docs: def which(cmd, mode=os.F_OK | os.X_OK, path=None):
      """Given a command, mode, and a PATH string, return the path which
      conforms to the given mode on the PATH, or None if there is no such
      file.
      `mode` defaults to os.F_OK | os.X_OK. `path` defaults to the result
      of os.environ.get("PATH"), or can be overridden with a custom search
      path.
      Note: This function was backported from the Python 3 source code.
      """
      # Check that a given file can be accessed with the correct mode.
      # Additionally check that `file` is not a directory, as on Windows
      # directories pass the os.access check.
      def _access_check(fn, mode):
          return (os.path.exists(fn) and os.access(fn, mode) and
                  not os.path.isdir(fn))
  
      # If we're given a path with a directory part, look it up directly
      # rather than referring to PATH directories. This includes checking
      # relative to the current directory, e.g. ./script
      if os.path.dirname(cmd):
          if _access_check(cmd, mode):
              return cmd
          return None
  
      if path is None:
          path = os.environ.get("PATH", os.defpath)
      if not path:
          return None
      path = path.split(os.pathsep)
  
      files = [cmd]
  
      seen = set()
      for dir in path:
          normdir = os.path.normcase(dir)
          if normdir not in seen:
              seen.add(normdir)
              for thefile in files:
                  name = os.path.join(dir, thefile)
                  if _access_check(name, mode):
                      return name
      return None