Attribute Reference: __getattr__ and __setattr__

The __getattr__ method intercepts attribute qualifications. More specifically, it’s called with the attribute name as a string whenever you try to qualify an instance with an undefined (nonexistent) attribute name. It is not called if Python can find the attribute using its inheritance tree search procedure. Because of its behavior, __getattr__ is useful as a hook for responding to attribute requests in a generic fashion. For example:

>>> class empty:
...     def __getattr__(self, attrname):
...         if attrname == "age":
...             return 40
...         else:
...             raise AttributeError(attrname)
...
>>> X = empty()
>>> X.age
40
>>> X.name
...error text omitted...
AttributeError: name