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Concept
Metaclass
Summary
In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Metaclasses can be implemented by having classes be first-class citizens, in which case a metaclass is simply an object that constructs classes. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.
In Python, the builtin class type is a metaclass. Consider this simple Python class:
class Car:
def init(self, make: str, model: str, year: int, color: str):
self.make = make
self.model = model
self.year = year
self.color = color
@property
def description(self) -> str:
"""Return a description of this car."""
return f"{self.color} {self.make} {self.model}"
At run time, Car itself is an instance of type. The source code of the Car class, shown above, does not include such details as the size in bytes of Car objects, their binary layout in memory, how they are allocated, that the init method is automatically called each time a Car is created, and so on. These details come into play not only when a new Car object is created, but also each time any attribute of a Car is accessed. In languages without metaclasses, these details are defined by the language specification and can't be overridden. In Python, the metaclass - type - controls these details of Car's behavior. They can be overridden by using a different metaclass instead of type.
The above example contains some redundant code to do with the four attributes make, model, year, and color. It is possible to eliminate some of this redundancy using a custom metaclass. In Python, a metaclass is most easily defined as a subclass of type.
class AttributeInitType(type):
def call(self, *args, **kwargs):
"""Create a new instance.
Official source
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