Why does setattr fail on a bound method

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The short answer: There is no way of adding custom attributes to bound methods.

The long answer follows.

In Python, there are function objects and method objects. When you define a class, the def statement creates a function object that lives within the class’ namespace:

>>> class c:
...     def m(self):
...         pass
...
>>> c.m
<function m at 0x025FAE88>

Function objects have a special __dict__ attribute that can hold user-defined attributes:

>>> c.m.i = 0
>>> c.m.__dict__
{'i': 0}

Method objects are different beasts. They are tiny objects just holding a reference to the corresponding function object (__func__) and one to its host object (__self__):

>>> c().m
<bound method c.m of <__main__.c object at 0x025206D0>>
>>> c().m.__self__
<__main__.c object at 0x02625070>
>>> c().m.__func__
<function m at 0x025FAE88>
>>> c().m.__func__ is c.m
True

Method objects provide a special __getattr__ that forwards attribute access to the function object:

>>> c().m.i
0

This is also true for the __dict__ property:

>>> c().m.__dict__['a'] = 42
>>> c.m.a
42
>>> c().m.__dict__ is c.m.__dict__
True

Setting attributes follows the default rules, though, and since they don’t have their own __dict__, there is no way to set arbitrary attributes.

This is similar to user-defined classes defining __slots__ and no __dict__ slot, when trying to set a non-existing slot raises an AttributeError (see the docs on __slots__ for more information):

>>> class c:
...     __slots__ = ('a', 'b')
...
>>> x = c()
>>> x.a = 1
>>> x.b = 2
>>> x.c = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'c' object has no attribute 'c'

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