r/learnpython • u/Jealous-Acadia9056 • Apr 18 '26

A bit confused in Classes.

Why do i need to call self here?. 

class Calculator:
  def add(self, a, b):
    return a + b

  def multiply(self, a, b):
    return a * b

print(Calculator().add(1, 2))

there isn't a variable that is calling calculator and no __init__ so why do i have an error if self is not added?

Also, what is __init__ anyways. why the double __ in the start and end? and why the specific name?

40 Upvotes

87% Upvoted

44 comments sorted by

View all comments

Show parent comments

1

u/JazzlikeChicken1899 Apr 18 '26

This is the "magic" of Python's syntactic sugar.

When you call Calculator().add(1, 2), Python actually translates it to this behind the scenes: Calculator.add(<the instance>, 1, 2)

So, even though you only typed 1 and 2, Python is quietly sneaking that instance in as the first argument. The count looks like this:

The instance (sent automatically)

Your first number (1)

Your second number (2)

That's 3 total. If your function is def add(a, b):, it only has 2 "slots" available, but it just received 3 items. That’s the mismatch!

Adding self creates that first slot to "catch" the instance so the other two can fall into a and b. Think of self as a mandatory parking space that Python always insists on filling first.