code: def which(prog):
      
      executable = lambda path: os.path.isfile(path) and os.access(path, os.X_OK)
  
      # prog can also be a path
      dirname, _ = os.path.split(prog)
      if dirname:
          if executable(prog):
              return prog
      elif "PATH" in os.environ:
          for search_path in os.environ["PATH"].split(os.pathsep):
              path = os.path.join(search_path.strip('"'), prog)
              if executable(path):
                  return path
  
      return None
docs: Pythonic ``which`` implementation. Returns the path to an executable *prog* by searching in
      *PATH*, or *None* when it could not be found.