Why is Button parameter "command" executed when declared? [duplicate]

Question

I'm new to Python and trying to write a program with tkinter. Why is the Hello-function below executed? As I understand it, the callback would only be executed when the button is pressed? I am very confused...

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

>>> hi=Button(frame,text="Hello",command=Hello())
Hi there!
>>> 

Answer 1

It is called while the parameters for Button are being assigned:

command=Hello()

If you want to pass the function (not it's returned value) you should instead:

command=Hello

in general function_name is a function object, function_name() is whatever the function returns. See if this helps further:

>>> def func():
...     return 'hello'
... 
>>> type(func)
<type 'function'>
>>> type(func())
<type 'str'>

---

If you want to pass arguments, you can use a lambda expression to construct a parameterless callable.

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

Simply put, because Goodnight("Moon") is in a lambda, it won't execute right away, instead waiting until the button is clicked.

Answer 2

You can also use a lambda expression as the command argument:

import tkinter as tk
def hello():
    print("Hi there!")

main = tk.Tk()
hi = tk.Button(main,text="Hello",command=lambda: hello()).pack()
main.mainloop()