exception - Python: Why is __getattr__ catching AttributeErrors?

Question

I'm struggling with __getattr__. I have a complex recursive codebase, where it is important to let exceptions propagate.

class A(object):
    @property
    def a(self):
        raise AttributeError('lala')

    def __getattr__(self, name):     
        print('attr: ', name)
        return 1      

print(A().a)

Results in:

('attr: ', 'a')
1

Why this behaviour? Why is no exception thrown? This behaviour is not documented (__getattr__ documentation). getattr() could just use A.__dict__. Any thoughts?

Answer 1

I just changed the code to

class A(object):
    @property
    def a(self):
        print "trying property..."
        raise AttributeError('lala')
    def __getattr__(self, name):     
        print('attr: ', name)
        return 1      

print(A().a)

and, as we see, indeed the property is tried first. But as it claims not to be there (by raising AttributeError), __getattr__() is called as "last resort".

It is not documented clearly, but can maybe be counted under "Called when an attribute lookup has not found the attribute in the usual places".