optimize compact_path() in pytest discovery for the common case#26016
optimize compact_path() in pytest discovery for the common case#26016vaclavHala wants to merge 1 commit into
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…ommon case before using expensive relative_to
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Hello @eleanorjboyd should I create a separate issue for this? |
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Pull request overview
This PR optimizes pytest discovery payload compaction by avoiding the relatively expensive pathlib.Path.relative_to() for the common case where a node’s absolute path is nested under the discovery root, using a cheaper string-prefix check and substring instead.
Changes:
- Adds a fast-path in
compact_path()usingstr.startswith()+ slicing, and passes precomputed base-path strings through the compaction pipeline to avoid repeated conversions. - Updates
compact_test_id()/compact_test_node()/create_compact_discovery_payload()signatures and call sites to propagate the precomputed base strings. - Updates an existing discovery compaction test to match the new function signatures.
Show a summary per file
| File | Description |
|---|---|
python_files/vscode_pytest/__init__.py |
Introduces the startswith/substring optimization and threads precomputed base-path strings through the compacting functions. |
python_files/tests/pytestadapter/test_discovery.py |
Updates existing assertions to call the new signatures for compact_path() / compact_test_id(). |
Review details
- Files reviewed: 2/2 changed files
- Comments generated: 2
- Review effort level: Low
| # pathlib.Path.relative_to is an expensive operation, | ||
| # for common cases where path is nested in path_base and path_base is therefore a prefix | ||
| # we skip the expensive check and just chop off the prefix to yield relative path | ||
| if path_str.startswith(path_base_str): | ||
| # +1 because pathlib.Path never ends by a trailing separator which we also need to chop off: | ||
| # path_base_str= /some/prefix | ||
| # path_str= /some/prefix/tests/mytest.py | ||
| rel_str = path_str[(len(path_base_str) + 1):] | ||
| return "." if rel_str == "" else rel_str |
| def test_compact_discovery_payload_keeps_paths_outside_base_absolute(tmp_path): | ||
| base_path = tmp_path / "workspace" | ||
| external_file = tmp_path / "external" / "test_external.py" | ||
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| assert vscode_pytest.compact_path(external_file, base_path) == os.fspath(external_file) | ||
| assert vscode_pytest.compact_path(external_file, base_path, str(base_path)) == os.fspath(external_file) | ||
| assert ( | ||
| vscode_pytest.compact_test_id(f"{os.fspath(external_file)}::test_external", base_path) | ||
| vscode_pytest.compact_test_id(f"{os.fspath(external_file)}::test_external", base_path, str(base_path)) | ||
| == f"{os.fspath(external_file)}::test_external" | ||
| ) |
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Hello @eleanorjboyd , I have now looked at the actual data that gets sent in the discovery payload, and I wonder if, rather than applying this band-aid, we could perhaps remove some unnecessary data completely and recalculate it once the message is received in the In particular:
Do you see some problem with this approach or should I try to implement it? The experiment I did shows for large parameterized suites there will be significant reduction in data that has to be transferred, and I expect computation time may also be lower as we will have to go through the code that deals with computing the relative paths fewer times |
I tested the latest changes to discovery where absolute paths get compacted to global prefix + relative path in each test node, and this change has made the discovery 2 to 3 times slower for us.
The problem is mainly the
pathlib.Path.relative_tooperation which is relatively expensive:This PR adds optimization which first checks using simple
str.startswithif the test is in some subfolder of the root, in which case creating of the relative path is done as trivial (and cheap) substring:I pass the base paths as both
strandpathlib.Path.relative_toso each operation can use whichever form of the base path is needed without having to convert to the other, i.e.str(pathlibPath), which in my testing also adds noticeable overhead.