User serializer with validate_password

serializers.py

class UserSerializer(serializers.ModelSerializer):

    def validate_password(self, value):
        if value.isalnum():
            raise serializers.ValidationError('password must have atleast one special character.')
        return value

    class Meta:
        model = User
        fields = ('username', 'email', 'first_name', 'last_name', 'password')
        extra_kwargs = {
            'password': {'write_only': True}
        }

Using validate_fieldname when serializer is a subclass of serializers.ModelSerializer seems to miss-validate of the field, falling back to the default validation of the ModelSerializer based on the model field attributes.

I had to implement a basic serializers.Serializer describing all the fields according to the model and then the implementation of thevalidate_fieldname methods did work.