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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def serialize(self, obj, fields):
 """ Serializes a single model instance to a Python dict, based on the specified list of fields. """


        data = {}
        remaining_fields = []
        for field in fields:
            if callable(field):  # Callable
                data[field.__name__] = field(obj)
            elif hasattr(self, field) and callable(getattr(self, field)):  # Method on the view
                data[field] = getattr(self, field)(obj)
            elif hasattr(obj, field):  # Callable/property/field on the model
                attr = getattr(obj, field)
                if isinstance(attr, Model):
                    data[field] = attr.pk
                elif isinstance(attr, Manager):
                    data[field] = [item['pk'] for item in attr.values('pk')]
                elif callable(attr):  # Callable on the model
                    data[field] = attr()
                else:
                    remaining_fields.append(field)
            else:
                raise AttributeError('Invalid field: %s' % field)

        # Add on db fields
        serializer = Serializer()
        serializer.serialize([obj], fields=list(remaining_fields))
        data.update(serializer.getvalue()[0]['fields'])

        # Any remaining fields should be properties on the model
        remaining_fields = set(remaining_fields) - set(data.keys())

        for field in remaining_fields:
            data[field] = getattr(obj, field)

        return data