fix(frontend): keep warnings about analysed sources off the analyzer's stderr

src/coretrace_python/frontend/parser.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import ast
+import warnings
 
 from coretrace_python.source import SourceFile
 
@@ -11,7 +12,12 @@ class ParseError(Exception):
 
 def parse_source(source: str, filename: str = "<unknown>") -> ast.Module:
     try:
-        return ast.parse(source, filename=filename)
+        with warnings.catch_warnings():
+            # A warning raised while parsing (an invalid escape sequence: a SyntaxWarning
+            # from 3.12, a DeprecationWarning on 3.11) is about the analysed source, not
+            # the analyzer, and would reach its stderr once per process that parses (#241).
+            warnings.simplefilter("ignore")
+            return ast.parse(source, filename=filename)
     except SyntaxError as error:
         line = error.lineno or 0
         column = error.offset or 0

tests/test_parse_warnings.py

@@ -0,0 +1,59 @@
+"""Acceptance tests for issue #241: a warning Python raises while parsing an analysed
+source is about that source, not about the analyzer, and never reaches its stderr.
+
+An invalid escape sequence is a ``SyntaxWarning`` from Python 3.12 on and a
+``DeprecationWarning`` on 3.11. With ``-j N`` the parent and a worker both parse every
+module, so an unfiltered warning was printed twice.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+import warnings
+from pathlib import Path
+
+import pytest
+
+from coretrace_python.frontend.parser import parse_source
+
+INVALID_ESCAPE = 'import re\n\nPATTERN = re.compile("\\d+")\n'
+
+
+def test_parsing_raises_no_warning_about_the_analysed_source() -> None:
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        tree = parse_source(INVALID_ESCAPE, "escapes.py")
+
+    assert caught == []
+    assert len(tree.body) == 2
+
+
+def test_parsing_leaves_the_warning_filters_of_the_process_alone() -> None:
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        parse_source(INVALID_ESCAPE, "escapes.py")
+        warnings.warn("raised by the analyzer itself", SyntaxWarning, stacklevel=1)
+
+    assert [str(warning.message) for warning in caught] == ["raised by the analyzer itself"]
+
+
+@pytest.mark.parametrize("jobs", ["1", "2"])
+def test_check_prints_nothing_on_stderr_about_the_analysed_sources(
+    tmp_path: Path, jobs: str
+) -> None:
+    root = tmp_path / "project"
+    root.mkdir()
+    (root / "escapes.py").write_text(INVALID_ESCAPE, encoding="utf-8")
+    (root / "other.py").write_text("def run():\n    return 1\n", encoding="utf-8")
+
+    checked = subprocess.run(
+        [sys.executable, "-m", "coretrace_python", "--check", str(root), "-j", jobs],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+
+    assert checked.stderr == ""
+    assert checked.stdout == "no findings\ncoverage: 2/2 files, 2/2 functions\n"
+    assert checked.returncode == 0