gh-127001: Fix PATHEXT issues in shutil.which() on Windows (GH-127035)

* Name without a PATHEXT extension is only searched if the mode does not include X_OK. * Support multi-component PATHEXT extensions (e.g. ".foo.bar"). * Support files without extensions in PATHEXT contains dot-only extension (".", "..", etc). * Support PATHEXT extensions that end with a dot (e.g. ".foo.").

Author: serhiy-storchaka

Doc/library/shutil.rst

@@ -491,12 +491,6 @@ Directory and files operations
  or ends with an extension that is in ``PATHEXT``; and filenames that
  have no extension can now be found.
 
- .. versionchanged:: 3.12.1
- On Windows, if *mode* includes ``os.X_OK``, executables with an
- extension in ``PATHEXT`` will be preferred over executables without a
- matching extension.
- This brings behavior closer to that of Python 3.11.
-
 .. exception:: Error
 
  This exception collects exceptions that are raised during a multi-file

Lib/shutil.py

@@ -1550,21 +1550,21 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
  if sys.platform == "win32":
  # PATHEXT is necessary to check on Windows.
  pathext_source = os.getenv("PATHEXT") or _WIN_DEFAULT_PATHEXT
- pathext = [ext for ext in pathext_source.split(os.pathsep) if ext]
+ pathext = pathext_source.split(os.pathsep)
+ pathext = [ext.rstrip('.') for ext in pathext if ext]
 
  if use_bytes:
  pathext = [os.fsencode(ext) for ext in pathext]
 
- files = ([cmd] + [cmd + ext for ext in pathext])
+ files = [cmd + ext for ext in pathext]
 
- # gh-109590. If we are looking for an executable, we need to look
- # for a PATHEXT match. The first cmd is the direct match
- # (e.g. python.exe instead of python)
- # Check that direct match first if and only if the extension is in PATHEXT
- # Otherwise check it last
- suffix = os.path.splitext(files[0])[1].upper()
- if mode & os.X_OK and not any(suffix == ext.upper() for ext in pathext):
- files.append(files.pop(0))
+ # If X_OK in mode, simulate the cmd.exe behavior: look at direct
+ # match if and only if the extension is in PATHEXT.
+ # If X_OK not in mode, simulate the first result of where.exe:
+ # always look at direct match before a PATHEXT match.
+ normcmd = cmd.upper()
+ if not (mode & os.X_OK) or any(normcmd.endswith(ext.upper()) for ext in pathext):
+ files.insert(0, cmd)
  else:
  # On other platforms you don't have things like PATHEXT to tell you
  # what file suffixes are executable, so just pass on cmd as-is.
@@ -1573,7 +1573,7 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
  seen = set()
  for dir in path:
  normdir = os.path.normcase(dir)
- if not normdir in seen:
+ if normdir not in seen:
  seen.add(normdir)
  for thefile in files:
  name = os.path.join(dir, thefile)