On Sun, Jun 17, 2018 at 02:02:07PM -0400, C W wrote: > Dear Python experts, > > I never figured out when to call a method with parenthesis, when is it not? > It seems inconsistent.
You *always* CALL a method (or function) with parentheses. But sometimes you can grab hold of a method (or function) *without* calling it, by referring to its name without parentheses. The trick is to understand that in Python, functions and methods are what they call "First-class citizens" or first-class values, just like numbers, strings, lists etc. https://en.wikipedia.org/wiki/First-class_citizen Let's take a simple example: def add_one(x=0): return x + 1 To CALL the function, it always needs parentheses (even if it doesn't require an argument): py> add_one(10) 11 py> add_one() 1 But to merely refer to the function by name, you don't use parentheses. Then you can treat the function as any other value, and print it, assign it to a variable, pass it to a different function, or stick it in a list. Anything you can do with anything else, really. For example: py> myfunction = add_one # assign to a variable py> print(myfunction) <function add_one at 0xb78ece84> py> L = [1, "a", add_one, None] # put it inside a list py> print(L) [1, 'a', <function add_one at 0xb78ece84>, None] If it isn't clear to you why anyone would want to do this in the first place, don't worry too much about it, it is a moderately advanced technique. But it is essential for such things as: - function introspection; - callback functions used in GUI programming; - functional programming; etc. For example, you may have seen the map() function. Here it is in action: py> list(map(add_one, [1, 10, 100, 1000])) [2, 11, 101, 1001] Can you work out what it does? If not, try opening the interactive interpreter, and enter: help(map) and see if that helps. Or ask here :-) -- Steve _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
