monkeypatch fixture in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the monkeypatch fixture in pytest?

The monkeypatch fixture allows you to safely modify or replace attributes, dictionaries, environment variables, or functions during a test, without affecting other tests.

beginner

How do you use monkeypatch to change an environment variable in a test?

Use monkeypatch.setenv('VAR_NAME', 'value') to set or change an environment variable temporarily during the test.

beginner

What happens to changes made by monkeypatch after the test finishes?

All changes made by monkeypatch are automatically undone after the test, restoring the original state to avoid side effects on other tests.

intermediate

Show an example of using monkeypatch to replace a function during a test.

Example:<br><pre>def fake_func():
    return 'fake'

import module

def test_func(monkeypatch):
    monkeypatch.setattr(module, 'func', fake_func)
    assert module.func() == 'fake'</pre>

intermediate

Why is monkeypatch preferred over directly modifying global variables or functions in tests?

Because monkeypatch safely isolates changes to the test scope and automatically reverts them, preventing unexpected side effects and making tests more reliable.

What does monkeypatch.setattr() do in pytest?

A Runs a test multiple times

B Replaces an attribute or function temporarily during a test

C Permanently changes a function in the codebase

D Skips a test

monkeypatch.setattr() temporarily replaces an attribute or function only during the test, then restores it after.

How does monkeypatch affect environment variables during a test?

A It permanently changes them

B It copies environment variables to a file

C It ignores environment variables

D It temporarily sets or deletes them for the test duration

monkeypatch.setenv() and monkeypatch.delenv() temporarily modify environment variables only during the test.

What happens if you forget to use monkeypatch and modify globals directly in tests?

A Other tests may fail due to leftover changes

B Tests run faster

C Nothing, tests are isolated automatically

D The test framework will fix it

Direct global changes can cause side effects, making other tests fail unpredictably.

Which of these is NOT a feature of monkeypatch?

A Temporarily replace functions or attributes

B Modify environment variables temporarily

C Automatically generate test data

D Automatically undo changes after test

monkeypatch does not generate test data; it only modifies code or environment temporarily.

How do you use monkeypatch in a pytest test function?

A Add <code>monkeypatch</code> as a parameter to the test function

B Import and call <code>monkeypatch()</code> inside the test

C Use a decorator <code>@monkeypatch</code>

D Call <code>pytest.monkeypatch()</code> globally

pytest injects the monkeypatch fixture when you add it as a parameter to your test function.

Explain how the monkeypatch fixture helps keep tests isolated and reliable.

Think about how changing something only during a test helps avoid problems.

Describe a scenario where you would use monkeypatch.setattr() in a test.

Imagine you want to test code that calls a function you can't or don't want to run.

Practice

1. What is the primary purpose of the monkeypatch fixture in pytest?

easy

A. To temporarily replace parts of the code during tests

B. To permanently change the source code

C. To speed up the pytest installation

D. To generate test reports automatically

Solution

1. Step 1: Understand monkeypatch usage

   The monkeypatch fixture is used to temporarily replace or modify parts of the code during test execution.

2. Step 2: Differentiate temporary vs permanent changes

   It does not permanently change source code but only affects the test runtime environment.

3. Final Answer:

   To temporarily replace parts of the code during tests -> Option A

4. Quick Check:

   Temporary code replacement = B [OK]

Hint: Remember monkeypatch changes code only during tests [OK]

Common Mistakes:

• Thinking monkeypatch changes code permanently
• Confusing monkeypatch with test report tools
• Assuming monkeypatch speeds up pytest installation

2. Which of the following is the correct way to use monkeypatch to replace a function get_data in a module my_module?

easy

A. monkeypatch.replace('my_module.get_data', lambda: 'mocked')

B. monkeypatch.setattr(my_module.get_data, lambda: 'mocked')

C. monkeypatch.setattr('my_module.get_data', lambda: 'mocked')

D. monkeypatch.patch('my_module.get_data', lambda: 'mocked')

Solution

1. Step 1: Recall monkeypatch syntax

   The correct method to replace an attribute or function is monkeypatch.setattr(target, value) where target is a string path.

2. Step 2: Identify correct target format

   The target must be a string like 'my_module.get_data', not the function object itself.

3. Final Answer:

   monkeypatch.setattr('my_module.get_data', lambda: 'mocked') -> Option C

4. Quick Check:

   Use setattr with string target = A [OK]

Hint: Use setattr with string path to replace functions [OK]

Common Mistakes:

• Passing function object instead of string path
• Using non-existent monkeypatch methods like replace or patch
• Confusing monkeypatch with unittest.mock syntax

3. Given the code below, what will be the output of the test?

def get_value():
    return 10

def test_value(monkeypatch):
    monkeypatch.setattr('__main__.get_value', lambda: 5)
    assert get_value() == ?

medium

A. 10

B. 5

C. AssertionError

D. TypeError

Solution

1. Step 1: Understand monkeypatch effect on get_value

   The monkeypatch replaces get_value in the __main__ module with a lambda returning 5.

2. Step 2: Evaluate the assertion

   Calling get_value() now returns 5, so assert get_value() == 5 passes.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Patched function returns 5 = A [OK]

Hint: Patched function returns new value, not original [OK]

Common Mistakes:

• Expecting original return value 10
• Thinking monkeypatch causes error
• Confusing module path for patching

4. What is wrong with this test code using monkeypatch?

def test_env(monkeypatch):
    monkeypatch.setenv('API_KEY')
    assert os.getenv('API_KEY') is None

medium

A. monkeypatch cannot patch environment variables

B. os.getenv cannot be used in tests

C. assertion should check for empty string, not None

D. setenv requires two arguments: key and value

Solution

1. Step 1: Check monkeypatch.setenv usage

   The setenv method requires both a key and a value to set an environment variable.

2. Step 2: Identify missing argument

   Here, only the key 'API_KEY' is given, missing the value argument.

3. Final Answer:

   setenv requires two arguments: key and value -> Option D

4. Quick Check:

   setenv needs key and value = C [OK]

Hint: Always provide key and value to setenv [OK]

Common Mistakes:

• Calling setenv with only one argument
• Assuming os.getenv is not usable in tests
• Thinking monkeypatch can't patch env variables

5. You want to test a function that reads a config file using open. How can you use monkeypatch to avoid reading a real file and instead provide fake content?

hard

A. Use monkeypatch.setattr to replace builtins.open with a function returning a StringIO with fake data

B. Use monkeypatch.setenv to set a fake file path environment variable

C. Use monkeypatch.setitem to replace the file path in a dictionary

D. Use monkeypatch.delattr to delete the open function

Solution

1. Step 1: Identify how to fake file reading

   Replacing open with a function returning a StringIO object simulates file reading without disk access.

2. Step 2: Use monkeypatch.setattr on builtins.open

   Monkeypatch can replace builtins.open with a lambda or function returning StringIO containing fake content.

3. Final Answer:

   Use monkeypatch.setattr to replace builtins.open with a function returning a StringIO with fake data -> Option A

4. Quick Check:

   Patch open with StringIO = D [OK]

Hint: Patch builtins.open with StringIO for fake file content [OK]

Common Mistakes:

• Trying to delete open instead of replacing it
• Using setenv which doesn't fake file reading
• Replacing dictionary keys instead of open function