)

Answer 1

You assigned a function to the attribute A.f (the attribute f of the class A). The attribute A.f was defined as part of the class. It is a function, so it is by default an instance method of that class.

Creating an instance (named a) of class A causes that instance to have an attribute f, and you access that by the name a.f. This is a bound method (cause it's bounded to the object a; further explanation here).

Every instance method, when it is called, automatically receives the instance as its first argument (conventionally named self). Other types of method are possible: - see class methods and static methods.

For this reason the error says that func takes no arguments (as it's defined as def func():) but received 1 (self).

To do what you want, you should tell python that you're using a static method

def func():
    pass

class A(object):
    f = staticmethod(func)