How to stop functions from giving errors when using variables not defined in the function?

I am creating several experiments in python and have various functions which will be common across these experiments. I thus wanted to create a script only containing these functions which I could import at the beginning of the experimental script to avoid half of the script being taken up with ‘generic setup lines’.

So far, I have created the functions and the script and can import them and use them. For example, in the following, I have a function which shows a blank screen which takes the duration I want (e.g., 4 seconds) and displays it on the window defined in the experimental script.

import functions 
win = visual.Window([1440,900], color=[-1,-1,-1], fullscr=True)
dur = 4
functions.blank_screen(duration=dur)

This all works fine but in the script containing the functions, there are several ‘errors’ the function uses the variable ‘win’ which is not defined in the function:

def blank_screen(duration):
    blank = TextStim(win, text='')
    blank.draw()
    win.flip()

But when I run the experimental script, as it is defined in the script, it all works. How can I get around this? I have this problem with several functions as a large majority uses variables which are defined in the experimental script and not in the functions. As I say, it all works but just annoys me that the script is covered in ‘errors’!

I’d be greatly appreciative of any help, thank you!