How do I override __getattr__ without breaking the default behavior?

getattr, python

Solution

Overriding `__getattr__` should be fine -- `__getattr__` is only called as a last resort i.e. if there are no attributes in the instance that match the name. For instance, if you access `foo.bar`, then `__getattr__` will only be called if `foo` has no attribute called `bar`. If the attribute is one you don't want to handle, raise `AttributeError`:

class Foo(object):
    def __getattr__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            raise AttributeError

However, unlike `__getattr__`, `__getattribute__` will be called first (only works for new style classes i.e. those that inherit from object). In this case, you can preserve default behaviour like so:

class Foo(object):
    def __getattribute__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            return object.__getattribute__(self, name)

See the Python docs for more.

Problem

Original source