Passing functions without () into map/filter/reduce

Question

In Python3.X, when passing methods into Map/Filter/Reduce, why is it done without '()'? Does this apply to all generators?

This is probably a simple question but I can't find a clear answer in docs. I'm hoping to understand the reasons rather than just "memorizing" syntax.

e.g.

map(str.upper, ['a', 'b', 'c']) #this works 
vs 
map(str.upper(), ['a', 'b', 'c']) #this gives an error

Answer 1

This is confusing at first, especially when you come from other programming languages. In Python the functions are objects and you can pass them along without invoking them. Check below example:

def func():  # function definition
    print("func was invoked")


a = func  # assign func object (without invoking it)
print(a, type(a))


a()  # invoking the function

map will use this function object to invoke the function for each item in the iterable.