[Tutor] What's the difference between calling a method with parenthesis vs. without it?

[Steven D'Aprano]

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