# Buggy code
def _slice_dataset(self, start, stop, step):
    """Return the element tags in the Dataset that match the slice.

    Parameters
    ----------
    start : int or 2-tuple of int or None
        The slice's starting element tag value, in any format accepted by
        pydicom.tag.Tag.
    stop : int or 2-tuple of int or None
        The slice's stopping element tag value, in any format accepted by
        pydicom.tag.Tag.
    step : int or None
        The slice's step size.

    Returns
    ------
    list of pydicom.tag.Tag
        The tags in the Dataset that meet the conditions of the slice.
    """
    # Check the starting/stopping Tags are valid when used
    if start is not None:
        start = Tag(start)
    if stop is not None:
        stop = Tag(stop)

    all_tags = sorted(self.tags.keys())
    # If the Dataset is empty, return an empty list
    if not all_tags:
        return []

    # Special case the common situations:
    #   - start and/or stop are None
    #   - step is 1

    if start is None:
        if stop is None:
            # For step=1 avoid copying the list
            return all_tags if step == 1 else all_tags[::step]
        else:  # Have a stop value, get values until that point
            step1_list = list(takewhile(lambda x: x < stop, all_tags))
            return step1_list if step == 1 else step1_list[::step]

    # Have a non-None start value.  Find its index
    i_start = bisect_left(all_tags, start)
    if stop is None:
        return all_tags[i_start::step]
    else:
        i_stop = bisect_left(all_tags, stop)
        return all_tags[i_start:i_stop:step]


# Traceback (AttributeError)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "[...]/pydicom/pydicom/dataset.py", line 1280, in update
for key, value in list(dictionary.items()):
File "[...]/pydicom/pydicom/dataset.py", line 532, in __getattr__
return super(Dataset, self).__getattribute__(name)
AttributeError: 'Dataset' object has no attribute 'items'

# Fixed code
def _slice_dataset(self, start, stop, step):
    """Return the element tags in the Dataset that match the slice.

    Parameters
    ----------
    start : int or 2-tuple of int or None
        The slice's starting element tag value, in any format accepted by
        pydicom.tag.Tag.
    stop : int or 2-tuple of int or None
        The slice's stopping element tag value, in any format accepted by
        pydicom.tag.Tag.
    step : int or None
        The slice's step size.

    Returns
    ------
    list of pydicom.tag.Tag
        The tags in the Dataset that meet the conditions of the slice.
    """
    # Check the starting/stopping Tags are valid when used
    if start is not None:
        start = Tag(start)
    if stop is not None:
        stop = Tag(stop)

    all_tags = sorted(self._dict.keys())
    # If the Dataset is empty, return an empty list
    if not all_tags:
        return []

    # Special case the common situations:
    #   - start and/or stop are None
    #   - step is 1

    if start is None:
        if stop is None:
            # For step=1 avoid copying the list
            return all_tags if step == 1 else all_tags[::step]
        else:  # Have a stop value, get values until that point
            step1_list = list(takewhile(lambda x: x < stop, all_tags))
            return step1_list if step == 1 else step1_list[::step]

    # Have a non-None start value.  Find its index
    i_start = bisect_left(all_tags, start)
    if stop is None:
        return all_tags[i_start::step]
    else:
        i_stop = bisect_left(all_tags, stop)
        return all_tags[i_start:i_stop:step]