**Question**: How to write DRY code for field validation in both form and serializer?

**Example**:
I have simple django app with model form, which validates `passengers` field for `Order`:

```
def clean_passengers(self):
    passengers = self.cleaned_data['passengers']
    if passengers > self.flight.available_seats:
        raise forms.ValidationError(
            _(u'''Passengers count can`t be greater then seats count'''))
    return passengers

```

And same code for validation in `Order` serializer:

```
def validate_passengers(self, attrs, source):
    passengers = attrs[source]
    if passengers > self.flight.available_seats:
        raise serializers.ValidationError(
            _(u'''Passengers count can`t be greater then seats count'''))
    return attrs

```

This isn`t DRY and I have write same logic twice. How I can avoid this? Maybe I can inherit serializer from form or something like this.