def save(self):

        user = self.cleaned_data['subscriber']
        object = self.instance

        if user not in self.subscriptions:
            object.subscribers.add(user)
        self.subscriptions = object.subscribers.all()

        return self.subscriptions
Subscribe
dataset/ETHPy150Open treeio/treeio/treeio/core/forms.py/SubscribeForm.save