🐛 fix(path): verify known implementations

docs/changelog/201.bugfix.rst

@@ -0,0 +1,2 @@
+Check the interpreter's reported implementation when a spec names a known implementation, regardless of its filename on
+``PATH``.

src/python_discovery/_discovery.py

@@ -12,7 +12,7 @@
 
 from ._compat import fs_path_id
 from ._py_info import PythonInfo
-from ._py_spec import PythonSpec
+from ._py_spec import KNOWN_IMPLEMENTATIONS, PythonSpec
 from ._specifier import DC_KW, SimpleVersion
 
 if TYPE_CHECKING:
@@ -494,18 +494,19 @@ def path_exe_finder(
     """Given a spec, return a function that can be called on a path to find all matching files in it."""
     pat = spec.generate_re(windows=sys.platform == "win32", all_implementations=all_implementations)
     direct = spec.str_spec
+    implementation = spec.implementation.lower() if spec.implementation is not None else None
+    impl_must_match = implementation in KNOWN_IMPLEMENTATIONS and implementation != "python"
     if sys.platform == "win32":  # pragma: win32 cover
         direct = f"{direct}.exe"
 
     def path_exes(path: Path) -> Generator[tuple[Path, bool], None, None]:
         direct_path = path / direct
         if direct_path.exists():
-            yield direct_path, False
+            yield direct_path, impl_must_match
 
         for exe in path.iterdir():
-            match = pat.fullmatch(exe.name)
-            if match:
-                yield exe.absolute(), match["impl"] == "python"
+            if pat.fullmatch(exe.name):
+                yield exe.absolute(), impl_must_match
 
     return path_exes
 

tests/test_discovery.py

@@ -13,7 +13,7 @@
 
 import pytest
 
-from python_discovery import DiskCache, PythonInfo, PythonSpec, SimpleSpecifierSet, get_interpreter
+from python_discovery import DiskCache, PythonInfo, PythonSpec, SimpleSpecifierSet, get_interpreter, iter_interpreters
 from python_discovery._discovery import IS_WIN, LazyPathDump, get_paths, propose_interpreters
 
 if TYPE_CHECKING:
@@ -85,6 +85,39 @@ def test_discovery_via_path_literal_symlink(tmp_path: Path) -> None:
     assert interpreter.executable == str(link)
 
 
+@pytest.mark.skipif(sys.implementation.name != "cpython", reason="requires CPython")
+@pytest.mark.parametrize("name", [pytest.param("pypy", id="pypy"), pytest.param("Python", id="mixed-case-python")])
+def test_pypy_spec_rejects_cpython_alias(name: str, tmp_path: Path, session_cache: DiskCache) -> None:
+    threaded = "t" if PythonInfo.current_system(session_cache).free_threaded else ""
+    version = f"{sys.version_info.major}.{sys.version_info.minor}{threaded}"
+    (tmp_path / f"{name}{version}{'.exe' if IS_WIN else ''}").symlink_to(sys.executable)
+    env = {"PATH": str(tmp_path), "UV_PYTHON_INSTALL_DIR": str(tmp_path / "missing")}
+
+    selected = get_interpreter(f"pypy{version}", [], cache=session_cache, env=env)
+    assert selected is None or selected.implementation == "PyPy"
+    assert all(
+        info.implementation == "PyPy" for info in iter_interpreters(f"pypy{version}", cache=session_cache, env=env)
+    )
+
+
+@pytest.mark.skipif(sys.implementation.name != "cpython", reason="requires CPython")
+def test_generic_python_spec_accepts_mixed_case_alias(tmp_path: Path, session_cache: DiskCache) -> None:
+    threaded = "t" if PythonInfo.current_system(session_cache).free_threaded else ""
+    version = f"{sys.version_info.major}.{sys.version_info.minor}{threaded}"
+    alias = tmp_path / f"Python{version}{'.exe' if IS_WIN else ''}"
+    alias.symlink_to(sys.executable)
+
+    selected = get_interpreter(
+        f"Python{version}",
+        [],
+        cache=session_cache,
+        env={"PATH": str(tmp_path), "UV_PYTHON_INSTALL_DIR": str(tmp_path / "missing")},
+    )
+
+    assert selected is not None
+    assert (selected.executable, selected.implementation) == (str(alias), "CPython")
+
+
 def test_discovery_via_path_in_nonbrowseable_directory(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
     bad_perm = tmp_path / "bad_perm"
     bad_perm.mkdir(mode=0o000)