Tokenize String in Python with Optional components -

i'm building simple webapp in flask , wanted able tokenize strings fit format , variables. example understand this

if input_string = 'play me song eminem'   artist = eminem   action = play  if input_string = 'play me sad song eminem'   artist = eminem   action = play   emotion = sad  if input_string = 'play me sad song'   emotion = sad   action = play 

i'm not looking @ natural language processing, rather picking tokens if exist. i'm looking @ format string [play/show/do] {[emotion]} [song/image/video] { [artist]} {[topic]}

where in {} optional, , should none/null if not present in string

i'm unsure how proceed though. regex way go?

this 1 (straight forward) possibility of 1000s, may give direction dig. notice there no error check:

class command(object):     def __init__(self, input_string):         words = input_string.split()         self.artist = self.get_artist(words)         self.action = self.get_action(words)         self.emotion = self.get_emotion(words)      def get_artist(self, words):         try:             index = words.index('by') + 1         except valueerror:             return          else:             return words[index]      def get_action(self, words):         return words[0]      def get_emotion(self, words):         index = words.index('song') - 1         emotion = words[index]         return emotion if emotion not 'a' else none  >>> tokenise import command >>> command = command('play me song eminem') >>> command.artist 'eminem' >>> command.action 'play' >>> command.emotion >>> command = command('play me sad song eminem') >>> command.artist 'eminem' >>> command.action 'play' >>> command.emotion 'sad' >>> command = command('play me sad song') >>> command.artist >>> command.action 'play' >>> command.emotion 'sad'