fixtures: fix catastrophic performance problem in reorder_items - #12409
Merged
bluetech merged 2 commits intoJun 4, 2024
Merged
Conversation
Copy link
Copy Markdown
Member
Sorry, something went wrong.
This makes some minor clarity and performance improvements to the code.
nicoddemus
approved these changes
Jun 3, 2024
nicoddemus
left a comment
Copy link
Copy Markdown
Member
There was a problem hiding this comment.
LGTM, thanks for taking the time to look into this!
Sorry, something went wrong.
Comment thread
changelog/12355.bugfix.rst
Outdated
| @@ -0,0 +1 @@ | |||
| Fix possible catasrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. | |||
Copy link
Copy Markdown
Member
There was a problem hiding this comment.
Suggested change
| Fix possible catasrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. | |
| Fix possible catastrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. |
Sorry, something went wrong.
Fix pytest-dev#12355. In the issue, it was reported that the `reorder_items` has quadratic (or worse...) behavior with certain simple parametrizations. After some debugging I found that the problem happens because the "Fix items_by_argkey order" loop keeps adding the same item to the deque, and it reaches epic sizes which causes the slowdown. I don't claim to understand how the `reorder_items` algorithm works, but if as far as I understand, if an item already exists in the deque, the correct thing to do is to move it to the front. Since a deque doesn't have such an (efficient) operation, this switches to `OrderedDict` which can efficiently append from both sides, deduplicate and move to front.
bluetech
force-pushed
the
reorder-items-perf
branch
from
7a230b0 to
e89d23b
Compare
June 4, 2024 07:15
webknjaz
reviewed
Mar 19, 2025
Comment thread
src/_pytest/fixtures.py
| other_scoped_items_by_argkey = items_by_argkey[other_scope] | ||
| for argkey in argkeys_by_item[other_scope].get(i, ()): | ||
| other_scoped_items_by_argkey[argkey][i] = None | ||
| other_scoped_items_by_argkey[argkey].move_to_end( |
Copy link
Copy Markdown
Member
There was a problem hiding this comment.
@bluetech I've hit some corner case on PyPy where move_to_end() seems to be raising a KeyError and crashing the item collection entirely...
Sorry, something went wrong.
Copy link
Copy Markdown
Member
There was a problem hiding this comment.
Oh, nevermind. This is a regression of v8.2.2 and this PR only hit release in v8.3.0.
Nope, it was backported into 8.2.2: #13312
Sorry, something went wrong.
webknjaz
added a commit
to webknjaz/multidict
that referenced
this pull request
Mar 19, 2025
This patch temporarily restricts pytest version below 8.2.2 under PyPy due to a discovered regression that it introduced [[1]]. The regression has been observed on at least `pypy3.9-7.3.16`, `pypy3.10-7.3.19` and `pypy3.11-7.3.19`. It can be triggered by running the following in affected runtimes: pytest --collect-only --no-cov tests/test_abc.py tests/test_copy.py tests/test_incorrect_args.py tests/test_multidict.py tests/test_mypy.py tests/test_pickle.py tests/test_types.py tests/test_update.py tests/test_version.py [1]: pytest-dev/pytest#13312 [2]: pytest-dev/pytest#12414 [3]: pytest-dev/pytest#12409
2 tasks
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Reviewers
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
First commit is a cleanup commit (no functional changes intended).
The second commit fixes #12355.
In the issue, it was reported that the reorder_items has quadratic (or worse...) behavior with certain simple parametrizations. After some debugging I found that the problem happens because the "Fix items_by_argkey order" loop keeps adding the same item to the deque, and it reaches epic sizes which causes the slowdown.
I don't claim to understand how the reorder_items algorithm works, but if as far as I understand, if an item already exists in the deque, the correct thing to do is to move it to the front. Since a deque doesn't have such an (efficient) operation, this switches to OrderedDict which can efficiently append from both sides, deduplicate and move to front.