python - Why is Button parameter "command" executed when declared? -

this question has answer here:

i'm new python , trying write program tkinter. why hello-function below executed? understand it, callback executed when button pressed? confused...

>>> def hello():         print("hi there!")  >>> hi=button(frame,text="hello",command=hello()) hi there! >>>

it called while parameters button being assigned:

command=hello()

if want pass function (not it's returned value) should instead:

command=hello

in general function_name function object, function_name() whatever function returns. see if helps further:

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

if want pass arguments, can use lambda expression construct parameterless callable.

>>> hi=button(frame, text="hello", command=lambda: goodnight("moon"))

simply put, because goodnight("moon") in lambda, won't execute right away, instead waiting until button clicked.

JVParth