How to Test Exceptions in Python with pytest.raises

Learn the pytest.raises pattern to assert that specific exceptions are raised and validate their messages.

Easy Python 3.9+ Aug 9, 2026 Errors & debugging 15 views 0 copies

pytest testing exceptions unit-testing

Requires third-party packages — install first

pip install pytest

Python code

23 lines

Python 3.9+

import pytest


def divide(a: int, b: int) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b


def test_divide_by_zero_raises():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(10, 0)


def test_divide_by_zero_raises_exact_match():
    with pytest.raises(ValueError) as exc_info:
        divide(10, 0)
    assert str(exc_info.value) == "Cannot divide by zero"
    assert exc_info.type is ValueError


if __name__ == "__main__":
    pytest.main([__file__, "-v"])

Output

stdout

============================= test session starts ==============================
platform linux -- Python 3.11.0, pytest-7.4.0, pluggy-1.0.0
rootdir: /path/to/tests
collected 2 items

test_exceptions.py ..                                                    [100%]

============================== 2 passed in 0.02s ==============================

How it works

pytest.raises(ValueError) acts as a context manager; the block inside must raise the expected exception or the test fails. Using match allows you to assert a regex pattern on the exception message, while capturing the exception with as exc_info lets you inspect its type and string value. Always keep the exception type specific to catch real bugs, not broad Exception.

Common mistakes

Forgetting to import pytest, causing a NameError.
Using a broad exception type like `Exception` when a specific one is expected, hiding unrelated failures.
Neglecting to check the exception message when exact validation is needed.

Variations

Use `pytest.raises(ValueError, match='^...$')` to enforce a full regex match.
Call `pytest.raises(ValueError)` as a function: `pytest.raises(ValueError, divide, 10, 0)` for simple cases.

Real-world use cases

Verifying that a payment gateway raises ValueError for invalid amounts.
Ensuring a config parser raises FileNotFoundError when a required file is missing.
Testing that API clients raise requests.HTTPError on non‑2xx responses.

How to Test Exceptions in Python with pytest.raises

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

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