Define `__init__` in a Python class to initialize each new instance. Python calls it when an instance is created, passing that instance as the first argument, conventionally named `self`. Despite its common label of “constructor,” `__init__` initializes an object; `__new__` creates it. Override `__new__` only for less common cases, such as subclassing an immutable type. section 15.1.1
Initialize a field
class Circle: def __init__(self, radius=1): self.radius = radius small = Circle() large = Circle(5) print(small.radius, large.radius) # 1 5 The default supplies a value when no argument is given. Assigning to `self.radius` creates the field on that instance; it can be changed later. section 15.1.1 section 15.8
Two practical rules:
- A subclass initializer must explicitly call its base-class initializer when it needs that initialization. For example, `super().__init__(x, y)` can set inherited position fields before the subclass sets its own radius. section 3.6 section 15.8
- Overriding `__setitem__` in a `dict` subclass does not check every way to populate it: initial key-value pairs can bypass that method. Override `__init__` too if initial contents need validation; one approach calls the parent initializer and then checks `self`. section 17.11
A Python class has only one `__init__` method. The similarly named `__init__.py` is a package file, not an instance initializer. section 15.1.1 section 18.6.2
