code: def which(executable_name, env_var='PATH'):
      
      exec_fp = None
  
      if env_var in os.environ:
          paths = os.environ[env_var]
  
          for path in paths.split(os.pathsep):
              curr_exec_fp = os.path.join(path, executable_name)
  
              if os.access(curr_exec_fp, os.X_OK):
                  exec_fp = curr_exec_fp
                  break
  
      return exec_fp
docs: Equivalent to ``which executable_name`` in a *nix environment.
  
      Will return ``None`` if ``executable_name`` cannot be found in ``env_var``
      or if ``env_var`` is not set. Otherwise will return the first match in
      ``env_var``.
  
      Note: this function will likely not work on Windows.
  
      Code taken and modified from:
          http://www.velocityreviews.com/forums/
          t689526-python-library-call-equivalent-to-which-command.html