Campo requerido condicionado

model.py

class ArticleForm(forms.ModelForm):
    brief_summary = forms.CharField(
        widget=TinyMCE(
            attrs={
                'cols': 80,
                'rows': 50
            }
        )
    )

    class Meta:
        model = Article
        fields = ()

    def clean(self):
        data = self.cleaned_data
        highlighted_article = data.get('highlighted_article')
        image = data.get('image')

        if highlighted_article and not image:
            raise forms.ValidationError(_('Provide an image for highlighted article.'))