This would not be possible without an explicit self.Īnother way of understanding the need for the explicit self is to see where Python adds some syntactical sugar. This works because the first parameter of length_global, can be re-used as the self parameter in length_new.
How can me make use of this? If we assume for a moment that we hadn’t written a length method for our Vector class, we could do this: Vector.length_new = length_global Return math.sqrt(vector.x ** 2 + vector.y ** 2) What should it look like when we were to define it as a global method/function? def length_global(vector): Return math.sqrt(self.x ** 2 + self.y ** 2) What would it look like if we wanted to define it inside the class? def length(self): We want to have a method which calculates the length. Let’s take a simple vector class: class Vector: That's why assigning to an instance attribute needs to know what instance to assign to, and that's why it needs self. Python's all for making things explicit, making it obvious what's what, and although it doesn't do it entirely everywhere, it does do it for instance attributes. Python could have done something else to distinguish normal names from attributes - special syntax like Ruby has, or requiring declarations like C++ and Java do, or perhaps something yet more different - but it didn't. That makes methods entirely the same as functions, and leaves the actual name to use up to you (although self is the convention, and people will generally frown at you when you use something else.) self is not special to the code, it's just another object. Python decided to do methods in a way that makes the instance to which the method belongs be passed automatically, but not received automatically: the first parameter of methods is the instance the method is called on. is because Python does not use the syntax to refer to instance attributes.
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