python - 如何在不带括号调用的函数中传递参数？

Question

这个问题已经有答案了:

How can I bind arguments to a function in Python? (7 个回答)

What is a callback function? (22 个回答)

已关闭 去年。

这是来自 pynput 的示例代码documentation 。它基本上会打印您按下的键。

from pynput import keyboard

def on_press(key):
    print('You pressed {}'.format(key))

    if key == keyboard.Key.esc:
        return False

with keyboard.Listener(
    on_press=on_press) as listener:
listener.join()

我需要将参数传递给 on_press函数，但调用时不带括号。我不明白这是为什么以及有什么作用。理想情况下，这就是我想要做的工作:

from pynput import keyboard

def on_press(key, addition):
    print('You pressed {}, {}'.format(key, addition))

    if key == keyboard.Key.esc:
        return False

string = 'congrats!'

with keyboard.Listener(
    on_press=on_press(key, string)) as listener:
listener.join()

Answer 1

with keyboard.Listener(on_press=on_press) as listener:

这实际上并没有调用on_press，而是将其传递给keyboard.Listener。 (Python 函数是一流的，即它们可以传递。)然后 Listener 对象稍后会调用它。这称为 callback .

您可以使用高阶函数来设置加法:

def outer(addition):
    def on_press(key):
        print('You pressed {}, {}'.format(key, addition))
        ...
    return on_press

with keyboard.Listener(on_press=outer('congrats!')) as listener:
    listener.join()

部分也可以正常工作。