Question

>>> class S(object):
...     def __init__(self):
...             self.x = 1
...     def x(self):
...             return self.x
...
>>> s = S()
>>> s.x
1
>>> s.x()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable

Why, in this example, is s.x a method, but also an integer? It seems to me that self.x = 1 should replace the def x(self): declaration of the attribute x during instantiation. Why is it that I can get and call, resulting in an integer and a method, respectively, the same attribute? My guess is that the variable look-up pattern in new-style classes is duck typed, so as to return the most relevant result to the caller. I would love to hear the whole story.

Answer 1

It looks like you’re having a misunderstanding of the error you’re seeing. When your s object is instantiated, its constructor replaces the method x by an integer, so in the s object, x is an integer, not a function. Trying to call it as a method results in an exception being thrown.

Python is duck-typed in the sense that method calls are resolved at runtime – the compiler has no problem with s.x() because x might have been created as a method dynamically. However, when the interpreter actually calls x as a method, it notices x is an integer and can’t be called, hence the TypeError.