'Validate that the username is alphanumeric and is not already in use.
Also validates that the username is not listed in
``USERENA_FORBIDDEN_USERNAMES`` list.'

def clean_username(self):

    try:
        user = get_user_model().objects.get(username__iexact=self.cleaned_data[u'username'])
    except get_user_model().DoesNotExist:
        pass
    else:
        if (userena_settings.USERENA_ACTIVATION_REQUIRED and UserenaSignup.objects.filter(user__username__iexact=self.cleaned_data[u'username']).exclude(activation_key=userena_settings.USERENA_ACTIVATED)):
            raise forms.ValidationError(_(u'This    username    is    already    taken    but    not    confirmed.    Please    check    your    email    for    verification    steps.'))
        raise forms.ValidationError(_(u'This    username    is    already    taken.'))
    if (self.cleaned_data[u'username'].lower() in userena_settings.USERENA_FORBIDDEN_USERNAMES):
        raise forms.ValidationError(_(u'This    username    is    not    allowed.'))
    return self.cleaned_data[u'username']
