A metaclass instantiates and defines behavior for a class just like a class instantiates and defines behavior for an instance.
Pseudocode:
>>> Class(...)
instance
The above should look familiar. Well, where does Class come from? It's an instance of a metaclass (also pseudocode):
>>> Metaclass(...)
Class
In real code, we can pass the default metaclass, type, everything we need to instantiate a class and we get a class:
>>> type('Foo', (object,), {}) # requires a name, bases, and a namespace
<class '__main__.Foo'>
You use a metaclass every time you create a class:
When you write a class definition, for example, like this,
class Foo(object):
'demo'
You instantiate a class object.
>>> Foo
<class '__main__.Foo'>
>>> isinstance(Foo, type), isinstance(Foo, object)
(True, True)
It is the same as functionally calling type with the appropriate arguments and assigning the result to a variable of that name:
name = 'Foo'
bases = (object,)
namespace = {'__doc__': 'demo'}
Foo = type(name, bases, namespace)
Note, some things automatically get added to the __dict__, i.e., the namespace:
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
'__module__': '__main__', '__weakref__': <attribute '__weakref__'
of 'Foo' objects>, '__doc__': 'demo'})
The metaclass of the object we created, in both cases, is type.
(A side-note on the contents of the class __dict__: __module__ is there because classes must know where they are defined, and __dict__ and __weakref__ are there because we don't define __slots__ - if we define __slots__ we'll save a bit of space in the instances, as we can disallow __dict__ and __weakref__ by excluding them. For example:
>>> Baz = type('Bar', (object,), {'__doc__': 'demo', '__slots__': ()})
>>> Baz.__dict__
mappingproxy({'__doc__': 'demo', '__slots__': (), '__module__': '__main__'})
... but I digress.)
So now when we create an object with this metaclass, the __repr__ echoed on the command line provides a much less ugly sight than the default:
>>> class Bar(object): pass
>>> Baz = Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
>>> Baz
Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
With a nice __repr__ defined for the class instance, we have a stronger ability to debug our code. However, much further checking with eval(repr(Class)) is unlikely (as functions would be rather impossible to eval from their default __repr__'s).
An expected usage: __prepare__ a namespace
If, for example, we want to know in what order a class's methods are created in, we could provide an ordered dict as the namespace of the class. We would do this with __prepare__ which returns the namespace dict for the class if it is implemented in Python 3:
from collections import OrderedDict
class OrderedType(Type):
@classmethod
def __prepare__(metacls, name, bases, **kwargs):
return OrderedDict()
def __new__(cls, name, bases, namespace, **kwargs):
result = Type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result
And usage:
class OrderedMethodsObject(object, metaclass=OrderedType):
def method1(self): pass
def method2(self): pass
def method3(self): pass
def method4(self): pass
And now we have a record of the order in which these methods (and other class attributes) were created:
>>> OrderedMethodsObject.members
('__module__', '__qualname__', 'method1', 'method2', 'method3', 'method4')
Note, this example was adapted from the documentation - the new enum in the standard librarydoes this.
So what we did was instantiate a metaclass by creating a class. We can also treat the metaclass as we would any other class. It has a method resolution order:
>>> inspect.getmro(OrderedType)
(<class '__main__.OrderedType'>, <class '__main__.Type'>, <class 'type'>, <class 'object'>)
And it has approximately the correct repr (which we can no longer eval unless we can find a way to represent our functions.):
>>> OrderedMethodsObject
OrderedType('OrderedMethodsObject', (object,), {'method1': <function OrderedMethodsObject.method1 at 0x0000000002DB01E0>, 'members': ('__module__', '__qualname__', 'method1', 'method2', 'method3', 'method4'), 'method3': <function OrderedMet
hodsObject.method3 at 0x0000000002DB02F0>, 'method2': <function OrderedMethodsObject.method2 at 0x0000000002DB0268>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'OrderedMethodsObject' objects>, '__doc__': None, '__d
ict__': <attribute '__dict__' of 'OrderedMethodsObject' objects>, 'method4': <function OrderedMethodsObject.method4 at 0x0000000002DB0378>})
