Running tests (pytest command)

Introduction

We run tests to check if our code works correctly. The pytest command helps us run all tests easily.

After writing new test functions to check your code

To quickly check if recent changes broke anything

Before sharing your code with others to ensure quality

When you want a summary of which tests passed or failed

Syntax

PyTest

pytest [options] [file_or_dir]

You can run pytest without arguments to run all tests in the current folder.

Use options like -v for more details or -k to run specific tests.

Examples

Runs all tests in the current directory and subdirectories.

PyTest

pytest

Runs all tests with detailed output showing each test name and result.

PyTest

pytest -v

Runs tests only in the file test_example.py.

PyTest

pytest test_example.py

Runs tests that match the name test_function_name.

PyTest

pytest -k test_function_name

Sample Program

This simple test checks if the add function returns correct sums. Running pytest -v will show each test and if it passed.

PyTest

def add(a, b):
    return a + b

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0

# Run this in terminal:
# pytest -v

Important Notes

Make sure your test files and functions start with test_ so pytest finds them.

You can stop tests early with Ctrl+C if needed.

Summary

The pytest command runs your tests automatically.

Use options like -v for detailed results or specify files to run specific tests.

Running tests often helps catch mistakes early and keeps your code working well.

Practice

1. What does the basic pytest command do when run in a project folder?

easy

A. It only checks the syntax of test files without running them.

B. It automatically finds and runs all test files and test functions.

C. It deletes all test files in the folder.

D. It installs pytest in the project folder.

Solution

1. Step 1: Understand the default behavior of pytest command

   Running pytest without arguments searches for test files and runs all tests found.

2. Step 2: Eliminate incorrect options

   Options A, C, and D describe actions pytest does not perform by default.

3. Final Answer:

   It automatically finds and runs all test files and test functions. -> Option B

4. Quick Check:

   pytest runs tests automatically = B [OK]

Hint: pytest runs all tests by default when no file is specified [OK]

Common Mistakes:

• Thinking pytest only checks syntax
• Assuming pytest deletes files
• Confusing pytest with package installer

2. Which of the following is the correct command to run pytest with detailed output?

easy

A. pytest -v

B. pytest --detail

C. pytest -d

D. pytest --verbose-output

Solution

1. Step 1: Recall pytest option for verbose output

   The -v option stands for verbose and shows detailed test results.

2. Step 2: Check other options for correctness

   Options -d, --detail, and --verbose-output are not valid pytest flags.

3. Final Answer:

   pytest -v -> Option A

4. Quick Check:

   Verbose flag is -v = C [OK]

Hint: Use -v for verbose pytest output [OK]

Common Mistakes:

• Using -d instead of -v
• Typing long incorrect flags
• Confusing verbose with debug flags

3. Given the command pytest test_math.py, what will happen?

medium

A. pytest runs all tests in all files in the folder.

B. pytest runs tests but ignores test_math.py.

C. pytest runs only the tests inside the file named test_math.py.

D. pytest runs tests only if they are named test_math.py inside any folder.

Solution

1. Step 1: Understand pytest file argument behavior

   When a filename is given, pytest runs tests only from that file.

2. Step 2: Eliminate incorrect options

   A: Runs all tests in all files (wrong when file specified). B: Ignores test_math.py (wrong). C: Misunderstands filename as test name search (wrong).

3. Final Answer:

   pytest runs only the tests inside the file named test_math.py. -> Option C

4. Quick Check:

   pytest filename runs tests in that file only = D [OK]

Hint: Specify filename to run tests only from that file [OK]

Common Mistakes:

• Assuming pytest runs all tests ignoring filename
• Confusing filename with test function name
• Thinking pytest ignores specified files

4. You run pytest -v test_example.py but get an error: pytest: error: unrecognized arguments: -v. What is the likely cause?

medium

A. pytest does not support the -v option.

B. You typed the option incorrectly; it should be --verbose.

C. The test file test_example.py does not exist.

D. You are running pytest with a wrong command or from a wrong environment.

Solution

1. Step 1: Understand the error message

   The error says -v is unrecognized, which is unusual because -v is a valid pytest option.

2. Step 2: Identify environment or command issues

   This usually means pytest is not properly installed or a different command is running (like python or another tool).

3. Final Answer:

   You are running pytest with a wrong command or from a wrong environment. -> Option D

4. Quick Check:

   Unrecognized option means wrong environment = A [OK]

Hint: Check pytest installation and environment if options fail [OK]

Common Mistakes:

• Assuming -v is invalid option
• Ignoring environment issues
• Blaming test file existence for option errors

5. You want to run only tests marked as slow in your project using pytest. Which command correctly runs only those tests?

hard

A. pytest -m slow

B. pytest --only slow

C. pytest --mark slow

D. pytest -s slow

Solution

1. Step 1: Recall pytest marker option

   The -m option runs tests matching a marker expression, such as slow.

2. Step 2: Verify other options

   Options --only, --mark, and -s do not filter tests by marker.

3. Final Answer:

   pytest -m slow -> Option A

4. Quick Check:

   Use -m to run tests by marker = A [OK]

Hint: Use -m marker to run tests with that marker [OK]

Common Mistakes:

• Using incorrect flags like --only or --mark
• Confusing -s (capture output) with marker filtering
• Not marking tests before filtering