Pytest discovery: create_class_node re-parses the file for every class on Python < 3.13 #26240

Type: Performance Issue

Environment

• Python extension: ms-python.python 2026.10.0
• VS Code: 1.134.0
• OS: Linux (Remote - SSH)
• Python: 3.10.22
• pytest: 9.1.1

Summary

To put a line number on class nodes, vscode_pytest.create_class_node() calls
inspect.getsourcelines(class_module.obj) once per test class. On Python < 3.13, classes have no
__firstlineno__, so inspect.findsource() re-parses the entire module with ast.parse()
and walks the whole AST on every call. A file containing N test classes is therefore parsed N times.

Test discovery that takes ~4.6s with plain pytest takes ~45s in VS Code. During that time the
Testing view is empty and no gutter run buttons are shown.

Repro

Generate 20 files × 50 classes × 20 tests (20,000 tests, 1,000 classes):

# gen.py
import pathlib

root = pathlib.Path("tests")
root.mkdir(exist_ok=True)
for f in range(20):
    lines = []
    for c in range(50):
        lines.append(f"class TestGroup{c}:")
        for m in range(20):
            lines.append(f"    def test_case_{m}(self):\n        assert {m} == {m}\n")
    (root / f"test_file_{f}.py").write_text("\n".join(lines))

Run discovery with and without the extension's plugin (P = the extension's python_files dir):

python gen.py
python -m pytest --collect-only -q -p no:cacheprovider tests

TEST_RUN_PIPE=/tmp/out.json PYTHONPATH=$P:$P/lib/python \
  python -m pytest -p vscode_pytest --collect-only -q -p no:cacheprovider tests

Results (warm runs, same machine)

Profile (Python 3.10, repro above)

ncalls  cumtime  function
     1   56.521  vscode_pytest/__init__.py:698(build_test_tree)
  1000   54.293  vscode_pytest/__init__.py:877(create_class_node)
  1000   54.263  inspect.py:1112(getsourcelines)
  1000   51.794  inspect.py:932(findsource)
  1000   30.349  ast.py:33(parse)
  1067   30.345  {built-in method builtins.compile}

Nearly all of discovery time goes to findsource → ast.parse for classes. Times are inflated by cProfile overhead (45s wall-clock without profiling); the proportions are what matter.

Root cause

python_files/vscode_pytest/__init__.py:

def create_class_node(class_module):
    ...
    _, lineno = inspect.getsourcelines(class_module.obj)

• Only lineno is needed. getsourcelines also extracts the full source block, which is extra work.
• On CPython < 3.13, inspect.findsource() for a class runs ast.parse(source) plus a
  _ClassFinder visit of the whole file on every call. Nothing caches it, so the cost is
  O(classes × file size).
• On 3.13+ it reads cls.__firstlineno__, which is why 3.13 is fine.