conditional mocking of open

blark.py

def foop():
    print(open('book.txt', 'r+').read())
    print(open('/home/vilmibm/tmp/blark.py').read())

mocking.py

from unittest.mock import Mock, patch
import blark

original_open = open
mock_file = Mock()
mock_file.read.return_value = "BOOOOOOOOKS MMMMMMMMMMMMMM YEAHHHHH\n"
def mock_open(*args):
    if args[0] == 'book.txt':
        return mock_file
    else:
        return original_open(*args)

with patch('builtins.open', ) as m:
    m.side_effect = mock_open
    blark.foop()