Mocking function defaults using decorator

Following the approach in this answer I am able to override the default values of a function using the approach below, see test_foo_defaults.

main.py:

def bar(x):
    return x * 2

def foo(a, b=bar, c=4):
    return f'{a} {b(a)} {c}'

main_test.py

import unittest
from unittest import mock
from unittest.mock import MagicMock
import main

def test_foo():
    assert '3 6 4' == main.foo(3)

def test_foo_defaults():
    m = MagicMock()
    m.return_value = 12
    with mock.patch.object(main.foo, '__defaults__', (m, 7)):
        assert '3 12 7' == main.foo(3)

@mock.patch.object(main.foo.__defaults__, 0)
def test_foo_defaults_2(m):
    m.return_value = 12
    assert '3 12 5' == main.foo(3)

However, I'd like to be able to do something like test_foo_defaults2, in which I only override one default parameter and get a handle to it as a mock passed to my test function. test_foo_defaults2 does not work unfortunately, but fails with: TypeError: getattr(): attribute name must be string.

Are there any solutions to this?

from Mocking function defaults using decorator