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__getattribute__ needs to be written carefully in order to avoid the infinite loop. For example:
class A: def __init__(self): self.x = 100 def __getattribute__(self, x): return self.x >>> a = A() >>> a.x # infinite looop RuntimeError: maximum recursion depth exceeded while calling a Python object class B: def __init__(self): self.x = 100 def __getattribute__(self, x): return self.__dict__[x] >>> b = B() >>> b.x # infinite looop RuntimeError: maximum recursion depth exceeded while calling a Python object
Hence we need to write the method in this way:
class C: def __init__(self): self.x = 100 def __getattribute__(self, x): # 1. error # AttributeError: type object 'object' has no attribute '__getattr__' # return object.__getattr__(self, x) # 2. works return object.__getattribute__(self, x) # 3. works too # return super().__getattribute__(x)
My question is why does
object.__getattribute__ method work? From where does
object get the
__getattribute__ method? And if
object does not have any
__getattribute__, then we are just calling the same method on class
C but via the super class. Why, then calling the method via super class does not result in an infinite loop?
You seem to be under the impression that your implementation of
__getattribute__ is merely a hook, that if you provide it Python will call it, and otherwise the interpreter will do its normal magic directly.
That is not correct. When python looks up attributes on instances,
__getattribute__ is the main entry for all attribute access, and
object provides the default implementation. Your implementation is thus overriding the original, and if your implementation provides no alternative means of returning attributes it fails. You cannot use attribute access in that method, since all attribute access to the instance (
self) is channelled again through
The best way around this is by calling the overridden original again. That’s where
super(C, self).__getattribute__(attr) comes in; you are asking the next class in the class-resolution order to take care of the attribute access for you.
Alternatively, you can call the unbound
object.__getattribute__() method directly. The C implementation of this method is the final stop for attribute access (it has direct access to
__dict__ and is thus not bound to the same limitations).
super() returns a proxy object that’ll look up whatever method can be found next in the method-resolution ordered base classes. If no such method exists, it’ll fail with an attribute error. It will never call the original method. Thus
Foo.bar() looking up
super(Foo, self).bar will either be a base-class implementation or an attribute error, never
When you do this:
return object.__getattribute__(self, x)
you are calling a specific function — the one defined in the object class, and not the one defined in A, so there is no recursion.
When you do this:
you are letting python choose which function to call, and it calls the one from A, and you have an infinite recursion.
Write it like this (C inherits from object):
class C(object): def __init__(self): self.x = 100 def __getattribute__(self, x): return object.__getattribute__(self, x)
Now you see why object.__getattribute__(self, x) works – you are calling parent object.