Why does setattr fail on a bound method
methods, python, python-3.x, setattr
Solution
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'
Problem
In the following, `setattr` succeeds in the first invocation, but fails in the second, with: ``` AttributeError: 'method' object has no attribute 'i' ``` Why is this, and is there a way of setting an attribute on a method such that it will only exist on one instance, not for each instance of the class? ``` class c: def m(self): print(type(c.m)) setattr(c.m, 'i', 0) print(type(self.m)) setattr(self.m, 'i', 0) ``` Python 3.2.2