Python计算器GUI按钮command参数中lambda函数写法疑问

我对下面Python计算器GUI按钮创建语法里的lambda函数用法有疑问：我理解的lambda函数写法是示例1这种，但为什么在Tkinter Button构造器的command选项里可以用示例2的写法？我原本以为得写成command=lambda num: button_press(1)才能生效，搞不懂原因。

示例1：我熟悉的Lambda写法 #

>>> add_one = lambda x: x + 1
>>> add_one(2)

示例2：计算器代码中的Lambda写法 #

button1 = Button(frame,text=1,height=4,width=9,font=35,
             command=lambda: button_press(1))

计算器完整代码 #

from tkinter import *

def button_press(num):                          
    global equation_text   

    equation_text = equation_text + str(num)  
    equation_label.set(equation_text)

def equals():
    global equation_text

    try:
        total = str(eval(equation_text))
        equation_label.set(total)
        equation_text = total   
    except SyntaxError:
        equation_label.set("syntax error")
        equation_text = ""           
    except ZeroDivisionError:
        equation_label.set("arithmetic error")
        equation_text = ""       

def clear():
    global equation_text

    equation_label.set("")
    equation_text = ""          

window = Tk()                       
window.title("Calculator program")                   
window.geometry("500x500")                      

equation_text = ""
equation_label = StringVar()
label = Label(window, textvariable=equation_label, font=('consolas',
20),bg='white',width=24,height=2)
label.pack()
frame = Frame(window)
frame.pack()

button1 = Button(frame,text=1,height=4,width=9,font=35,
                 command=lambda: button_press(1))
button1.grid(row=0,column=0)

button2 = Button(frame,text=2,height=4,width=9,font=35,
                 command=lambda: button_press(2))
button2.grid(row=0,column=1)

button3 = Button(frame,text=3,height=4,width=9,font=35,
                 command=lambda: button_press(3))
button3.grid(row=0,column=2)

button4 = Button(frame,text=4,height=4,width=9,font=35,
             command=lambda: button_press(4))
button4.grid(row=1,column=0)

button5 = Button(frame,text=5,height=4,width=9,font=35,
                 command=lambda: button_press(5))
button5.grid(row=1,column=1)

button6 = Button(frame,text=6,height=4,width=9,font=35,
                 command=lambda: button_press(6))
button6.grid(row=1,column=2)

button7 = Button(frame,text=7,height=4,width=9,font=35,
                 command=lambda: button_press(7))
button7.grid(row=2,column=0)

button8 = Button(frame,text=8,height=4,width=9,font=35,
                 command=lambda: button_press(8))
button8.grid(row=2,column=1)

button9 = Button(frame,text=9,height=4,width=9,font=35,
                 command=lambda: button_press(9))
button9.grid(row=2,column=2)

button0 = Button(frame,text=0,height=4,width=9,font=35,
                 command=lambda: button_press(0))
button0.grid(row=3,column=0)

plus = Button(frame,text='+',height=4,width=9,font=35,
                 command=lambda: button_press('+'))
plus.grid(row=0,column=3)

minus = Button(frame,text='-',height=4,width=9,font=35,
                 command=lambda: button_press('-'))
minus.grid(row=1,column=3)

multiply = Button(frame,text='*',height=4,width=9,font=35,
                 command=lambda: button_press('*'))
multiply.grid(row=2,column=3)

divide = Button(frame,text='/',height=4,width=9,font=35,
                 command=lambda: button_press('/'))
divide.grid(row=3, column=3)

equal = Button(frame,text='=',height=4,width=9,font=35,
                 command=equals)
equal.grid(row=3,column=2)

decimal = Button(frame,text='.',height=4,width=9,font=35,
                 command=lambda: button_press('.'))
decimal.grid(row=3,column=1)

clear = Button(window,text='clear',height=4,width=9,font=35,
                 command=clear)
clear.pack()

window.mainloop()

问题解答 #

核心原因是Tkinter触发按钮点击时，会调用command指定的函数，且不会传递任何参数：

1. 示例1的lambda x: x + 1是带参数的lambda，它要求调用时必须传入一个参数，但Tkinter不会传参，这种写法放到command里会直接报错，因为lambda期望1个参数却没收到。
2. 示例2的lambda: button_press(1)是无参数lambda，刚好匹配Tkinter的调用规则：点击按钮时，Tkinter调用这个lambda，它不需要任何参数，内部直接把固定值1传给button_press执行。

你之前想的lambda num: button_press(1)完全多余：这个lambda定义了参数num，但Tkinter不会给它传值，而且lambda内部也没用到这个参数，纯粹多此一举。

另外补充：为什么不直接写command=button_press(1)？因为这样写的话，Python在创建按钮的瞬间就会执行button_press(1)，而不是等到按钮点击时才触发。lambda的作用就是延迟执行，把函数调用包装成一个可调用对象，只有触发点击事件时才会运行内部的代码。

内容的提问来源于stack exchange，提问作者Casey Wong