python: Constructor goes into infinite loop

Question

My purpose is to override some of the functions of 'First' class run-time for certain cases. So I want to derive a class from the original one. Here is the code snippet.

class First(object):

    def __init__(self):
        print "First"
        super(First, self).__init__()

    def foo(self):
        print "foo"

class Second(First):

    def __init__(self):
        print "second"
        super(Second, self).__init__()

    def foo(self):
        print "want to override this"

First = Second

o = First()

Why the constructor goes into infinite loop? What wrong thing am doing?

Answer 1

Python names are not looked up at compile time, name lookups happen when the code is executed.

The thing to watch out for is

First = Second

• Because of the assignment, First() will create an instance of class Second
• Second.__init__() will call First.__init__() .
• in First.__init__() , First will be looked up by name in the global context.
• Since you reassigned First = Second , the name First points to class Second . Which will get its __init__() called and that gives you your infinite recursion.

In short: Don't do this ...