Ryan Thompson Ryan Thompson - 1 month ago 13
Python Question

How do I properly override __setattr__ and __getattribute__ on new-style classes in Python?

I want to override my Python class's

__getattribute__
and
__setattr__
methods. My use case is the usual one: I have a few special names that I want to handle, and I want the default behavior for anything else. For
__getattribute__
, it seems that I can request the default behavior simply by raising
AttributeError
. However, how can I achieve the same in
__setattr__
? Here is a trivial example, implementing a class with immutable fields "A", "B", and "C".

class ABCImmutable(SomeSuperclass):
def __getattribute__(self, name):
if name in ("A", "B", "C"):
return "Immutable value of %s" % name
else:
# This should trigger the default behavior for any other
# attribute name.
raise AttributeError()

def __setattr__(self, name, value):
if name in ("A", "B", "C"):
raise AttributeError("%s is an immutable attribute.")
else:
# How do I request the default behavior?
???


What goes in place of the question marks? With old-style classes, the answer was apparently
self.__dict__[name] = value
, but documentation indicates that this is wrong for new-style classes.

Answer

It's

super(ABCImmutable, self).__setattr__(name, value)

in Python 2, or

super().__setattr__(name, value)

in Python 3.

Also, raising AttributeError is not how you fall back to the default behavior for __getattribute__. You fall back to the default with

return super(ABCImmutable, self).__getattribute__(name)

on Python 2 or

return super().__getattribute__(name)

on Python 3.

Raising AttributeError skips the default handling and goes to __getattr__, or just produces an AttributeError in the calling code if there's no __getattr__.

See the documentation on Customizing Attribute Access.