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.'))
Validate that the username is alphanumeric and is not already
dataset/ETHPy150Open davemerwin/blue-channel/external_apps/registration/forms.py/RegistrationForm.clean_username