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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_username(form, field):
 """Wrap username validator for WTForms."""

        try:
            validate_username(field.data)
        except ValueError as e:
            raise ValidationError(e)

        try:
            user_profile = UserProfile.get_by_username(field.data)
            if current_userprofile.is_anonymous or \
                    (current_userprofile.user_id != user_profile.user_id and
                     field.data != current_userprofile.username):
                # NOTE: Form validation error.
                raise ValidationError(_('Username already exists.'))
        except NoResultFound:
            return