How to Test Environment Variables with pytest monkeypatch in Python

Shows how to use pytest's monkeypatch fixture to set and delete environment variables for isolated tests.

Easy Python 3.9+ Aug 9, 2026 Testing & modern typing 12 views 0 copies

pytest monkeypatch environment variables testing os.getenv

Requires third-party packages — install first

pip install pytest

Python code

13 lines

Python 3.9+

import os
import pytest

def get_database_url():
    return os.getenv("DATABASE_URL", "postgres://default")

def test_database_url_with_env(monkeypatch):
    monkeypatch.setenv("DATABASE_URL", "postgres://test-db")
    assert get_database_url() == "postgres://test-db"

def test_database_url_default(monkeypatch):
    monkeypatch.delenv("DATABASE_URL", raising=False)
    assert get_database_url() == "postgres://default"

Output

stdout

Run the snippet in the editor to inspect the return value or side effect.

How it works

The monkeypatch fixture automatically reverts changes after each test, keeping tests isolated. monkeypatch.setenv sets an environment variable for the duration of the test, while monkeypatch.delenv removes it. The code under test reads the variable with os.getenv, which returns the default if unset. This pattern makes it easy to test behavior under different configurations without touching the real environment.

Common mistakes

Forgetting to use `raising=False` with `delenv` to avoid errors when the variable doesn't exist
Not using the monkeypatch fixture, relying on manual `os.environ` cleanup that can leak state between tests

Variations

Use `monkeypatch.setenv` with a lambda for dynamic values
Use `monkeypatch.setitem` on `os.environ` directly

Real-world use cases

Testing configuration loading functions that depend on environment variables.
Simulating different deployment environments (prod, staging) in integration tests.
Ensuring default values are used when environment variables are missing.

How to Test Environment Variables with pytest monkeypatch in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Testing & modern typing

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