# Buggy code
def __getitem__(self, key):
    """Operator for Dataset[key] request.

    Any deferred data elements will be read in and an attempt will be made
    to correct any elements with ambiguous VRs.

    Examples
    --------
    Indexing using DataElement tag
    >>> ds = Dataset()
    >>> ds.SOPInstanceUID = '1.2.3'
    >>> ds.PatientName = 'CITIZEN^Jan'
    >>> ds.PatientID = '12345'
    >>> ds[0x00100010]
    'CITIZEN^Jan'

    Slicing using DataElement tag
    All group 0x0010 elements in the dataset
    >>> ds[0x00100000:0x0011000]
    (0010, 0010) Patient's Name                      PN: 'CITIZEN^Jan'
    (0010, 0020) Patient ID                          LO: '12345'

    All group 0x0002 elements in the dataset
    >>> ds[(0x0002, 0x0000):(0x0003, 0x0000)]

    Parameters
    ----------
    key
        The DICOM (group, element) tag in any form accepted by
        pydicom.tag.Tag such as [0x0010, 0x0010], (0x10, 0x10), 0x00100010,
        etc. May also be a slice made up of DICOM tags.

    Returns
    -------
    pydicom.dataelem.DataElement or pydicom.dataset.Dataset
        If a single DICOM element tag is used then returns the
        corresponding DataElement. If a slice is used then returns a
        Dataset object containing the corresponding DataElements.
    """
    # If passed a slice, return a Dataset containing the corresponding
    #   DataElements
    if isinstance(key, slice):
        return self._dataset_slice(key)

    if isinstance(key, BaseTag):
        tag = key
    else:
        tag = Tag(key)
    data_elem = self.tags[tag]

    if isinstance(data_elem, DataElement):
        return data_elem
    elif isinstance(data_elem, tuple):
        # If a deferred read, then go get the value now
        if data_elem.value is None:
            from pydicom.filereader import read_deferred_data_element

            data_elem = read_deferred_data_element(
                self.fileobj_type, self.filename, self.timestamp, data_elem
            )

        if tag != BaseTag(0x00080005):
            character_set = self.read_encoding or self._character_set
        else:
            character_set = default_encoding
        # Not converted from raw form read from file yet; do so now
        self[tag] = DataElement_from_raw(data_elem, character_set)

        # If the Element has an ambiguous VR, try to correct it
        if "or" in self[tag].VR:
            from pydicom.filewriter import correct_ambiguous_vr_element

            self[tag] = correct_ambiguous_vr_element(self[tag], self, data_elem[6])

    return self.tags.get(tag)


# 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 __getitem__(self, key):
    """Operator for Dataset[key] request.

    Any deferred data elements will be read in and an attempt will be made
    to correct any elements with ambiguous VRs.

    Examples
    --------
    Indexing using DataElement tag
    >>> ds = Dataset()
    >>> ds.SOPInstanceUID = '1.2.3'
    >>> ds.PatientName = 'CITIZEN^Jan'
    >>> ds.PatientID = '12345'
    >>> ds[0x00100010]
    'CITIZEN^Jan'

    Slicing using DataElement tag
    All group 0x0010 elements in the dataset
    >>> ds[0x00100000:0x0011000]
    (0010, 0010) Patient's Name                      PN: 'CITIZEN^Jan'
    (0010, 0020) Patient ID                          LO: '12345'

    All group 0x0002 elements in the dataset
    >>> ds[(0x0002, 0x0000):(0x0003, 0x0000)]

    Parameters
    ----------
    key
        The DICOM (group, element) tag in any form accepted by
        pydicom.tag.Tag such as [0x0010, 0x0010], (0x10, 0x10), 0x00100010,
        etc. May also be a slice made up of DICOM tags.

    Returns
    -------
    pydicom.dataelem.DataElement or pydicom.dataset.Dataset
        If a single DICOM element tag is used then returns the
        corresponding DataElement. If a slice is used then returns a
        Dataset object containing the corresponding DataElements.
    """
    # If passed a slice, return a Dataset containing the corresponding
    #   DataElements
    if isinstance(key, slice):
        return self._dataset_slice(key)

    if isinstance(key, BaseTag):
        tag = key
    else:
        tag = Tag(key)
    data_elem = self._dict[tag]

    if isinstance(data_elem, DataElement):
        return data_elem
    elif isinstance(data_elem, tuple):
        # If a deferred read, then go get the value now
        if data_elem.value is None:
            from pydicom.filereader import read_deferred_data_element

            data_elem = read_deferred_data_element(
                self.fileobj_type, self.filename, self.timestamp, data_elem
            )

        if tag != BaseTag(0x00080005):
            character_set = self.read_encoding or self._character_set
        else:
            character_set = default_encoding
        # Not converted from raw form read from file yet; do so now
        self[tag] = DataElement_from_raw(data_elem, character_set)

        # If the Element has an ambiguous VR, try to correct it
        if "or" in self[tag].VR:
            from pydicom.filewriter import correct_ambiguous_vr_element

            self[tag] = correct_ambiguous_vr_element(self[tag], self, data_elem[6])

    return self._dict.get(tag)