# Buggy code
def __setitem__(self, key, value):
    """Operator for Dataset[key] = value.

    Check consistency, and deal with private tags.

    Parameters
    ----------
    key : int
        The tag for the element to be added to the Dataset.
    value : pydicom.dataelem.DataElement or pydicom.dataelem.RawDataElement
        The element to add to the Dataset.

    Raises
    ------
    NotImplementedError
        If `key` is a slice.
    ValueError
        If the `key` value doesn't match DataElement.tag.
    """
    if isinstance(key, slice):
        raise NotImplementedError(
            "Slicing is not supported for setting Dataset elements."
        )

    # OK if is subclass, e.g. DeferredDataElement
    if not isinstance(value, (DataElement, RawDataElement)):
        raise TypeError("Dataset contents must be DataElement instances.")
    if isinstance(value.tag, BaseTag):
        tag = value.tag
    else:
        tag = Tag(value.tag)
    if key != tag:
        raise ValueError("DataElement.tag must match the dictionary key")

    data_element = value
    if tag.is_private:
        # See PS 3.5-2008 section 7.8.1 (p. 44) for how blocks are reserved
        logger.debug("Setting private tag %r" % tag)
        private_block = tag.elem >> 8
        private_creator_tag = Tag(tag.group, private_block)
        if private_creator_tag in self and tag != private_creator_tag:
            if data_element.is_raw:
                data_element = DataElement_from_raw(data_element, self._character_set)
            data_element.private_creator = self[private_creator_tag].value
    self.tags[tag] = data_element


# 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 __setitem__(self, key, value):
    """Operator for Dataset[key] = value.

    Check consistency, and deal with private tags.

    Parameters
    ----------
    key : int
        The tag for the element to be added to the Dataset.
    value : pydicom.dataelem.DataElement or pydicom.dataelem.RawDataElement
        The element to add to the Dataset.

    Raises
    ------
    NotImplementedError
        If `key` is a slice.
    ValueError
        If the `key` value doesn't match DataElement.tag.
    """
    if isinstance(key, slice):
        raise NotImplementedError(
            "Slicing is not supported for setting Dataset elements."
        )

    # OK if is subclass, e.g. DeferredDataElement
    if not isinstance(value, (DataElement, RawDataElement)):
        raise TypeError("Dataset contents must be DataElement instances.")
    if isinstance(value.tag, BaseTag):
        tag = value.tag
    else:
        tag = Tag(value.tag)
    if key != tag:
        raise ValueError("DataElement.tag must match the dictionary key")

    data_element = value
    if tag.is_private:
        # See PS 3.5-2008 section 7.8.1 (p. 44) for how blocks are reserved
        logger.debug("Setting private tag %r" % tag)
        private_block = tag.elem >> 8
        private_creator_tag = Tag(tag.group, private_block)
        if private_creator_tag in self and tag != private_creator_tag:
            if data_element.is_raw:
                data_element = DataElement_from_raw(data_element, self._character_set)
            data_element.private_creator = self[private_creator_tag].value
    self._dict[tag] = data_element