Hello,
While working on #12444, I used pytest.approx to compare two dictionaries with different keys.
The error message is pretty confusing. Instead of telling me that the keys don't match, I get an AssertionError with a representation issue and a KeyError for the missing key.
What I expected: A clearer message saying the dictionaries have different keys, something like: "comparison failed. dictionary has different keys: expected (a, c), got (a, b)".
What I got instead:
AssertionError: assert approx({'a': ... 4 ± 4.0e-06}) == {'a': 1, 'c': 3}
(pytest_assertion plugin: representation of details failed: /home/cmnemoi/code/repro_bugs/.venv/lib/python3.13/site-packages/_pytest/python_api.py:271: KeyError: 'b'.
Probably an object has a faulty __repr__.)
How to reproduce
import pytest
def test_approx_dicts_with_mismatch_on_keys() -> None:
expected = {"a": 1, "c": 3}
actual = {"a": 1, "b": 4}
assert pytest.approx(actual) == expectedEnvironment
- pytest version: 8.4.2
- Operating System: Pop_OS! 22.04 (GNU/Linux)
Installed Packages
Package Version Editable project location
----------------------- --------- -----------------------------
annotated-types 0.7.0
certifi 2025.10.5
charset-normalizer 3.4.4
click 8.1.8
click-option-group 0.5.9
coverage 7.10.7
Deprecated 1.2.18
dotty-dict 1.3.1
filelock 3.20.0
gitdb 4.0.12
GitPython 3.1.45
idna 3.11
importlib_resources 6.5.2
iniconfig 2.1.0
Jinja2 3.1.6
markdown-it-py 4.0.0
MarkupSafe 3.0.3
mdurl 0.1.2
mypy 1.18.2
mypy_extensions 1.1.0
packaging 25.0
pathspec 0.12.1
pip 25.2
pluggy 1.6.0
pydantic 2.12.2
pydantic_core 2.41.4
Pygments 2.19.2
pytest 8.4.2
pytest-cov 7.0.0
pytest-mypy 1.0.1
pytest-watcher 0.4.3
python-gitlab 6.4.0
python-semantic-release 10.4.1
requests 2.32.5
requests-toolbelt 1.0.1
rich 14.2.0
ruff 0.14.0
shellingham 1.5.4
smmap 5.0.2
tomlkit 0.13.3
typing_extensions 4.15.0
typing-inspection 0.4.2
urllib3 2.5.0
watchdog 6.0.0
wrapt 1.17.3
Hello,
While working on #12444, I used
pytest.approxto compare two dictionaries with different keys.The error message is pretty confusing. Instead of telling me that the keys don't match, I get an
AssertionErrorwith a representation issue and aKeyErrorfor the missing key.What I expected: A clearer message saying the dictionaries have different keys, something like: "comparison failed. dictionary has different keys: expected (a, c), got (a, b)".
What I got instead:
How to reproduce
Environment
Installed Packages