'Returns ``User`` instance based on the given identification.'

def clean_user(self):

    identification = self.cleaned_data[u'user']
    user_model = get_user_model()
    try:
        username_field = user_model.USERNAME_FIELD
    except AttributeError:
        username_field = u'username'
    try:
        user = user_model.objects.get(**{username_field: identification})
        return user
    except user_model.DoesNotExist:
        raise forms.ValidationError(self.fields[u'user'].error_messages[u'does_not_exist'])
