How to Mock Environment Variables in pytest
Sometimes tests need to change environment variables. This is fairly straightforward in pytest, thanks to os.environ
quacking like a dict
, and the mock.patch.dict
decorator/context manager.
(If you’re not using pytest, or use TestCase
classes with pytest, see the unittest
edition of this post.)
mock.patch
or monkeypatch
?
pytest comes with a monkeypatch
fixture which does some of the same things as mock.patch
. This post uses mock.patch
, since it’s a more powerful and general purpose tool. But you might prefer monkeypatch
- check out the monkeypatch documentation for environment variables.
Adding Environment Variables
If you want to write a test that sets one or more environment variables, overriding existing values, you can use mock.patch.dict
like this:
import os
from unittest import mock
from example.settings import Colour, get_settings
@mock.patch.dict(os.environ, {"FROBNICATION_COLOUR": "ROUGE"})
def test_frobnication_colour():
colour = get_settings().frobnication_colour
assert colour == Colour.ROUGE
You can apply this to all tests in a module by creating a local auto-used pytest fixture that uses mock.patch.dict
:
import os
from unittest import mock
import pytest
@pytest.fixture(autouse=True)
def mock_settings_env_vars():
with mock.patch.dict(os.environ, {"FROBNICATION_COLOUR": "ROUGE"}):
yield
def test_frobnication_colour():
assert os.environ["FROBNICATION_COLOUR"] == "ROUGE"
def test_frobnication_shade():
assert os.environ["FROBNICATION_COLOUR"] == "ROUGE"
Dynamic Values
If you don’t know the keys or values you want to mock at import time, you’ll need to use the context manager form of mock.patch.dict
within your test function:
import os
from unittest import mock
from example.settings import Colour, get_settings
from tests.fixtures import get_mock_colour
def test_frobnication_colour():
with mock.patch.dict(os.environ, {"FROBNICATION_COLOUR": get_mock_colour()}):
colour = get_settings().frobnication_colour
assert colour == Colour.ROUGE
Clearing
If you want to clear everything from os.environ
so only the given variables are set, you can do so by passing clear=True
to mock.patch.dict
:
import os
from unittest import mock
from example.settings import get_settings
@mock.patch.dict(os.environ, {"FROBNICATION_COLOUR": "ROUGE"}, clear=True)
def test_frobnication_colour():
settings = get_settings()
assert settings.modified_names == {"FROBNICATION_COLOUR"}
Removing
If you want to remove only a few variables, it gets a little more tricky. mock.patch.dict
doesn’t have a way of removing select keys, so you need to build a dictionary of the keys to preserve, and use that with clear=True
:
import os
from unittest import mock
from example.settings import get_settings
def test_frobnication_colour():
names_to_remove = {"FROBNICATION_COLOUR"}
modified_environ = {k: v for k, v in os.environ.items() if k not in names_to_remove}
with mock.patch.dict(os.environ, modified_environ, clear=True):
settings = get_settings()
assert settings.modified_names == set()
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Tags: python