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'

Show Hint

Practice

1.

What is the main purpose of monkeypatch.setenv in pytest?

easy

A. Create new environment variables outside tests

B. Permanently change environment variables on the system

C. Temporarily change environment variables during a test

D. Delete environment variables permanently

Solution

1. Step 1: Understand the role of monkeypatch in pytest

   Monkeypatch is used to temporarily modify or replace parts of the environment during tests without permanent effects.

2. Step 2: Focus on setenv method

   The setenv method specifically changes environment variables only for the duration of the test.

3. Final Answer:

   Temporarily change environment variables during a test -> Option C

4. Quick Check:

   monkeypatch.setenv = temporary env change [OK]

Hint: Remember: monkeypatch.setenv only affects test-time env variables [OK]

Common Mistakes:

• Thinking setenv changes env variables permanently
• Confusing setenv with creating new system variables
• Assuming changes persist after test ends

2.

Which of the following is the correct syntax to set an environment variable API_KEY to 12345 using monkeypatch.setenv in a pytest test function?

def test_api_key(monkeypatch):
    # Your code here
    assert ...

easy

A. monkeypatch.setenv('API_KEY', '12345')

B. monkeypatch.setenv('API_KEY', 12345)

C. monkeypatch.setenv(API_KEY='12345')

D. monkeypatch.setenv('API_KEY') = '12345'

Solution

1. Step 1: Check method signature of setenv

   setenv takes two string arguments: the variable name and its value.

2. Step 2: Identify correct string usage

   The value must be a string, so '12345' is correct, not integer 12345.

3. Final Answer:

   monkeypatch.setenv('API_KEY', '12345') -> Option A

4. Quick Check:

   setenv('VAR', 'value') syntax [OK]

Hint: Always pass env var values as strings in setenv [OK]

Common Mistakes:

• Passing integer instead of string as value
• Using keyword arguments instead of positional
• Trying to assign value with '=' outside method call

3.

What will be the output of the following pytest test?

import os

def test_env(monkeypatch):
    monkeypatch.setenv('MODE', 'test')
    mode = os.getenv('MODE')
    assert mode == 'test'
    print(mode)

medium

A. AssertionError

B. None

C. MODE

D. test

Solution

1. Step 1: Understand monkeypatch.setenv effect

   It sets environment variable 'MODE' to 'test' temporarily during the test.

2. Step 2: Check os.getenv call and assertion

   os.getenv('MODE') returns 'test', so assertion passes and print outputs 'test'.

3. Final Answer:

   test -> Option D

4. Quick Check:

   setenv sets env var, getenv reads it [OK]

Hint: setenv changes env var for test, getenv reads it [OK]

Common Mistakes:

• Expecting None because env var not set globally
• Thinking assertion fails
• Confusing variable name with value

4.

Identify the error in this pytest test using monkeypatch.setenv:

def test_path(monkeypatch):
    monkeypatch.setenv(PATH)
    assert os.getenv('PATH') == '/usr/bin'

medium

A. Missing import of os module

B. setenv missing value argument

C. Incorrect assertion syntax

D. Using monkeypatch outside test function

Solution

1. Step 1: Check setenv method call

   setenv requires two arguments: variable name and value. Here only one argument is given.

2. Step 2: Verify assertion and imports

   Assertion syntax is correct and os module import is assumed; error is in setenv call.

3. Final Answer:

   setenv missing value argument -> Option B

4. Quick Check:

   setenv needs two args: name and value [OK]

Hint: setenv always needs two strings: var name and value [OK]

Common Mistakes:

• Calling setenv with only one argument
• Forgetting to import os module
• Miswriting assertion statement

5.

You want to test a function that behaves differently based on the environment variable FEATURE_FLAG. Which pytest test code correctly uses monkeypatch.setenv to test the function with FEATURE_FLAG set to enabled and ensures the environment is restored after the test?

def test_feature(monkeypatch):
    # Your code here
    result = feature_function()
    assert result == 'Feature Enabled'

hard

A. monkeypatch.setenv('FEATURE_FLAG', 'enabled')

B. os.environ['FEATURE_FLAG'] = 'enabled'

C. monkeypatch.setenv(FEATURE_FLAG='enabled')

D. monkeypatch.setenv('FEATURE_FLAG') = 'enabled'

Solution

1. Step 1: Identify correct usage of monkeypatch.setenv

   setenv requires two string arguments: variable name and value, passed positionally.

2. Step 2: Understand environment restoration

   Using monkeypatch.setenv ensures the env var is set only during the test and restored after, unlike direct os.environ assignment.

3. Final Answer:

   monkeypatch.setenv('FEATURE_FLAG', 'enabled') -> Option A

4. Quick Check:

   Use monkeypatch.setenv for temporary env changes [OK]

Hint: Use monkeypatch.setenv for temp env vars, not os.environ [OK]

Common Mistakes:

• Directly modifying os.environ without monkeypatch
• Using keyword arguments in setenv
• Trying to assign value outside setenv call