How to differentiate between hasattr and normal attribute access in __getattr__?

Question

class a_class:
    def __getattr__(self, name):
        # if called by hasattr(a, 'b') not by a.b
        # print("I am called by hasattr")
        print(name)

a = a_class()
a.b_attr

hasattr(a, 'c_attr')

Please take a look the comment inside __getattr__. How do I do that? I am using Python 3. The reason is I want to create attribute dynamically but I don't want to do that when using hasattr. Thanks.

Answer 1

You can't, without cheating. As the documentation says:

This [that is, hasattr] is implemented by calling getattr(object, name) and seeing whether it raises an exception or not.

In other words, you can't block hasattr without also blocking getattr, which basically means you can't block hasattr at all if you care about accessing attributes.

By "cheating" I mean one of these solutions that clever people like to post on here that involve an end-run around essentially all of Python. They typically involve reassigning builtins, inspecting/manipulating the call stack, using introspection to peek at the literal source code, modifying "secret" internal attributes of objects, and so on. For instance, you could look at the call stack to see if hasattr is in the call chain. This type of solution is possible, but extremely fragile, with possibility of breaking in future Python versions, on non-CPython implementations, or in situations where another equally ugly and devious hack is also being used.

You can see a similar question and some discussion here.