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

Question:

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.

Asked By: Ryan C. Thompson

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Source

Answers:

SomeSuperclass.__setattr__(self, name, value) ?

Answered By: Jeannot

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.

Answered By: Hank Gay

The following seems to do the trick:

class ABCImmutable:
    def __getattribute__(self, name:str):
        if name in ("A", "B", "C"):
            return "Immutable value of %s" % name
        else:
            return super(ABCImmutable, self).__getattribute__(name)

    def __setattr__(self, name:str, value):
        if name in ("A", "B", "C"):
            msg = "%s is an immutable attribute." % name
            # msg = f"{name} is an immutable attribute."
            # msg = "{} is an immutable attribute.".format(name)
            raise AttributeError(msg)
        else:
            super().__setattr__(name, value)
Answered By: Samuel Muldoon