Can't define __call__ within __init__?

[Neal Becker]

Want to switch __call__ behavior.  Why doesn't this work?  What is the 
correct way to write this?

class X (object):
    def __init__(self, i):
        if i == 0:
            def __call__ (self):
                return 0
        else:
            def __call_ (self):
                return 1


x = X(0)

x()
TypeError: 'X' object is not callable