Advanced Python - metaclasses

https://github.com/ingleby/prescons

metaclass

Examples: - add base_fields attribute to Django models - class Meta: abstract = True -> meta class throws error - ForeignKey -> add xyz_set manager object

classes recap: - isinstance, type - create classes dynamically

name = 'ExampleClass'
bases = (object,)
attrs = {
    '__init__': lambda self: print('hello from __init__')
}
ExampleClass = type(name, bases, attrs)

def create_class(name, bases, attrs):
    # ...
    return type(name, bases, attrs)

can add to attrs, bases etc

• Q: what is type?
• type is a class
• classes are factories for creating objects

we can subclass type

class FormType(type):
    def __new__(cls, name, bases, attrs):
    fields = {k: v for k, v in attrs.items() if isinstance(v, forms.Field)}
    attrs['base_fields'] = fields
    return type.__new__(cls, name, bases, attrs)

PonyForm = FormType(name, bases, attrs)

in Django - search for metaclass - see ModelBase in django/db/models.py for lots of big/hairy metaclass stuff

Python 3

class Form(metaclass=FormType):
    ...

For supporting both 2.x and 3.x, see library called six.

key points:

• classes are factories for creating objects
• metaclasses are factories for creating classes
• (missed the last one)