Monkey patch a python instance method using the original method from the class

Question

I want to monkey patch a method of a library class to define a different default for a param. This fails:

from functools import partial

class A(object):
    def meth(self, foo=1):
        print(foo)

A.meth = partial(A.meth, foo=2)

a = A()
a.meth()

with:

Traceback (most recent call last):
  File "...", line 10, in <module>
    a.meth()
TypeError: meth() missing 1 required positional argument: 'self'

what is the correct way of doing this?

(Original code is using getattr on the method names in a loop)

The answers in the question linked involve defining a new module-level function - I would like to avoid a new function definition

Possible duplicate of Python: replacing a function within a class of a module — Torxed
– Torxed, Commented Jan 31, 2019 at 12:24
Ignore my override statement, that might be my brain mixing in some Java syntax.. ouff.. That declaration might just be eye-candy-fluff. Sorry for the confusion. You should be able to just in-line replace A.meth = new_func and it should work. — Torxed
– Torxed, Commented Jan 31, 2019 at 12:27
@Mr_and_Mrs_D Note that A.meth is not a simple function, it is a descriptor. Hence you need to substitute it with an object that implements the descriptor protocol. — a_guest
– a_guest, Commented Jan 31, 2019 at 12:37

tayfun · Accepted Answer · 2019-01-31 12:31:14Z

4

In [32]: from functools import partialmethod

In [33]: A.meth = partialmethod(A.meth, foo=2)

In [34]: a = A()

In [35]: a.meth()
2

answered Jan 31, 2019 at 12:30

tayfun

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1 Comment

Oh! What is the difference between partial and partialmethod (didn't know about the latter)