Pytest monkey patching builtins open

pytest_monkey_open.py

import builtins
import pytest
from io import StringIO
import yaml

TEST_CONFIG = """
environment: prod
"""


def read_file():
    with open("config.yaml", 'r') as stream:
    try:
        data = yaml.load(stream)
        return data


def test_read_file(monkeypatch):
    mopen = lambda x, y: StringIO(initial_value=TEST_CONFIG)
    monkeypatch.setattr(builtins, 'open', mopen)

    config = read_environment_config()
    assert config.environment == 'prod'