assert str1 == str2 takes forever with long strings that differ by a short prefix #8998
Description
Environment and versions:
platform linux -- Python 3.9.5, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python
cachedir: .pytest_cache
rootdir: /home/edited, configfile: pytest.ini
plugins: xdist-2.3.0, timeout-1.4.2, forked-1.3.0
timeout: 10.0s
timeout method: signal
timeout func_only: False
To reproduce: download test.py and run pytest test.py --timeout 10 -vv
EXPECTED: both test methods (test1 and test2) fail quickly with assert False
OBSERVED: test1 fails quickly and test2 timeouts after 10 seconds (or runs forever if no --timeout flag passed)
The difference between test1 and test2 is simple
- test1 does
x = ' long string' == 'long string'; assert xand behaves as expected, but - test2 does straight
assert ' long string' == 'long string'and pytest substitutes '==' with its logic that, apparently, has the bug.
The "long string" part of the literal strings is the same four times: twice in test1 and twice in test2.
The two strings that are compared with == both times differ by one leading space character only.
The actual strings in test.py are a bit longer (but not crazy long, some 159000 ANSI characters/bytes), but they differ just by one leading space still.
Full output attached as output.txt, that includes the interesting stack trace which hints at where the execution is happening at the moment when the execution is cancelled by the timeout. I will paste that stack trace here for convenience:
../../.local/lib/python3.9/site-packages/_pytest/assertion/rewrite.py:485: in _call_reprcompare
custom = util._reprcompare(ops[i], each_obj[i], each_obj[i + 1])
../../.local/lib/python3.9/site-packages/_pytest/assertion/__init__.py:141: in callbinrepr
hook_result = ihook.pytest_assertrepr_compare(
../../.local/lib/python3.9/site-packages/pluggy/hooks.py:286: in __call__
return self._hookexec(self, self.get_hookimpls(), kwargs)
../../.local/lib/python3.9/site-packages/pluggy/manager.py:93: in _hookexec
return self._inner_hookexec(hook, methods, kwargs)
../../.local/lib/python3.9/site-packages/pluggy/manager.py:84: in <lambda>
self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
../../.local/lib/python3.9/site-packages/_pytest/assertion/__init__.py:179: in pytest_assertrepr_compare
return util.assertrepr_compare(config=config, op=op, left=left, right=right)
../../.local/lib/python3.9/site-packages/_pytest/assertion/util.py:153: in assertrepr_compare
explanation = _compare_eq_any(left, right, verbose)
../../.local/lib/python3.9/site-packages/_pytest/assertion/util.py:176: in _compare_eq_any
explanation = _diff_text(left, right, verbose)
../../.local/lib/python3.9/site-packages/_pytest/assertion/util.py:241: in _diff_text
explanation += [
../../.local/lib/python3.9/site-packages/_pytest/assertion/util.py:241: in <listcomp>
explanation += [
/usr/lib/python3.9/difflib.py:912: in compare
yield from g
/usr/lib/python3.9/difflib.py:979: in _fancy_replace
cruncher.ratio() > best_ratio:
/usr/lib/python3.9/difflib.py:651: in ratio
matches = sum(triple[-1] for triple in self.get_matching_blocks())
/usr/lib/python3.9/difflib.py:486: in get_matching_blocks
i, j, k = x = self.find_longest_match(alo, ahi, blo, bhi)
Thanks!