Test discovery rules in PyTest

Introduction

Test discovery rules help pytest find your test files and test functions automatically. This saves time and avoids mistakes in running tests.

When you want pytest to find and run all your tests without listing them manually.

When organizing tests in multiple files and folders.

When adding new tests and you want them to run automatically.

When you want to avoid running non-test code by mistake.

When you want consistent test naming to keep your project clean.

Syntax

PyTest

pytest discovers tests by looking for:
- Files named test_*.py or *_test.py
- Functions named test_* inside those files
- Classes named Test* without an __init__ method
- Methods inside those classes named test_*

Test files must be named with test_ prefix or _test suffix.

Test functions and methods must start with test_ to be discovered.

Examples

This file and function follow discovery rules and will be found by pytest.

PyTest

# File: test_math.py

def test_add():
    assert 1 + 1 == 2

Class name starts with Test and method starts with test_, so pytest finds this test.

PyTest

# File: math_test.py

class TestMath:
    def test_subtract(self):
        assert 2 - 1 == 1

This file and function do not follow naming rules, so pytest will ignore them.

PyTest

# File: example.py

def add():
    return 1 + 1

Sample Program

This file has two tests that pytest will find and run. The method not_a_test is ignored because it does not start with test_.

PyTest

# test_sample.py

def test_true_is_true():
    assert True is True

class TestNumbers:
    def test_one_plus_one(self):
        assert 1 + 1 == 2

    def not_a_test(self):
        assert False  # This will not run

# Run pytest in terminal with: pytest

Important Notes

pytest ignores files and functions that do not follow the naming rules.

You can customize discovery rules with command line options or configuration files if needed.

Keeping consistent naming helps pytest find tests quickly and avoids confusion.

Summary

pytest finds tests by looking for files, classes, and functions with specific name patterns.

Test files: start or end with test, test functions and methods: start with test_.

Following these rules means pytest runs your tests automatically without extra setup.

Practice

1. Which of the following file names will pytest recognize as a test file by default?

easy

A. example.py

B. test_example.py

C. example_test.pyc

D. TestExample.py

Solution

1. Step 1: Understand pytest file naming rules

   pytest looks for files starting or ending with 'test' and ending with '.py'.

2. Step 2: Check each option

   'test_example.py' starts with 'test' and ends with '.py', so pytest will find it. Others either don't end with '.py' or don't start/end with 'test'.

3. Final Answer:

   test_example.py -> Option B

4. Quick Check:

   File starts or ends with 'test' + '.py' = test_example.py [OK]

Hint: Test files start or end with 'test' and end with '.py' [OK]

Common Mistakes:

• Thinking pytest finds .pyc files
• Ignoring case sensitivity in file names
• Assuming any .py file is a test file

2. Which function name will pytest recognize as a test function by default?

easy

A. test_login()

B. testLogin()

C. check_login()

D. login_test()

Solution

1. Step 1: Recall pytest test function naming

   pytest looks for functions starting with 'test_'.

2. Step 2: Evaluate each function name

   'test_login()' starts with 'test_' exactly, so pytest will find it. Others do not follow this pattern.

3. Final Answer:

   test_login() -> Option A

4. Quick Check:

   Function starts with 'test_' = test_login() [OK]

Hint: Test functions start with 'test_' exactly [OK]

Common Mistakes:

• Using camelCase instead of snake_case
• Placing 'test' at the end of function name
• Missing underscore after 'test'

3. Given the following files and functions, which tests will pytest discover and run?

File: test_math.py
  def test_add(): pass
  def add_test(): pass

File: math_test.py
  def test_subtract(): pass
  def subtract_test(): pass

File: math.py
  def test_multiply(): pass

medium

A. test_add, test_subtract

B. test_add, test_subtract, test_multiply

C. test_add, subtract_test

D. add_test, subtract_test

Solution

1. Step 1: Identify test files pytest finds

   pytest finds files starting or ending with 'test' and ending with '.py'. So 'test_math.py' and 'math_test.py' are test files. 'math.py' is not.

2. Step 2: Identify test functions pytest finds in those files

   In 'test_math.py', pytest finds 'test_add' (starts with 'test_'). 'add_test' is ignored. In 'math_test.py', pytest finds 'test_subtract'. 'subtract_test' is ignored. 'test_multiply' in 'math.py' is ignored because file is not a test file.

3. Final Answer:

   test_add, test_subtract -> Option A

4. Quick Check:

   Files and functions follow naming rules = test_add, test_subtract [OK]

Hint: Test files and functions must follow naming rules to be discovered [OK]

Common Mistakes:

• Assuming any function starting with 'test' is found regardless of file name
• Including functions in non-test files
• Confusing function name endings with beginnings

4. You have a test file named TestExample.py with a function def test_func():. pytest does not run this test. What is the likely cause?

medium

A. Function name does not start with 'test_'

B. pytest requires test classes to run tests

C. File name does not follow pytest discovery rules

D. Function is missing a return statement

Solution

1. Step 1: Check pytest file discovery rules

   pytest finds files starting or ending with 'test' in lowercase and ending with '.py'. 'TestExample.py' starts with uppercase 'T', so it is not recognized.

2. Step 2: Verify function name and other options

   Function name 'test_func' is correct. Return statement is not required. pytest does not require test classes to run functions.

3. Final Answer:

   File name does not follow pytest discovery rules -> Option C

4. Quick Check:

   File name case-sensitive for discovery = D [OK]

Hint: File names must be lowercase starting or ending with 'test' [OK]

Common Mistakes:

• Ignoring case sensitivity in file names
• Thinking function needs return statement
• Assuming test classes are mandatory

5. You want pytest to discover tests in a file named mytests.py containing functions def test_one(): and def test_two():. By default, pytest does not find this file. How can you rename the file to follow pytest discovery rules without changing function names?

hard

A. Rename to my_test_s.py

B. Rename to mytestsTest.py

C. Rename to testmytests.py

D. Rename to test_mytests.py

Solution

1. Step 1: Recall pytest file discovery naming

   pytest finds files starting with 'test_' or ending with '_test.py'.

2. Step 2: Check options for correct naming

   'test_mytests.py' starts with 'test_' [check]. 'mytestsTest.py' ends with 'Test.py' (uppercase 'T' not matching lowercase 'test'). 'testmytests.py' lacks '_' after 'test'. 'my_test_s.py' does not start with 'test_' or end with '_test.py'. Only the first option works.

3. Final Answer:

   Rename to test_mytests.py -> Option D

4. Quick Check:

   File starts or ends with 'test' = test_mytests.py [OK]

Hint: File must start or end with 'test' exactly [OK]

Common Mistakes:

• Removing 'test' from file name
• Adding 'test' inside word without underscore
• Using camelCase instead of snake_case