code: def tokenize(self):
          self.token_list = []
          ps = self.parse_string.strip()
          i = 0
          last_token = None
          while i < len(ps) and ps[i].isspace():
              i += 1
          while i < len(ps):
              token = ''
              if ps[i].isalpha():
                  while i < len(ps) and (ps[i].isalnum() or ps[i] == '_'):
                      token += ps[i]
                      i += 1
              elif ps[i].isdigit():
                  while i < len(ps) and (ps[i].isdigit() or
                                         ps[i] == '.' or
                                         ps[i] == 'e' or
                                         ps[i] == 'E' or
                                         (ps[i] == '+' and (ps[i-1] == 'e' or ps[i-1] == 'E')) or
                                         (ps[i] == '-' and (ps[i-1] == 'e' or ps[i-1] == 'E'))):
                      token += ps[i]
                      i += 1
              elif ps[i] == '.':
                  if ps[i+1].isdigit():
                      while i < len(ps) and (ps[i].isdigit() or ps[i] == '.'):
                          token += ps[i]
                          i += 1
                  else:
                      while i < len(ps) and (ps[i].isalpha() or ps[i] == '.'):
                          token += ps[i]
                          i += 1
              else:
                  token += ps[i]
                  i += 1
              if token == '-' and \
                 (last_token == None or last_token == '(' or self.is_op(last_token)):
                  token = '~'
              self.token_list += [token]
              last_token = token
              while i < len(ps) and ps[i].isspace():
                  i += 1