def clean_username(self):
        try:
            user = User.objects.get(username__iexact=self.cleaned_data['username'])
        except User.DoesNotExist:
            return self.cleaned_data['username']
        raise forms.ValidationError(_(u'This username is already taken. Please choose another.'))
Handler that supports param values from URIs.
dataset/ETHPy150Open davemerwin/blue-channel/external_apps/registration/forms.py/RegistrationForm.clean_username--crossover with--dataset/ETHPy150Open aws/aws-cli/awscli/argprocess.py/uri_param