Lex/yacc

# ------------------------------------------------------------
 # calclex.py
 #
 # tokenizer for a simple expression evaluator for
 # numbers and +,-,*,/
 # ------------------------------------------------------------
 import ply.lex as lex
 
 # List of token names.   This is always required
 tokens = (
    'NUMBER',
    'PLUS',
    'MINUS',
    'TIMES',
    'DIVIDE',
    'LPAREN',
    'RPAREN',
 )
 
 # Regular expression rules for simple tokens
 t_PLUS    = r'\+'
 t_MINUS   = r'-'
 t_TIMES   = r'\*'
 t_DIVIDE  = r'/'
 t_LPAREN  = r'\('
 t_RPAREN  = r'\)'
 
 # A regular expression rule with some action code
 def t_NUMBER(t):
     r'\d+'
     t.value = int(t.value)    
     return t
 
 # Define a rule so we can track line numbers
 def t_newline(t):
     r'\n+'
     t.lexer.lineno += len(t.value)
 
 # A string containing ignored characters (spaces and tabs)
 t_ignore  = ' \t'
 
 # Error handling rule
 def t_error(t):
     print("Illegal character '%s'" % t.value[0])
     t.lexer.skip(1)
 
 # Build the lexer
 lexer = lex.lex()
 
 
# Test it out
 data = '''
 3 + 4 * 10
   + -20 *2
 '''
 
 # Give the lexer some input
 lexer.input(data)
 
 # Tokenize
 while True:
     tok = lexer.token()
     if not tok: 
         break      # No more input
     print(tok)

class Lexer:
...
...
    def input(self,s):
...
 
 
def lex():
 
    lexobj = Lexer()
...    
    global input
...
    input = lexobj.input