How to modify class dict (a mappingproxy)?

Question

I want to apply a decorator to every method in a class. I don't have source code of the class, so I can't directly apply the decorator. I want to call some function that accepts the class and adds the decorators.

But the problem is testclass.__dict__ is a mappingproxy object and it doesn't support any assignment or modification, directly at least. So the question is how to avoid that restriction and apply decorator?

Here is code of the failed attempt:

class qwer:
    def test(self):
        print('test')

def decor(func):
    def w(*args, **named_args):
        print('decor')
        func(*args, **named_args)
    return w

qwer.__dict__['test'] = decor(qwer.__dict__['test'])

Error:

TypeError: 'mappingproxy' object does not support item assignment

IIRC The purpose of this artificial restriction is to force keys to be strings. — julaine, Jul 07 '23 at 09:26
see here: https://stackoverflow.com/questions/32720492/why-is-a-class-dict-a-mappingproxy?noredirect=1&lq=1 — julaine, Jul 07 '23 at 09:43

score 22 · Accepted Answer · edited Jan 13 '21 at 21:06

Use setattr to set an attribute on a class:

>>> setattr(qwer, 'test', decor(qwer.__dict__['test']))
>>> qwer.test
<function decor.<locals>.w at 0xb5f10e3c>

Demo:

>>> class A:                 
    pass
... 
>>> A.__dict__['foo'] = 'bar'
Traceback (most recent call last):
  File "<ipython-input-117-92c06357349d>", line 1, in <module>
    A.__dict__['foo'] = 'bar'
TypeError: 'mappingproxy' object does not support item assignment

>>> setattr(A, 'foo', 'bar')
>>> A.foo
'bar'

How to modify class dict (a mappingproxy)?

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How to modify class __dict__ (a mappingproxy)?

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How to modify class dict (a mappingproxy)?