# -----------------------------------------------------------------------------# calc.py## A simple calculator with variables.   This is from O'Reilly's# "Lex and Yacc", p. 63.# -----------------------------------------------------------------------------tokens = (    'NAME','NUMBER',    'PLUS','MINUS','TIMES','DIVIDE','EQUALS',    'LPAREN','RPAREN',    )# Tokenst_PLUS    = r'\+'t_MINUS   = r'-'t_TIMES   = r'\*'t_DIVIDE  = r'/'t_EQUALS  = r'='t_LPAREN  = r'\('t_RPAREN  = r'\)'t_NAME    = r'[a-zA-Z_][a-zA-Z0-9_]*'def t_NUMBER(t):    r'\d+'    try:        t.value = int(t.value)    except ValueError:        print "Integer value too large", t.value        t.value = 0    return tt_ignore = " \t"def t_newline(t):    r'\n+'    t.lineno += t.value.count("\n")    def t_error(t):    print "Illegal character '%s'" % t.value[0]    t.skip(1)    # Build the lexerimport lexlex.lex()# Parsing rulesprecedence = (    ('left','PLUS','MINUS'),    ('left','TIMES','DIVIDE'),    ('right','UMINUS'),    )# dictionary of namesnames = { }def p_statement_assign(t):    'statement : NAME EQUALS expression'    names[t[1]] = t[3]def p_statement_expr(t):    'statement : expression'    print t[1]def p_expression_binop(t):    '''expression : expression PLUS expression                  | expression MINUS expression                  | expression TIMES expression                  | expression DIVIDE expression'''    if t[2] == '+'  : t[0] = t[1] + t[3]    elif t[2] == '-': t[0] = t[1] - t[3]    elif t[2] == '*': t[0] = t[1] * t[3]    elif t[2] == '/': t[0] = t[1] / t[3]def p_expression_uminus(t):    'expression : MINUS expression %prec UMINUS'    t[0] = -t[2]def p_expression_group(t):    'expression : LPAREN expression RPAREN'    t[0] = t[2]def p_expression_number(t):    'expression : NUMBER'    t[0] = t[1]def p_expression_name(t):    'expression : NAME'    try:        t[0] = names[t[1]]    except LookupError:        print "Undefined name '%s'" % t[1]        t[0] = 0def p_error(t):    print "Syntax error at '%s'" % t.valueimport yaccyacc.yacc()while 1:    try:        s = raw_input('calc > ')    except EOFError:        break    yacc.parse(s)