Overriding in class inheriting

As Python is a dynamic language, there are a lot you can play with. However, it also produces a lot of confusion. As one of those, inheriting (or extending) of classes is sometimes different from expected. The following snippet shows a case.

>>> class A():
...     def __init__(self, arg):
...         self.arg = arg
...         self.afunc = None
>>> class B(A):
...     def __init__(self, arg):
...         A.__init__(self, arg)
...     def afunc():
...         print 'haha'
>>> b = B(5)
>>> b.afunc()
Traceback (most recent call last):
 File "<input>", line 1, in <module>
 TypeError: 'NoneType' object is not callable

The purpose was to extend the the original class meanwhile fixing the implementation of afunc(). However, it fails. The function definition of afunc() is overrode by the assigning statement in A.__init__(). Thus the sequence of name binding is like BaseClass -> Subclass -> Base.__init__() -> Subclass.__init__().

Tagged: python

Posted in: Python