Function callback in event binding, w/ and w/o parentheses

问题

I just started with my first Python program and ran into a pretty strange issue with function callback. Here is the code that matches my expectation:

from tkinter import *

def say_hello():
    print('hello')

root = Tk()
Button(root, text='say hello', command=say_hello).pack()
root.mainloop()

Now if I add parentheses to the function name

Button(root, text='say hello', command=say_hello()).pack()

'hello' will be printed only once when the program starts, but nothing further happens when the button is clicked. Why?

Thanks!

回答1:

When you add parentheses, you call the function (immediately printing "hello"), and its return value (not the function itself) is used as the callback.

The return value of None is a valid callback, indicating that there is no callback function for the Button. If say_hello returned, say, an int, you will probably get an error when you click the button to the effect that an int is not a callable value.

回答2:

say_hello is function. In 1st case you're providing it as argument, saying "Here's button, her's function say_hello. Execute (call) this function when you're pressed".

2nd case — if you're writing parentheses after function, this is the function call. So you're not providing your button with something to call later, but giving raw value instead.

The idea of callbacks overall — you provide something callable (function say_hello in your case) to object (Button in your case), so that object can call back in future, when it decides to do so (in your case, when pressed)

来源：https://stackoverflow.com/questions/54421018/function-callback-in-event-binding-w-and-w-o-parentheses

标签

python

tkinter