Test discovery rules in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Pytest test discovery with file naming

Given the following files in a directory, which files will pytest discover as test files by default?

PyTest

test_example.py
example_test.py
testexample.py
example.py

A All four files

B test_example.py only

C test_example.py, example_test.py, and testexample.py

D test_example.py and example_test.py only

💡 Hint

Pytest looks for files starting with 'test_' or ending with '_test.py'.

Pytest discovers test files that match the pattern test_*.py or *_test.py. So test_example.py and example_test.py are discovered. testexample.py and example.py do not match these patterns.

❓ assertion

intermediate

Pytest test function discovery rules

Which of the following function names will pytest discover as test functions inside a test file?

PyTest

def test_add(): pass
def add_test(): pass
def Test_add(): pass
def testAdd(): pass

A test_add and add_test

B Only test_add

C test_add, add_test, and Test_add

D All four functions

💡 Hint

Pytest looks for functions matching the 'test_*' pattern (starts with 'test_').

Pytest discovers test functions whose names match the default test_* pattern (using fnmatch), meaning they must start with test_ (lowercase 'test' followed by an underscore). Only test_add matches. add_test does not start with test_, Test_add starts with uppercase 'T', and testAdd lacks the underscore after 'test'.

🔧 Debug

advanced

Why is this test function not discovered by pytest?

Consider this test file named test_math.py: ```python def _test_subtract(): assert 5 - 3 == 2 ``` Why does pytest not run this test function?

PyTest

def _test_subtract():
    assert 5 - 3 == 2

A Function name starts with an underscore, so pytest ignores it

B Function name must be exactly 'test_subtract' to be discovered

C Function is missing a return statement

D Function is missing the @pytest.mark.test decorator

💡 Hint

Pytest ignores functions starting with underscore by default.

Pytest discovers functions starting with 'test' but not those starting with an underscore. The underscore prefix makes pytest ignore the function.

🧠 Conceptual

advanced

Pytest test class discovery rules

Which of the following classes will pytest discover test methods from inside a test file?

PyTest

class TestCalculator:
    def test_add(self): pass

class CalculatorTest:
    def test_subtract(self): pass

class testCalculator:
    def test_multiply(self): pass

class Test_calculator:
    def test_divide(self): pass

A TestCalculator and Test_calculator only

B All four classes

C TestCalculator, CalculatorTest, and Test_calculator

D TestCalculator only

💡 Hint

Pytest discovers classes starting with 'Test' (uppercase T) and ignores others.

Pytest discovers classes whose names start with 'Test' exactly (uppercase T). So TestCalculator and Test_calculator are discovered. CalculatorTest ends with 'Test' but does not start with it, so it is ignored. testCalculator starts with lowercase 't' so it is ignored.

❓ framework

expert

Customizing pytest test discovery patterns

You want pytest to discover test files that start with 'check_' instead of 'test_'. Which pytest configuration option should you set in pytest.ini to achieve this?

A

[pytest]
file_patterns = check_*.py

B

[pytest]
python_functions = check_*

C

[pytest]
python_files = check_*.py

D

[pytest]
test_file_pattern = check_*.py

💡 Hint

The option to change test file discovery pattern is 'python_files'.

In pytest.ini, the option python_files controls which file patterns pytest uses to discover test files. Setting python_files = check_*.py makes pytest look for files starting with 'check_'.

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