from tkinter.ttk import *
def calc(event):
a = float(t1.get())
b = float(t2.get())
dic = {0:a+b,1:a-b,2:a*b,3:a/b}
c = dic[comb.current()]
lbl.config(text=str(c))
root = Tk()
root.title('四则运算')
root.geometry('320x240')
t1 = Entry(root)
t1.place(relx=0.1,rely=0.1,relwidth=0.2,relheight=0.1)
t2 = Entry(root)
t2.place(relx=0.5,rely=0.1,relwidth=0.2,relheight=0.1)
var = StringVar()
comb = Combobox(root,textvariable=var,values=['加','减','乘','除',])
comb.place(relx=0.1,rely=0.5,relwidth=0.2)
comb.bind('<>',calc)
lbl=Label(root,text='结果')
lbl.place(relx=0.5,rely=0.7,relwidth=0.2,relheight=0.3)
root.mainloop()