解释器模式 python 表达式计算

GoF定义：
定义语言的文法 ,并且建立一个解释器来解释该语言中的句子.

在GOF的书中指出：如果一种特定类型的问题发生的频率足够高，那么可能就值得将该问题的各个实例表述为一个简单语言中的句子。这样就可以构建一个解释器，该解释器通过解释这些句子来解决该问题。而且当文法简单、效率不是关键问题的时候效果最好。 

更多相关资料：http://www.cnblogs.com/cbf4life/archive/2009/12/17/1626125.html

import sysclass priority:#定义符号优先级    symbol = '#+-*/%()'    def getisp(self,a):#栈内优先权        if '#'.find(a)!=-1: return 0        if '('.find(a)!=-1: return 1        if '-+'.find(a)!=-1: return 3        if '*/%'.find(a)!=-1: return 5        if ')'.find(a)!=-1: return 6    def geticp(self,a): #栈外优先权        if '#'.find(a)!=-1: return 0        if '('.find(a)!=-1: return 6        if '-+'.find(a)!=-1: return 2        if '*/%'.find(a)!=-1: return 4        if ')'.find(a)!=-1: return 1class Expression:#抽象类    def interpreter():        passclass VarExpression(Expression):#数字,终结表达式    def __init__(self,_key):        self.key=_key    def interpreter(self):        return self.keyclass SymbolExpression(Expression):#运算类基类    def __init__(self,_left,_right):        self.left = _left        self.right = _rightclass SubExpression(SymbolExpression):#减    def __init__(self,_left,_right):        self.parent=SymbolExpression(_left,_right)    def interpreter(self):        return self.parent.left.interpreter()-self.parent.right.interpreter()class AddExpression(SymbolExpression):#加    def __init__(self,_left,_right):        self.parent=SymbolExpression(_left,_right)    def interpreter(self):        return self.parent.left.interpreter()+self.parent.right.interpreter()class MulExpression(SymbolExpression):#乘    def __init__(self,_left,_right):        self.parent=SymbolExpression(_left,_right)    def interpreter(self):        return self.parent.left.interpreter()*self.parent.right.interpreter()class DivExpression(SymbolExpression):#除    def __init__(self,_left,_right):        self.parent=SymbolExpression(_left,_right)    def interpreter(self):        return self.parent.left.interpreter()/self.parent.right.interpreter()class ModExpression(SymbolExpression):#取余    def __init__(self,_left,_right):        self.parent=SymbolExpression(_left,_right)    def interpreter(self):        return self.parent.left.interpreter()%self.parent.right.interpreter()class Calculator:    exp=''    order=[]    def __init__(self,_exp):        _exp = _exp.replace(' ','')        self.exp=_exp        _exp=_exp+'#'        i=0        f=0        p=1        tmp = 0        stack='#'        order=[]        pty = priority()        while len(stack)>0:            if pty.symbol.find(_exp[i])!=-1:                if f>0:order = order+[tmp]                f = 0                p=1                tmp = 0                ch = _exp[i]                ch1 = stack[-1]                #print (ch1,' ',pty.getisp(ch1),' ',pty.geticp(ch))                if pty.getisp(ch1)<pty.geticp(ch):                    stack=stack+ch                    i=i+1                elif pty.getisp(ch1)>pty.geticp(ch):                    order = order+[stack[-1]]                    stack = stack[0:-1]                else :                    if stack[-1]=='(': i=i+1                    stack = stack[0:-1]            else :                if f==0: f = 1                if _exp[i]=='.':                    f = 2                elif f==1:                    tmp = tmp*10 +int(_exp[i])-int('0')                else:                    p = p*0.1                    tmp = tmp+(int(_exp[i])-int('0'))*p                i=i+1        print (order) #order 为后缀表达式        self.order=order        stack=[]        for i in order:#创建递归树            if pty.symbol.find(str(i))!=-1:                right = stack[-1] # 把栈中连个元素取出                left = stack[-2]                stack = stack[0:-2]                if '+'.find(str(i))!=-1:                    stack = stack+[AddExpression(left,right)]                elif '-'.find(str(i))!=-1:                    stack = stack+[SubExpression(left,right)]                elif '*'.find(str(i))!=-1:                    stack = stack+[MulExpression(left,right)]                elif '/'.find(str(i))!=-1:                    stack = stack+[DivExpression(left,right)]                elif '%'.find(str(i))!=-1:                    stack = stack+[ModExpression(left,right)]            else :                stack=stack+[VarExpression(i)]        self.expression = stack[0]        def run(self):#计算递归树        return self.expression.interpreter()def Client():    a=input("请输入正确表达式：eg:1*(2+3):\n")    cal = Calculator(a)    print (a,"=",cal.run())Client()