Check the argument to --python is executable

Author: pfmoore

src/pip/_internal/cli/main_parser.py

@@ -71,17 +71,20 @@ def identify_python_interpreter(python: str) -> Optional[str]:
         if py:
             return py
 
-    # TODO: On Windows, `--python .venv/Scripts/python` won't pass the
-    #       exists() check (no .exe extension supplied). But it's pretty
-    #       obvious what the user intends. Should we allow this?
+    # If the named file exists, and is executable, use it.
+    # If it's a directory, assume it's a virtual environment and
+    # look for the environment's Python executable.
     if os.path.exists(python):
-        if not os.path.isdir(python):
+        # Do the directory check first because directories can be executable
+        if os.path.isdir(python):
+            # bin/python for Unix, Scripts/python.exe for Windows
+            # Try both in case of odd cases like cygwin.
+            for exe in ("bin/python", "Scripts/python.exe"):
+                py = os.path.join(python, exe)
+                if os.path.exists(py):
+                    return py
+        elif os.access(python, os.X_OK):
             return python
-        # Might be a virtual environment
-        for exe in ("bin/python", "Scripts/python.exe"):
-            py = os.path.join(python, exe)
-            if os.path.exists(py):
-                return py
 
     # Could not find the interpreter specified
     return None