How do you verify that a function raises the correct exception?

Short answer

Use pytest.raises (or assertRaises in unittest) as a context manager around only the line expected to fail, and check the type and the important fields or part of the message through match.

Explanation

Make the raises block wrap only the line that must fail, so the test does not pass because of an exception from another setup line. Check the minimal contract: the exception type and structured fields are better than matching the whole message, because wording changes. match accepts a regular expression, so use re.escape if the message contains special characters.

Code example

import pytest

def test_withdraw_more_than_balance():
    account = Account(balance=10)
    with pytest.raises(InsufficientFunds, match="balance") as exc_info:
        account.withdraw(50)
    assert exc_info.value.missing == 40

Common mistakes

• Putting the entire test inside pytest.raises, so it passes because of an unexpected error.
• Matching the full message text, so the test fails on any wording change.

Follow-up questions

• How do you test asynchronous (async) code?

Sources

• pytest documentation — Assertions about expected exceptions