django-validation.py

# forms.py
class Form(ModelForm):
    class Meta:
        model = Repository
        fields = ['repository']
    
    def clean_repository(self):
        if self.instance.check_repository_exists():
            raise ValidationError('Repository exists')
        return self.cleaned_data
        
        
# models.py
class Repository(models.Model):
    repository = models.CharField(max_length=100)
    
    def get_dir_name(self):
        return os.path.join(settings.REPOSITORY_DIR, self.repository)
        
    def check_repository_exists(self):
        return os.path.exists(self.get_dir_name())