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show where your regex match assertion failed!


pip install re-assert


re-assert provides a helper class to make assertions of regexes simpler.

re_assert.Matches(pattern: str, *args, **kwargs)

construct a Matches object.

note: under the hood, re-assert uses the regex library for matching, any *args / **kwargs that regex.compile supports will work. in general, the regex library is 100% compatible with the re library (and will even accept its flags, etc.)

re_assert.Matches.from_pattern(pattern: Pattern[str]) -> Matches

construct a Matches object from an already-compiled regex.

this is useful (for instance) if you're testing an existing compiled regex.

>>> import re
>>> reg = re.compile('foo')
>>> Matches.from_pattern(reg) == 'fork'
>>> Matches.from_pattern(reg) == 'food'

Matches.__eq__(other) (==)

the equality operator is overridden for use with assertion frameworks such as pytest

>>> pat = Matches('foo')
>>> pat == 'bar'
>>> pat == 'food'

Matches.__repr__() (repr(...))

a side-effect of an equality failure changes the repr(...) of a Matches object. this allows for useful pytest assertion messages:

>       assert Matches('foo') == 'fork'
E       AssertionError: assert Matches('foo'...ork\n    #    ^ == 'fork'
E         -Matches('foo')\n
E         -    # regex failed to match at:\n
E         -    #\n
E         -    #> fork\n
E         -    #    ^
E         +'fork'

Matches.assert_matches(s: str)

if you're using some other test framework, this method is useful for producing a readable traceback

>>> Matches('foo').assert_matches('food')
>>> Matches('foo').assert_matches('fork')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/asottile/workspace/re-assert/", line 63, in assert_matches
    assert self == s, self._fail
AssertionError:  regex failed to match at:

> fork
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