python - Why is Button parameter “command” executed when declared? -

my code is:

from tkinter import *  admin = tk() def button(an):     print     print 'het'  b = button(admin, text='as', command=button('hey')) b.pack() mainloop() 

the button doesn't work, prints 'hey' , 'het' once without command, , then, when press button nothing happens.

the command option takes reference function, fancy way of saying need pass name of function. when button('hey') calling function button, and result of being given command option.

to pass reference must use name only, without using parenthesis or arguments. example:

b = button(... command = button) 

if want pass parameter such "hey" must use little code:

• you can create intermediate function can called without argument , calls button function,
• you can use lambda create referred anonymous function. in every way it's function except doesn't have name. when call lambda command returns reference created function, means can used value of command option button.
• you can use functools.partial

for me, lambda simplest since doesn't require additional imports functools.partial does, though people think functools.partial easier understand.

to create lambda function calls button function argument this:

lambda: button('hey') 

you end function functionally equivalent to:

def some_name():     button('hey') 

as said earlier, lambda returns reference nameless function. since reference command option expects can use lambda direction in creation of button:

b = button(... command = lambda: button('hey')) 

there's question on site has lot of interesting comments lambda, in general. see question why python lambdas useful?. same discussion has an answer shows how use lambdas in loop when need pass in variable callback.

finally, see section titled tkinter callbacks on effbot.org nice tutorial. coverage of lambda pretty lean information there might still useful.