Python Metaclass example

metaclass.py

"""
Metaclass example

>>> class MyKlass(Klass):
...     my_field = Field()
...
...     class Meta:
...         model = "Model"
...
>>> MyKlass._meta.model
'Model'
>>> 'my_field' in MyKlass.declared_fields
True
"""


class Field(object):
    pass


class KlassOptions(object):
    model = None

    def __new__(cls, meta=None):
        overrides = {}

        if meta:
            for override_name in dir(meta):
                if not override_name.startswith('_'):
                    overrides[override_name] = getattr(meta, override_name)

        return object.__new__(type('KlassOptions', (cls,), overrides))


class DeclarativeMetaclass(type):

    def __new__(cls, name, bases, attrs):
        declared_fields = {}

        for field_name, obj in attrs.items():
            if isinstance(obj, Field):
                field = attrs.pop(field_name)
                declared_fields[field_name] = field

        attrs['declared_fields'] = declared_fields
        new_class = super(DeclarativeMetaclass, cls).__new__(cls, name,
                bases, attrs)
        opts = getattr(new_class, 'Meta', None)
        new_class._meta = KlassOptions(opts)

        return new_class


class Klass(object):
    __metaclass__ = DeclarativeMetaclass


if __name__ == '__main__':
    import doctest
    doctest.testmod()