from django.db.models import CharField

class ModifyingFieldDescriptor(object):
    """ Modifies a field when set using the field's (overriden) .to_python() method. """
    def __init__(self, field):  
        self.field = field  
    def __get__(self, instance, owner=None):
        if instance is None:
            raise AttributeError('Can only be accessed via an instance.')  
        return instance.__dict__[self.field.name]
    def __set__(self, instance, value):
        instance.__dict__[self.field.name] = self.field.to_python(value)

class LowerCaseCharField(CharField):
    def to_python(self, value):
        value = super(LowerCaseCharField, self).to_python(value)
        if isinstance(value, basestring):
            return value.lower()
        return value
    def contribute_to_class(self, cls, name):
        super(LowerCaseCharField, self).contribute_to_class(cls, name)
        setattr(cls, self.name, ModifyingFieldDescriptor(self))
