self is the conventional first parameter of a Python instance method. It refers to whichever object receives the method call, so self.radius means that particular circle's radius. Think of self as a placeholder for the instance being created or used. section 31.2
A circle remembers its own radius
class Circle(object): def __init__(self): self.radius = 0 def change_radius(self, radius): self.radius = radius def get_radius(self): return self.radius The first method creates a radius attribute for each Circle. change_radius updates the current object's radius, while get_radius returns it. In one_circle.change_radius(4), Python supplies one_circle as self; 4 supplies the radius parameter. section 31.3 section 31.4
Keep these three uses separate:
- Declare self first in an instance method, even when the method needs no other parameter. section 31.2 section 31.3
- Use self.attribute to store or access data belonging to the current object. section 31.2
- Pass only the method's additional arguments at the call site; Python automatically supplies self from the object before the dot. section 31.4
- Remember that each instance has separate self-defined attributes, so changing one circle does not change another. section 31.4
class Circle(object):
def __init__(self):
self.radius = 0
Inspect the smallest complete example: __init__ uses self.radius to give every Circle instance its own starting radius. section 31.2
