^^&&^^

Edit File: calc.py

#!/usr/bin/env python

# -----------------------------------------------------------------------------
# calc.py
#
# A simple calculator with variables.   This is from O'Reilly's
# "Lex and Yacc", p. 63.
#
# Class-based example contributed to PLY by David McNab.
#
# Modified to use new-style classes.   Test case.
# -----------------------------------------------------------------------------

import sys
sys.path.insert(0,"../..")

if sys.version_info[0] >= 3:
    raw_input = input

import ply.lex as lex
import ply.yacc as yacc
import os

class Parser(object):
    """
    Base class for a lexer/parser that has the rules defined as methods
    """
    tokens = ()
    precedence = ()

def __init__(self, **kw):
        self.debug = kw.get('debug', 0)
        self.names = { }
        try:
            modname = os.path.split(os.path.splitext(__file__)[0])[1] + "_" + self.__class__.__name__
        except:
            modname = "parser"+"_"+self.__class__.__name__
        self.debugfile = modname + ".dbg"
        self.tabmodule = modname + "_" + "parsetab"
        #print self.debugfile, self.tabmodule

# Build the lexer and parser
        lex.lex(module=self, debug=self.debug)
        yacc.yacc(module=self,
                  debug=self.debug,
                  debugfile=self.debugfile,
                  tabmodule=self.tabmodule)

def run(self):
        while 1:
            try:
                s = raw_input('calc > ')
            except EOFError:
                break
            if not s: continue     
            yacc.parse(s)

class Calc(Parser):

tokens = (
        'NAME','NUMBER',
        'PLUS','MINUS','EXP', 'TIMES','DIVIDE','EQUALS',
        'LPAREN','RPAREN',
        )

# Tokens

t_PLUS    = r'\+'
    t_MINUS   = r'-'
    t_EXP     = 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(self, t):
        r'\d+'
        try:
            t.value = int(t.value)
        except ValueError:
            print("Integer value too large %s" % t.value)
            t.value = 0
        #print "parsed number %s" % repr(t.value)
        return t

t_ignore = " \t"

def t_newline(self, t):
        r'\n+'
        t.lexer.lineno += t.value.count("\n")
    
    def t_error(self, t):
        print("Illegal character '%s'" % t.value[0])
        t.lexer.skip(1)

# Parsing rules

precedence = (
        ('left','PLUS','MINUS'),
        ('left','TIMES','DIVIDE'),
        ('left', 'EXP'),
        ('right','UMINUS'),
        )

def p_statement_assign(self, p):
        'statement : NAME EQUALS expression'
        self.names[p[1]] = p[3]

def p_statement_expr(self, p):
        'statement : expression'
        print(p[1])

def p_expression_binop(self, p):
        """
        expression : expression PLUS expression
                  | expression MINUS expression
                  | expression TIMES expression
                  | expression DIVIDE expression
                  | expression EXP expression
        """
        #print [repr(p[i]) for i in range(0,4)]
        if p[2] == '+'  : p[0] = p[1] + p[3]
        elif p[2] == '-': p[0] = p[1] - p[3]
        elif p[2] == '*': p[0] = p[1] * p[3]
        elif p[2] == '/': p[0] = p[1] / p[3]
        elif p[2] == '**': p[0] = p[1] ** p[3]

def p_expression_uminus(self, p):
        'expression : MINUS expression %prec UMINUS'
        p[0] = -p[2]

def p_expression_group(self, p):
        'expression : LPAREN expression RPAREN'
        p[0] = p[2]

def p_expression_number(self, p):
        'expression : NUMBER'
        p[0] = p[1]

def p_expression_name(self, p):
        'expression : NAME'
        try:
            p[0] = self.names[p[1]]
        except LookupError:
            print("Undefined name '%s'" % p[1])
            p[0] = 0

def p_error(self, p):
        if p:
            print("Syntax error at '%s'" % p.value)
        else:
            print("Syntax error at EOF")

if __name__ == '__main__':
    calc = Calc()
    calc.run()