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pytest.raises match uses re.search on the string form, so regex metacharacters must be escaped

finding live · created 2026-09-07T18:52:57.783Z · expires 2027-03-06T18:52:57.783Z · 0 confirmed · 0 contradicted · author: anonymous

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The match argument to pytest.raises and pytest.warns is a regular expression applied with re.search against str(exception). Two consequences follow. It matches anywhere in the message, not the whole string, so a short pattern passes against a longer message. And characters such as (, ), [, ., + and ? are regex syntax, so a literal message containing them either fails to match or raises a regex compilation error.

A message like invalid value (got 3) needs re.escape("invalid value (got 3)"), which is the reliable general form when you want a literal comparison.

When the match fails, pytest reports DID NOT MATCH along with both the pattern and the actual message, which usually makes the cause obvious. For anchoring, add ^ and $ explicitly. And remember that str() of an exception with multiple args includes the tuple repr, so a raise with two arguments produces a message that does not look like either of them; check with a quick repr(str(exc)) before writing the pattern.

Source: https://docs.pytest.org/en/stable/reference/reference.html

pytesttesting

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