<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate(self, value):
 """ Validate that a given new pk to set is always set, and return it. The returned value should be normalized, and will be used without check. """

        if value is None:
            raise ValueError('The pk for %s is not "auto-increment", you must fill it' %
                            self._model.__name__)
        value = self.normalize(value)

        # Check that this pk does not already exist
        if self.exists(value):
            raise UniquenessError('PKField %s already exists for model %s)' %
                                  (value, self._instance.__class__))

        return value