Patching has no effect when applied to a parameter of a function called by the patched function

Question

On python 3.9.6 the following program prints "a" to stdout. I'd expect both print_a and print_b to behave the same.

import io
from unittest.mock import patch
import sys

def print_a(file=sys.stdout):
    """
    Does not get redirected to the mocked stdout
    """
    print("a", file=file)

def print_b():
    """
    Does get redirected to the mocked stdout
    """
    print("b", file=sys.stdout)

@patch("sys.stdout", new_callablle=io.StringIO)
def test_print(func, mock_stdout):
    func()

# Expect no output to stdout
test_print(print_a)
test_print(print_b)

Answer 1

Making the default file=None and then setting file conditionally produces the expected result.

import io
from unittest.mock import patch
import sys

def print_a(file=None):
    """
    Does get redirected to the mocked stdout
    """
    file = file or sys.stdout
    print("a", file=file)

def print_b():
    """
    Does get redirected to the mocked stdout
    """
    print("b", file=sys.stdout)

@patch("sys.stdout", new_callablle=io.StringIO)
def test_print(func, mock_stdout):
    func()

# Expect no output to stdout
test_print(print_a)
test_print(print_b)