How to use functools.partial for a class method?

Question

I'd like to apply partial from functools to a class method.

from functools import partial

class A:
    def __init__(self, i):
        self.i = i

    def process(self, constant):
        self.result = self.i * constant

CONST = 2
FUNC = partial(A.process, CONST)

When I try:

FUNC(A(4))

I got this error:

'int' object has no attribute 'i'

It seems like CONST has been exchange with the object A.

Answer 1

You're binding one positional argument with partial which will go to the first argument of process , self . When you then call it you're passing A(4) as the second positional argument, constant . In other words, the order of arguments is messed up. You need to bind CONST to constant explicitly:

FUNC = partial(A.process, constant=CONST)

Alternatively this would do the same thing:

FUNC = lambda self: A.process(self, CONST)

Answer 2

Try creating a partial from an instance of A .

CONST = 2
a = A()
FUNC = partial(a.process, CONST)

You can then call FUNC like so:

FUNC(A(4))