Python 2 __getattr__ max recursion depth

Question

for example i use this code:

class A(object):
    def __init__(self):
        self.dict1 = {
            'A': 3,
            'B': self.A}
    def __getattr__(self, key):
        if key in self.dict1:
            return self.dict1[key]
a = A()

and when it's runned it throws maximum recursion depth exceeded. Can someone please tell me what am i doing wrong here

Monastery

Answer 1

The reference to self.dict1 inside your __getattr__ method causes __getattr__ to be called again, and so on, hence the infinite recursion. The only safe way to access attributes of self inside __getattr__ is by using references to self.__dict__. Try

def __getattr__(self, key):
    if key in self.__dict__['dict1']:
        return self.__dict__['dict1'][key]

Note also that the absence of an else clause will mean undefined attributes appear to have the value None.

Nice

Tried you solution and it raise KeyError instead – Monastery

That's because in trying to construct self.dict1 in the __init__ method you attempt tp access self.A. This triggers a call to __getattr__, which assumes the existence of self.dict1. – Nice

Maybe not very ellegant, but it works if i just set 'B' after dict1 is created like this: self.dict1['B'] = self.A – Monastery

As long as you've got something that works. Elegance can come later! – Nice

Don't forget as well that if you implement __setattr__ the __init__ method should use self.__dict__['dict1'] = {}. – Nice