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"""
Convenience routines for creating non-trivial Field subclasses, as well as
backwards compatibility utilities.
Add SubfieldBase as the metaclass for your Field subclass, implement
to_python() and the other necessary methods and everything will work
seamlessly.
"""
class SubfieldBase(type):
"""
A metaclass for custom Field subclasses. This ensures the model's attribute
has the descriptor protocol attached to it.
"""
def __new__(cls, name, bases, attrs):
new_class = super(SubfieldBase, cls).__new__(cls, name, bases, attrs)
new_class.contribute_to_class = make_contrib(
new_class, attrs.get('contribute_to_class')
)
return new_class
class Creator(object):
"""
A placeholder class that provides a way to set the attribute on the model.
"""
def __init__(self, field):
self.field = field
def __get__(self, obj, type=None):
if obj is None:
raise AttributeError('Can only be accessed via an instance.')
return obj.__dict__[self.field.name]
def __set__(self, obj, value):
obj.__dict__[self.field.name] = self.field.to_python(value)
def make_contrib(superclass, func=None):
"""
Returns a suitable contribute_to_class() method for the Field subclass.
If 'func' is passed in, it is the existing contribute_to_class() method on
the subclass and it is called before anything else. It is assumed in this
case that the existing contribute_to_class() calls all the necessary
superclass methods.
"""
def contribute_to_class(self, cls, name):
if func:
func(self, cls, name)
else:
super(superclass, self).contribute_to_class(cls, name)
setattr(cls, self.name, Creator(self))
return contribute_to_class
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