<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def user_pk_to_url_str(user):
 """ This should return a string. """

    User = get_user_model()
    if issubclass(type(User._meta.pk), models.UUIDField):
        if isinstance(user.pk, six.string_types):
            return user.pk
        return user.pk.hex

    ret = user.pk
    if isinstance(ret, six.integer_types):
        ret = int_to_base36(user.pk)
    return str(ret)