Run Python tests with the project's virtualenv.

Mutator's interpreter does not have the project installed, so the baseline failed before any mutant ran.

README.md

@@ -110,7 +110,7 @@ The baseline command has to pass before any mutant runs. A mutant's timeout is t
 | Go | `go test -count=1` of the file's package | nearest `go.mod` |
 | TypeScript | `npm test` | nearest `package.json` |
 | Rust | `cargo test` | nearest `Cargo.toml` |
-| Python | `python -m pytest`, or `unittest discover` | nearest project file |
+| Python | the project's `.venv` or `venv` Python running `pytest`, or `unittest discover` | nearest project file |
 
 Coverage is generated with crapper's commands unless `--use-existing-coverage` or `--no-coverage` is set. A file missing from the report is treated as uncovered, and the run says so.
 

src/mutator/runner.py

@@ -128,7 +128,22 @@ def _clojure_command(directory: Path) -> str:
     return "clj -M:test"
 
 
+def _project_interpreter(directory: Path) -> str:
+    """The project's virtualenv, or this process when the project has none.
+
+    The path stays absolute and the symlink is left in place. A worker does
+    not link ``.venv``, and resolving ``bin/python`` would leave the virtualenv.
+    """
+
+    for name in (".venv", "venv"):
+        candidate = (directory / name / "bin" / "python").absolute()
+        if candidate.is_file() and os.access(candidate, os.X_OK):
+            return str(candidate)
+    return sys.executable
+
+
 def _python_command(directory: Path) -> str:
+    interpreter = _project_interpreter(directory)
     pyproject = _read(directory / "pyproject.toml")
     if (
         (directory / "pytest.ini").is_file()
@@ -136,8 +151,8 @@ def _python_command(directory: Path) -> str:
         or "pytest" in pyproject
         or (directory / "setup.cfg").is_file() and "pytest" in _read(directory / "setup.cfg")
     ):
-        return f"{sys.executable} -m pytest"
-    return f"{sys.executable} -m unittest discover"
+        return f"{interpreter} -m pytest"
+    return f"{interpreter} -m unittest discover"
 
 
 def test_plan(root: Path, source: Path, language: str, override: str | None) -> tuple[str, Path]:

tests/test_runner.py

@@ -44,6 +44,51 @@ def test_clojure_commands(tmp_path):
     assert _clojure_command(plain) == "clj -M:test"
 
 
+def _executable(path):
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text("#!/bin/sh\n", encoding="utf-8")
+    path.chmod(0o755)
+
+
+def test_python_command_uses_an_absolute_project_interpreter(tmp_path):
+    project = tmp_path / "proj"
+    dot = project / ".venv" / "bin" / "python"
+    _executable(dot)
+    (project / "pytest.ini").write_text("[pytest]\n", encoding="utf-8")
+    assert _python_command(project) == f"{dot.absolute()} -m pytest"
+
+    other = tmp_path / "other"
+    plain = other / "venv" / "bin" / "python"
+    _executable(plain)
+    assert _python_command(other) == f"{plain.absolute()} -m unittest discover"
+
+    _executable(other / ".venv" / "bin" / "python")
+    assert _python_command(other) == f"{(other / '.venv' / 'bin' / 'python').absolute()} -m unittest discover"
+
+
+def test_python_command_keeps_the_virtualenv_symlink(tmp_path):
+    project = tmp_path / "proj"
+    target = project / "real-python"
+    _executable(target)
+    link = project / ".venv" / "bin" / "python"
+    link.parent.mkdir(parents=True)
+    link.symlink_to(target)
+    (project / "pytest.ini").write_text("[pytest]\n", encoding="utf-8")
+    assert _python_command(project) == f"{link.absolute()} -m pytest"
+
+
+def test_python_command_falls_back_when_the_virtualenv_cannot_run(tmp_path):
+    project = tmp_path / "proj"
+    binary = project / ".venv" / "bin" / "python"
+    binary.parent.mkdir(parents=True)
+    binary.write_text("", encoding="utf-8")
+    assert _python_command(project) == f"{sys.executable} -m unittest discover"
+
+    missing = tmp_path / "missing"
+    missing.mkdir()
+    assert _python_command(missing) == f"{sys.executable} -m unittest discover"
+
+
 def test_python_commands(tmp_path):
     pytest_dir = tmp_path / "py"
     pytest_dir.mkdir()