Minimal example of mocking environment variables using PyTest

test_env_vars.py

def test_example(monkeypatch):
    # Print the value of $FOO. If not defined will be None
    print("FOO = %s" % os.getenv('FOO'))
    
    monkeypatch.setenv('FOO', '3.14')
    print("FOO = %s" % os.getenv('FOO'))  # Expected result: FOO = 3.14
    with monkeypatch.context() as m:
        m.setenv('FOO', '2.71')
        print("FOO = %s" % os.getenv('FOO'))  # Expected result: FOO = 2.71
    print("FOO = %s" % os.getenv('FOO'))  # Expected result: FOO = 3.14
    assert 1 == 2  # So you could see the captured output above ;)