def validate(self, value):
        errors = {}
        if self.field:
            if hasattr(value, 'iteritems') or hasattr(value, 'items'):
                sequence = value.iteritems()
            else:
                sequence = enumerate(value)
            for k, v in sequence:
                try:
                    self.field._validate(v)
                except ValidationError, error:
                    errors[k] = error.errors or error
                except (ValueError, AssertionError), error:
                    errors[k] = error

            if errors:
                field_class = self.field.__class__.__name__
                self.error('Invalid %s item (%s)' % (field_class, value),
                           errors=errors)
        # Don't allow empty values if required
        if self.required and not value:
            self.error('Field is required and cannot be empty')
If field is provided ensure the value is valid.
dataset/ETHPy150Open MongoEngine/mongoengine/mongoengine/base/fields.py/ComplexBaseField.validate