What is the difference between calling function with parentheses and without in python?

Question

I have a question. Lets assume that we have function hello(). What is the difference between calling it with parentheses and without? When I call hello() it is referring to

Answer 1

Short answer: see https://nedbatchelder.com/text/names.html to get a better understanding of the difference between objects and the names used to refer to objects.

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A function is called if and only if you use parentheses. hello() calls the function; hello is simply a name bound to the function, and can be used, for example, to pass the function object as an argument to another function.

def caller(f):
    f()

def hello():
    print("hi")

def goodbye():
    print("bye")

caller(hello)  # Prints "hi"
caller(goodbye)  # Prints "bye"

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Regarding your update, id returns different values because each call to id receives a completely separate object as its argument. With id(hello), id gets the function object itself. With id(hello()), id is getting the object returned by the call to hello; it's the same as

x = hello()
print(id(x))