django-eav2/eav/widgets.py

from django.core import validators
from django.core.exceptions import ValidationError
from django.forms.widgets import Textarea

EMPTY_VALUES = validators.EMPTY_VALUES + ('[]',)


class CSVWidget(Textarea):
    is_hidden = False

    def prep_value(self, value):
        """Prepare value before effectively render widget"""
        if value in EMPTY_VALUES:
            return ""
        elif isinstance(value, str):
            return value
        elif isinstance(value, list):
            return ";".join(value)
        raise ValidationError('Invalid format.')

    def render(self, name, value, **kwargs):
        value = self.prep_value(value)
        return super().render(name, value, **kwargs)