用于将字符串转换为Python语句执行，可以直接计算数学表达式。
result = eval（expression) #expression是字符串
数学表达式计算：
from tkinter import *
import numpy as np
def cal():
out.configure(text = "结果：" + str(eval（equ.get())))
root = Tk()
root.title("Ex")
label = Label(root, text = "请输入数学表达式：")
label.pack()
equ = Entry(root)
equ.pack(padx = 20, pady = 5)
out = Label(root)
out.pack()
btn = Button(root, text = "计算", command = cal)
btn.pack(pady = 5)
root.mainloop()
