Python Metaclass

In backtrader py3.py class, there is a magic method which is used extensively by the code.

# This is from Armin Ronacher from Flash simplified later by six
def with_metaclass(meta, *bases):
    """Create a base class with a metaclass."""
    # This requires a bit of explanation: the basic idea is to make a dummy
    # metaclass for one level of class instantiation that replaces itself with
    # the actual metaclass.
    class metaclass(meta):
        def __new__(cls, name, this_bases, d):
            return meta(name, bases, d)
    return type.__new__(metaclass, str('temporary_class'), (), {})

The with_metaclass() function makes use of the fact that metaclasses are

a) inherited by subclasses, and
b) a metaclass can be used to generate new classes and
c) when you subclass from a base class with a metaclass, creating the actual subclass object is delegated to the metaclass.
d) type.__new__(metaclass, 'temporary_class', (), {}) uses the metaclass metaclass to create a new class object named temporary_class that is entirely empty otherwise. type.__new__(metaclass, ...) is used instead of metaclass(...) to avoid using the special metaclass.__new__() implementation that is needed for the slight of hand in a next step to work.
If metaclass(...) is used there will be an extra temporary_class base class.
```
# print(Foo.__mro__)
(<class 'metaclass_tests.Foo'>, <class 'metaclass_tests.temporary_class'>, <class 'object'>)
```

It effectively creates a new, temporary base class with a temporary metaclass metaclass that, when used to create the subclass swaps out the temporary base class.

and it could be used as

class Meta(type):
    def __new__(cls, clsname, bases, attrs):
        print("Meta __new__")
        return super(Meta, cls).__new__(cls, clsname, bases, attrs)

    def __init__(cls, name, bases, attrs):
        print("Meta __init__")
        type.__init__(cls, name, bases, attrs)

    def __call__(cls, *args, **kwargs):
        print("Meta __call__")
        return super(Meta, cls).__call__(*args, **kwargs)


temporary_class = with_metaclass(Meta, object) # L1


class Foo(temporary_class): # L2
    def __new__(cls, *args, **kwargs):
        print("Rectangle __new__")
        return super(Foo, cls).__new__(cls, *args, **kwargs)

    def __int__(self, *args, **kwargs):
        print("Foo __int__")

At L1 a class named temporary_class has been created from metaclass and assigned to a local variable also named temporary_class.
At L2 when the temporary_class is used as the base class, creating the actual Foo class will be delegated to the temporary_class
the def __new__(cls, name, this_bases, d) function will be called and metaclass, Foo, temporary_class and {} will be passed in. Then meta(name, bases, d) will be used to create the actual Foo class. Note that as no instance of metaclass is return in the __new__ method, the __init__ method inside the metaclass will never be called.

Tests

import unittest


def with_metaclass_old(meta, *bases):
    class metaclass(meta):
        def __new__(cls, name, this_bases, d):
            print("metaclass __new__")
            return meta(name, bases, d)

    print("with_metaclass")
    return metaclass(str('temporary_class'), (), {})


def with_metaclass(meta, *bases):
    class metaclass(meta):
        def __new__(cls, name, this_bases, d):
            print("metaclass __new__")
            # return super(meta, cls).__new__(cls, name, bases, d) # metaclass.__init__ being called
            # return type.__new__(cls, name, bases, d) # metaclass.__init__ being called
            # return meta.__new__(cls, name, bases, d) # metaclass.__init__ being called
            return meta(name, bases, d)

        def __init__(cls, name, bases, attrs):
            print("metaclass __init__")

    print("with_metaclass")
    return type.__new__(metaclass, str('temporary_class'), (), {})


class Meta(type):
    def __new__(cls, clsname, bases, attrs):
        print("Meta __new__")
        return super(Meta, cls).__new__(cls, clsname, bases, attrs)

    def __init__(cls, name, bases, attrs):
        print("Meta __init__")
        type.__init__(cls, name, bases, attrs)

    def __call__(cls, *args, **kwargs):
        print("Meta __call__")
        return super(Meta, cls).__call__(*args, **kwargs)


temporary_class = with_metaclass(Meta, object)


class Foo(temporary_class):
    def __new__(cls, *args, **kwargs):
        print("Foo __new__")
        return super(Foo, cls).__new__(cls, *args, **kwargs)

    def __int__(self, *args, **kwargs):
        print("Rectangle __int__")


class MetaclassTests(unittest.TestCase):
    def setUp(self):
        print(">>>> classes have been setup")
        print(type(Foo))
        print(Foo.__mro__)

    def test_create_instance(self):
        print(">>>> test_create_instance")
        foo = Foo()


    def tearDown(self):
        pass


if __name__ == "__main__":
    unittest.main()

Reference

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Last updated 5 months ago