Why does function run without paramater?

Question

I'm currently learning curses in python, and I found this piece of code online that is confusing me.

import curses

def draw_menu(stdscr):
    # do stuff
    # if you want more code just let me know


def main():
    curses.wrapper(draw_menu)


if __name__ == "__main__":
    main()

When I run this I don't get the expected missing 1 required positional argument error, since there is no parameter being passed in the curses.wrapper(draw_menu) line. Is this a curses thing? Any help is greatly appreciated.

`curses.wrapper(draw_menu)` passes a parameter. The parameter is the function `draw_menu` itself. Important: It is the function, not the result of calling the function. This function might be called somewhere in `curses.wrapper` and `curses.wrapper` has to provide a parameter for that call. — Matthias, Feb 18 '21 at 22:51
You aren't calling the function anywhere, so you shouldn't expect that error. — juanpa.arrivillaga, Feb 18 '21 at 23:02

Loïc · Accepted Answer · 2021-02-18T23:37:12.083

A function is a datatype, just as much as strings, integers, and so on.

def my_function(txt):
  print(txt)

here type(my_function) # => <class 'function'>

You invoke the code inside the function when you call it with parenthesis : my_function('hello') # => prints hello

Until then you can perfectly pass a function as an argument to another function. And that last one can call the one you passed giving it some parameters.

Like in your case, I'd guess that curses.wrapper() creates a screen interface that it passes as argument your draw_menu() function. And you can probably use that screen object to build your curse app.

See this : Python function as a function argument?

score 1 · Answer 2 · answered Feb 18 '21 at 22:51

1

There's a big difference between curses.wrapper(draw_menu) and curses.wrapper(draw_menu()). curses.wrapper(draw_menu) calls curses.wrapper and passes the function draw_menu into it as an argument. In contrast, curses.wrapper(draw_menu()) would call draw_menu and pass its return value into curses.wrapper.

curses.wrapper will call the function you pass it. From that link:

Initialize curses and call another callable object, func, which should be the rest of your curses-using application.

E.g., it will call draw_menu when curses is completely initialized.

answered Feb 18 '21 at 22:51

T.J. Crowder

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Does the `wrapper` function call the `draw_menu` with a parameter then? – Have a nice day Feb 18 '21 at 22:55
@Haveaniceday - Yes. It says that in the first paragraph of the linked documentation. – T.J. Crowder Feb 19 '21 at 08:35

score 0 · Answer 3 · answered Feb 18 '21 at 22:53

Here is the signature for curses.wrapper from here.

curses.wrapper(func, /, *args, **kwargs)

It says that you need to give curses.wrapper a function reference argument followed by zero or more arguments and keyword arguments. Your code satisfies those requirements.

Python allows function signatures like this to enable developers a lot of flexibility regarding what can be passed in by the caller.

Why does function run without paramater?

3 Answers3