How to get unbounded classmethod

user_733545 0
Viewed 56

I'm trying to "wrap" an existing classmethod, i.e.,

def Foo:
  @classmethod
  def bar(cls, x): return x + 2

old_bar = Foo.bar

def wrapped_bar(cls, x):
  result = old_bar(cls, x) # Results in an error
  return result

Foo.bar = wrapped_bar

It seems that Foo.bar is already bound with cls = Foo, how do I get the unbound version of the function bar?

[I'm not allowed to modify Foo, it exists in another codebase that I'm patching]

python-3.x python
2 Answers

Suppose, you have:

>>> class Foo:
...     @classmethod
...     def bar(cls, x):
...         return x*42
...
>>> Foo.bar(2)
84

Then one way is to access the name-space of your class directly. Then you should be able to access the classmethod object and obtain the decorated function available at the __func__ attribute:

>>> vars(Foo)['bar']
<classmethod object at 0x103eec520>
>>> vars(Foo)['bar'].__func__
<function Foo.bar at 0x1043e49d0>

Alternatively, it is accessible on the bound-method object itself:

>>> bound = Foo.bar
>>> bound
<bound method Foo.bar of <class '__main__.Foo'>>
>>> bound.__func__
<function Foo.bar at 0x1043e49d0>
user_50144550

The __func__ attribute holds the wrapped method. Note that your wrapper will also need the @classmethod decorator in order to attach properly (otherwise it'll act as an instance method).

@classmethod
def wrapped_bar(cls, x):
    result = Foo.bar.__func__(cls, x)
    return result

Foo.bar = wrapped_bar
user_2574650
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