How can I override class attribute access in python?
P.S. Is there a way to leave regular access to class attributes alone but calling a more specific exception on missing attribute?
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2 Answers
The __getattr__ magic method is called when the attribute doesn't exist on the instance / class / parent classes. You'd use it to raise a special exception for a missing attribute:
class Foo:
def __getattr__(self, attr):
# only called when self.attr doesn't exist
raise MyCustonException(attr)
If you want to customize access to class attributes, you need to define __getattr__ on the metaclass / type:
class BooType(type):
def __getattr__(self, attr):
print attr
return attr
class Boo(metaclass=BooType):
pass
boo = Boo()
Boo.asd # prints asd
boo.asd # raises an AttributeError like normal
If you want to customize all attribute access, use the __getattribute__ magic method.
6 Comments
Roman A. Taycher
Doesn't that modify instance not class attribute access?
run_the_race
"If you want to customize all attribute access, use the getattribute magic method." is misleading: to quote a comment from another question: "getattribute is only invoked on instance attribute access - if you want it invoked on class attribute access, it needs to be on the meta-class."
Sarah Messer
As @run_the_race said. See docs.python.org/3/reference/…
run_the_race
Note:
class Boo(object): Code does not work with Python3, use class Boo(metaclass=BooType): passrun_the_race
@agf Would you please clarify what you mean by "all attrbiute access" ? (since it does not work for static attributes)
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agf's answer is correct, and in fact the answer I'm about to give is based on it and owes it some credit.
Just to give more detail on the class-attribute side, which is the part that's actually under question here (as opposed to the difference between __getattr__ and __getattribute__), I'd like to add a reference to an answer I wrote to a different similar question, which goes into more detail on the difference between class and instance attributes, how this is implemented, and why you need to use a metaclass to influence class attribute lookup.
1 Comment
agf
Hey, your answer to the other question is nice, but this isn't really an answer to this question. You should either add some content here or post this as a comment on the question or my answer.