fix(interpreter): verify py launcher path exists before returning

changelog.d/1723.bugfix.md

@@ -0,0 +1,3 @@
+Fixed `find_py_launcher_python` to verify that the path returned by the Windows `py` launcher actually exists before returning it.
+
+The `py` launcher can return stale registry entries for Python versions that have been uninstalled or moved, causing pipx to fail with "did not find executable at path" errors.

src/pipx/interpreter.py

@@ -121,7 +121,9 @@ def find_python_interpreter(python_version: str, fetch_missing_python: bool = Fa
 
 def find_py_launcher_python(python_version: Optional[str] = None) -> Optional[str]:
     py = shutil.which("py")
-    if py and python_version:
+    if not py:
+        return None
+    if python_version:
         python_semver = python_version
         if python_version.startswith("python"):
             logger.warning(
@@ -134,6 +136,11 @@ def find_py_launcher_python(python_version: Optional[str] = None) -> Optional[st
             text=True,
             check=True,
         ).stdout.strip()
+        # Verify the path returned by py launcher actually exists
+        # (it may return stale registry entries for uninstalled Python versions)
+        if not Path(py).is_file():
+            logger.info(f"py launcher returned non-existent path: {py}")
+            return None
     return py
 
 

tests/test_interpreter.py

@@ -199,3 +199,33 @@ def which(name):
     else:
         assert python_path.endswith("python3")
     subprocess.run([python_path, "-c", "import sys; print(sys.executable)"], check=True)
+
+
+def test_py_launcher_stale_path_returns_none(monkeypatch):
+    """Test that find_py_launcher_python returns None when py launcher returns a non-existent path.
+
+    This can happen when Python is uninstalled but registry entries remain,
+    causing the py launcher to return stale paths.
+    """
+    from pipx.interpreter import find_py_launcher_python
+
+    def which(name):
+        if name == "py":
+            return "py"
+        return None
+
+    class MockCompletedProcess:
+        def __init__(self, stdout):
+            self.stdout = stdout
+
+    # Simulate py launcher returning a non-existent path
+    non_existent_path = "/nonexistent/path/to/python.exe"
+    monkeypatch.setattr(shutil, "which", which)
+    monkeypatch.setattr(
+        subprocess,
+        "run",
+        lambda *args, **kwargs: MockCompletedProcess(non_existent_path),
+    )
+
+    result = find_py_launcher_python("3.14")
+    assert result is None