Metaclasses in Python

Metaclasses in Python allow you to deeply modify the behaviour of Python classes (in terms of how they’re defined, instantiated, accessed, and more) by replacing the type metaclass that new classes use by default.

Metaclasses in Python: Basic Metaclasses

When type is called with three arguments it behaves as the (meta)class it is, and creates a new instance, ie. it produces a new class/type.

Dummy = type('OtherDummy', (), dict(x=1))
Dummy.class #
Dummy().class.class #

It is possible to subclass type to create an custom metaclass.

class mytype(type):
def init(cls, name, bases, dict):

call the base initializer

type.init(cls, name, bases, dict)
perform custom initialization…
cls.custom_attribute = 2

Now, we have a new custom mytype metaclass which can be used to create classes in the same manner as type.

MyDummy = mytype('MyDummy', (), dict(x=2))
MyDummy.class #
MyDummy().class.class # MyDummy.custom_attribute # 2

When we create a new class using the class keyword the metaclass is by default chosen based on upon the baseclasses.

class Foo(object):
…pass
type(Foo)
type

In the above example the only baseclass is object so our metaclass will be the type of object, which is type. It is possible override the default, however it depends on whether we use Python 2 or Python 3:

Python 2.x Version ≤ 2.7

A special class-level attribute metaclass can be used to specify the metaclass.

class MyDummy(object):
metaclass = mytype
type(MyDummy) #
Python 3.x Version ≥ 3.0

A special metaclass keyword argument specify the metaclass.

class MyDummy(metaclass=mytype):
pass
type(MyDummy) #

Any keyword arguments (except metaclass) in the class declaration will be passed to the metaclass. Thus class MyDummy(metaclass=mytype, x=2) will pass x=2 as a keyword argument to the mytype constructor.

Read this in-depth description of python meta-classes for more details.

Metaclasses in Python: Singletons using metaclasses

A singleton is a pattern that restricts the instantiation of a class to one instance/object. For more info on python singleton design patterns, see here.

class SingletonType(type):
def call(cls, *args, *kwargs): try: return cls.instance except AttributeError: cls.__instance = super(SingletonType, cls).__call(args, **kwargs)
return cls.__instance

Python 2.x Version ≤ 2.7

class MySingleton(object):
metaclass = SingletonType

Python 3.x Version ≥ 3.0

class MySingleton(metaclass=SingletonType):
pass
MySingleton() is MySingleton() # True, only one instantiation occurs

Metaclasses in Python: Using a metaclass

Metaclass syntax

Python 2.x Version ≤ 2.7

class MyClass(object):
metaclass = SomeMetaclass

Python 3.x Version ≥ 3.0

class MyClass(metaclass=SomeMetaclass):
pass

Python 2 and 3 compatibility with six

import six
class MyClass(six.with_metaclass(SomeMetaclass)):
pass

Metaclasses in Python: Introduction to Metaclasses

What is a metaclass?

In Python, everything is an object: integers, strings, lists, even functions and classes themselves are objects. And every object is an instance of a class.

To check the class of an object x, one can call type(x), so:

type(5)
type(str)
type([1, 2, 3])
class C(object):
…pass

type(C)

Most classes in python are instances of type. type itself is also a class. Such classes whose instances are also classes are called metaclasses.

The Simplest Metaclass

OK, so there is already one metaclass in Python: type. Can we create another one?

class SimplestMetaclass(type):
pass
class MyClass(object):
metaclass = SimplestMetaclass

That does not add any functionality, but it is a new metaclass, see that MyClass is now an instance of

SimplestMetaclass:

type(MyClass)

A Metaclass which does Something

A metaclass which does something usually overrides type’s new, to modify some properties of the class to be created, before calling the original new which creates the class:

class AnotherMetaclass(type):
def new(cls, name, parents, dct):
cls is this class
name is the name of the class to be created
parents is the list of the class's parent classes
dct is the list of class's attributes (methods, static variables)
here all of the attributes can be modified before creating the class, e.g.
dct['x'] = 8 # now the class will have a static variable x = 8

return value is the new class. super will take care of that

return super(AnotherMetaclass, cls).new(cls, name, parents, dct)

Custom functionality with metaclasses

Functionality in metaclasses can be changed so that whenever a class is built, a string is printed to standard output, or an exception is thrown. This metaclass will print the name of the class being built.

class VerboseMetaclass(type):
def new(cls, class_name, class_parents, class_dict):
print("Creating class ", class_name)
new_class = super().new(cls, class_name, class_parents, class_dict)
return new_class

You can use the metaclass like so:

class Spam(metaclass=VerboseMetaclass):
def eggs(self):
print("[insert example string here]")
s = Spam()
s.eggs()

The standard output will be:

Creating class Spam

[insert example string here]

The default metaclass

You may have heard that everything in Python is an object. It is true, and all objects have a class:

type(1)
int

The literal 1 is an instance of int. Let’s declare a class:

class Foo(object):
…pass

Now let’s instantiate it:

bar = Foo()

What is the class of bar?

type(bar)
Foo

Nice, bar is an instance of Foo. But what is the class of Foo itself?

type(Foo)
type

Ok, Foo itself is an instance of type. How about type itself?

type(type)
type

So what is a metaclass? For now let’s pretend it is just a fancy name for the class of a class. Takeaways:

Everything is an object in Python, so everything has a class

The class of a class is called a metaclass

The default metaclass is type, and by far it is the most common metaclass

But why should you know about metaclasses? Well, Python itself is quite “hackable”, and the concept of metaclass is important if you are doing advanced stuff like meta-programming or if you want to control how your classes are initialized.

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