What is difference between A.fun and A().fun?

Question

class A(object):
    def fun(self):
        pass
ins_a = A.fun
ins_b = A().fun

I came across this piece of code and I am unable to understand the difference between the 2 objects.

Answer 1

Just try the above code in the interactive interpreter:

>>> class A(object):
...     def fun(self):
...             pass
... 
>>> ins_a = A.fun
>>> ins_b = A().fun
>>> ins_a
<unbound method A.fun>
>>> ins_b
<bound method A.fun of <__main__.A object at 0x7f694866a6d0>>

As you can see, it is a matter of bound/unbound methods. A bound method is a method "tied" to an object. You can have a more thorough explanation in this SO answer.

Answer 2

The biggest difference is if you try to call the methods: If we add a print "hello world", it will make it more obvious.

class A(object):
    def fun(self):
        print ("hello world")

ins_a = A.fun
ins_b = A().fun

Now try calling both:

In [10]: ins_a()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-10-52906495cc43> in <module>()
----> 1 ins_a()

TypeError: unbound method fun() must be called with A instance as first argument (got nothing instead)

In [11]: ins_b()
hello world

In python 3 they are different types as the unbound method type is gone:

In [2]: type(ins_a)
Out[2]: builtins.function

In [3]: type(ins_b)
Out[3]: builtins.method