Here's an example of why mborch's statement is true:

    class A:
        def __init__(self, value=0):
            self.b = value
        def __str__(self):
            if self.b == 0:
                global a
                a = A(1)
            elif self.b == 1:
                import __builtin__, __main__
                __builtin__.a = 2
                del __main__.a
            return str(self.b)
    a = A()
    
    def foo():
        print a
        print a
        print a
        
    foo()

This generates:

    1
    2
    3

but it's a different 'a' in all three cases; the last isn't even from the module's namespace.