monkeypatch.setenv in PyTest - Build an Automation Script

Test function behavior with modified environment variable using monkeypatch.setenv

Preconditions (2)

Step 1: Use monkeypatch.setenv to set 'APP_MODE' to 'test'

Step 2: Call the function that reads 'APP_MODE'

Step 3: Verify the function returns 'test'

Step 4: Use monkeypatch.setenv to set 'APP_MODE' to 'production'

Step 5: Call the function again

Step 6: Verify the function returns 'production'

✅ Expected Result: The function returns the environment variable value as set by monkeypatch.setenv in each case

Automation Requirements - pytest

Assertions Needed:

Assert the function returns 'test' after monkeypatch.setenv('APP_MODE', 'test')

Assert the function returns 'production' after monkeypatch.setenv('APP_MODE', 'production')

Best Practices:

Use the pytest monkeypatch fixture parameter in the test function

Use monkeypatch.setenv to temporarily set environment variables

Keep tests isolated and independent

Use clear and descriptive assertion messages

Automated Solution

PyTest

import os
import pytest

def get_app_mode():
    return os.getenv('APP_MODE', 'default')


def test_get_app_mode(monkeypatch):
    # Set APP_MODE to 'test' and verify
    monkeypatch.setenv('APP_MODE', 'test')
    assert get_app_mode() == 'test', "Expected APP_MODE to be 'test'"

    # Set APP_MODE to 'production' and verify
    monkeypatch.setenv('APP_MODE', 'production')
    assert get_app_mode() == 'production', "Expected APP_MODE to be 'production'"

This test uses the monkeypatch fixture provided by pytest to temporarily set environment variables.

First, it sets APP_MODE to 'test' and calls get_app_mode(). The assertion checks that the function returns 'test'.

Then, it sets APP_MODE to 'production' and asserts the function returns 'production'.

Using monkeypatch.setenv ensures the environment variable changes only affect this test and are cleaned up automatically.

Common Mistakes - 3 Pitfalls

Not using the monkeypatch fixture and setting environment variables globally

Forgetting to import os and using os.getenv in the function

Not asserting the function output after setting environment variables

Bonus Challenge

Now add data-driven testing with 3 different APP_MODE values: 'test', 'production', and 'development'

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