all Python 3OOPterm

self

self

A reference to the current instance of the class. The first parameter of instance methods (the name is a convention).

Syntax

def method(self): self.x = ...

self isn't a keyword but an ordinary first parameter: Python fills it with the instance automatically when you write b.double(), so you never pass it yourself. Forgetting the self. prefix is a common bug — size = 2 makes a local variable that vanishes when the method ends, while self.size = 2 actually changes the object.

Examples

class Box:
    def __init__(self, size):
        self.size = size
    def double(self):
        self.size *= 2
b = Box(5)
b.double()  # то же, что Box.double(b): self — это b
print(b.size)  # → 10

class Node:
    def __init__(self, val):
        self.val = val
        self.next = None
    def set_next(self, node):
        self.next = node
n1 = Node(1)
n2 = Node(2)
n1.set_next(n2)
print(n1.next.val)  # → 2
class Builder:
    def __init__(self):
        self.items = []
    def add(self, item):
        self.items.append(item)
        return self  # цепочка!
    def build(self):
        return self.items
b2 = Builder().add(1).add(2).add(3).build()
print(b2)  # → [1, 2, 3]

# Передача self явно
class Calc:
    def sq(self):
        return self.n ** 2
c2 = Calc()
c2.n = 5
print(Calc.sq(c2))  # → 25 (явно)

class Logger:
    prefix = '[LOG]'
    def log(self, msg):
        print(f'{self.prefix} {msg}')
l = Logger()
l.prefix = '[INFO]'
l.log('test')  # → [INFO] test

See also

Open in the full glossarydocs.python.orgReport an inaccuracy