code: def parse(constraints):
      
      if not constraints:
          return None
  
      if type(constraints) is dict:
          # Fowards compatibilty: already parsed
          return constraints
  
      constraints = {
          normalize_key(k): (
              normalize_list_value(v) if k in LIST_KEYS else
              normalize_value(v)
          ) for k, v in [s.split("=") for s in constraints.split(" ")]}
  
      return constraints
docs: Constraints must be expressed as a string containing only spaces
      and key value pairs joined by an '='.