Why do I need parentheses when instantiating a class?

Question

class Human:
    def exist(self)
        return print("exists")

H1 = Human # Why do I need parentheses here? I get error if i dont put parentheses here.

H1.exist() # I get an error that says parameter 'self' unfilled.

Class Human does not take any arguments, there is no even __init__ method. So I can't give any arguments when making an instance of Human so why do I need parentheses there?

I don't even know what I don't understand but I feel like I am missing something. Probably something about self.

Simply because the syntax of the language requires them. Without them you would assign the class to a new name. — Klaus D., Jul 12 '21 at 10:53
`Human` is a class, `Human()` instantiates a new object of that class. — bereal, Jul 12 '21 at 10:53
How would you differentiate referencing the class and an instance of the class without them? — Sayse, Jul 12 '21 at 10:54
https://docs.python.org/3/tutorial/classes.html#a-first-look-at-classes — Gino Mempin, Jul 12 '21 at 10:54
Also, there *is* an `__init__` method, you'll be using the base objects instead of your inherited one — Sayse, Jul 12 '21 at 10:54
You also need to put `:` after `def exist(self)`. And You probably want to `print("exists")` instead of `return print("exists")` . — joostblack, Jul 12 '21 at 10:57
Classes are callable in Python, and their return value is an instance of the class. — martineau, Jul 12 '21 at 11:00

score 1 · Accepted Answer · answered Jul 12 '21 at 11:11

In languages where the calling of a function/method/macro takes the form

foo(param1, param2, param3)

then it is almost universally true that, if foo were to take no parameters, the call would be

foo()

and further that, depending on the language

foo

is either incorrect, or means something else.

Now lets consider your example, expanded a bit

H1 = Human
H2 = Human
H3 = Human

In this case, nothing was called. H1, H2 and H3 have all been assigned the same reference, which is a reference to the class Human and not to any instance of that class.

H1 = Human()
H2 = Human()
H3 = Human()

In this case, the class was instantiated 3 times, that means that the __init__ function was run 3 sepearate times, and the result of each is a different instance of Human. H1, H2 and H3 now point to three different objects, all instances of Human. Since they are instances, they have a self.

score 0 · Answer 2 · answered Jul 12 '21 at 11:13

We need to clarify some points:

To call/invoke a class/function in the major of programming languages you have to use parentheses
The class/function name simply refers to its object in the memory, therefore without using parentheses we actually referring to the class/function object
When we use parentheses we actually call/invoke the class/function code and code got executed

Why do I need parentheses when instantiating a class?

2 Answers2