#!/usr/bin/env python
# -*- coding:utf-8 -*- def make_sum(a, b):
return ['+', a, b] def make_prod(a, b):
return ['*', a, b] def make_diff(a, b):
return ['-', a, b] def make_div(a, b):
return ['/', a, b] def is_basic_exp(a):
return not isinstance(a, list) def is_number(x):
return (isinstance(x, int) or isinstance(x, float) or isinstance(x, complex)) #表达式计算
def eval_exp(e, values):
if is_basic_exp(e):
#如果e位于字典里则返回键对应的值，否则直接返回e
if e in values.keys():
return values[e]
else:
return e
op, a, b = e[0], eval_exp(e[1], values), eval_exp(e[2], values)
if op=='+':
return eval_sum(a, b)
elif op=='-':
return eval_diff(a, b)
elif op=='*':
return eval_prod(a, b)
elif op=='/':
return eval_div(a, b)
else:
raise ValueError("Unknown operator:", op) #加法
def eval_sum(a, b):
if is_number(a) and is_number(b):
return a+b
if is_number(a) and a==0:
return b
if is_number(b) and b==0:
return a
return make_sum(a, b) #减法
def eval_diff(a, b):
if is_number(a) and is_number(b):
return a - b
if is_number(a) and a==0:
return -b
if is_number(b) and b==0:
return a
return make_diff(a, b) #乘法
def eval_prod(a, b):
if is_number(a) and is_number(b):
return a * b
if is_number(a) and a==0:
return 0
if is_number(b) and b==0:
return 0
return make_prod(a, b) #除法
def eval_div(a, b):
if is_number(a) and is_number(b):
return a / b
if is_number(a) and a==0:
return 0
if is_number(b) and b==1:
return a
if is_number(b) and b==0:
raise ZeroDivisionError
return make_div(a, b) #取出表达式里所有变量的集合
var_list = [] #这里使用全局变量
def varibles(exp):
for i in range(1,3):
if is_basic_exp(exp[i]):
var_list.append(exp[i])
else:
varibles(exp[i])
return var_list if __name__=='__main__':
e1 = make_prod(make_sum('a',3), make_sum('b',make_sum(4,6)))
print(e1)
print("变量集合：",varibles(e1))
values = {}
values['a'] = 3
values['b'] = 4
print(values)
print(eval_exp(e1, values))