Why is Button Parameter "command" Executed When Declared?

In Python, the "command" parameter of the Button widget is responsible for defining a callback function. However, users may be perplexed when this callback function appears to be executed immediately upon declaring the Button.

This issue arises when the "command" parameter is assigned a function call expression instead of a function object. For example:

def Hello():
    print("Hi there!")

Button(frame, text="Hello", command=Hello())  # Function call expression

In this code, the expression "Hello()" calls the Hello function immediately, returning its return value. As a result, the callback function is executed before the Button is created, resulting in the "Hi there!" message being printed to the console.

To avoid this issue and assign the function object to the "command" parameter, use the function name without parentheses:

Button(frame, text="Hello", command=Hello)  # Function object

Function objects hold references to their code, which will be executed when the callback is invoked. Additionally, if arguments need to be passed, lambda expressions can be employed:

Button(frame, text="Hello", command=lambda: Goodnight("Moon"))

In this case, the lambda expression wraps the Goodnight("Moon") call, delaying its execution until the button is clicked.