Why Is Button Parameter \"Command\" Executed When Declared in tkinter?

Button Parameter "Command" Execution Explained

In Python's tkinter, a frequent question arises: why is the "command" parameter of a Button executed when declared? This behavior can be confusing to beginners.

To understand this, let's examine the Python code snippet causing the issue:

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

hi = Button(frame, text="Hello", command=Hello())

As written, the "command" parameter is set to the result of calling the Hello function. However, in Python, calling a function with parentheses, such as Hello(), returns the value the function produces. In the case of Hello, it returns None because it doesn't have a return statement.

Therefore, when the Button object is created, it interprets "command=Hello()" as "command=None." As a result, the Hello function is executed immediately, printing "Hi there!" to the console.

To avoid this issue and only execute the Hello function when the button is pressed, you should pass the function object itself as the command, without parentheses:

hi = Button(frame, text="Hello", command=Hello)

This will ensure that the Hello function is called when the button is clicked, rather than at declaration time.

If you need to pass arguments to the function, use a lambda expression to create a parameterless callable. This allows you to avoid evaluating the function immediately and pass it to the command parameter:

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

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