setting parameters for functions and nested functions

[mepyyeti]

Think of a function as a sort of black box. You give it some data to work with, it does *something*, and then gives you a return value.

Yes. And it works wonderfully well when functions are part of a class.

A function shouldn't use any value that isn't passed to it.

this is where I get frustrated lol. Pls see code in op above. Entire script runs off foo(). Not a single parameter, but quite a few variable gets assigned values in its body....
So this is what keeps getting me and textbooks seems to not care about the discrepency

def foo():
    a = 'moo'
    print(a)
    bar()

def bar():
    print('bar function')

if __name__=='__main__':
    foo()
else:
    print('choosing not to run')

this script runs...this bothers me.

Maybe it's a detail that comes from experience but I never feel certain when I can make do with foo() and when I need foo(param)....