在python3中繼承自collections.abc的MutableMapping
有5個抽象方法需要實作: __getitem__, __setitem__, __delitem__, __iter__, __len__

    def __init__(self, data=None, **kwargs):
        self._store = OrderedDict()
        if data is None:
            data = {}
        self.update(data, **kwargs)

    def __setitem__(self, key, value):
        # Use the lowercased key for lookups, but store the actual
        # key alongside the value.
        self._store[key.lower()] = (key, value)

    def __getitem__(self, key):
        return self._store[key.lower()][1]
        
    def lower_items(self):
        """Like iteritems(), but with all lowercase keys."""
        return (
            (lowerkey, keyval[1])
            for (lowerkey, keyval)
            in self._store.items()
        )

內部儲存使用OrderedDict，使用lowercased key操作元素`，提供lower_items方法回傳lowercased key與其值。可以透過test確認使用方式

class TestCaseInsensitiveDict:

    @pytest.fixture(autouse=True)
    def setup(self):
        """CaseInsensitiveDict instance with "Accept" header."""
        self.case_insensitive_dict = CaseInsensitiveDict()
        self.case_insensitive_dict['Accept'] = 'application/json'

    def test_list(self):
        assert list(self.case_insensitive_dict) == ['Accept']

    possible_keys = pytest.mark.parametrize('key', ('accept', 'ACCEPT', 'aCcEpT', 'Accept'))

    @possible_keys
    def test_getitem(self, key):
        assert self.case_insensitive_dict[key] == 'application/json'

    @possible_keys
    def test_delitem(self, key):
        del self.case_insensitive_dict[key]
        assert key not in self.case_insensitive_dict

    def test_lower_items(self):
        assert list(self.case_insensitive_dict.lower_items()) == [('accept', 'application/json')]

    def test_repr(self):
        assert repr(self.case_insensitive_dict) == "{'Accept': 'application/json'}"

    def test_copy(self):
        copy = self.case_insensitive_dict.copy()
        assert copy is not self.case_insensitive_dict
        assert copy == self.case_insensitive_dict

    @pytest.mark.parametrize(
        'other, result', (
            ({'AccePT': 'application/json'}, True),
            ({}, False),
            (None, False)
        )
    )
    def test_instance_equality(self, other, result):
        assert (self.case_insensitive_dict == other) is result

主要使用在request和response的header

def default_headers():
    """
    :rtype: requests.structures.CaseInsensitiveDict
    """
    return CaseInsensitiveDict({
        'User-Agent': default_user_agent(),
        'Accept-Encoding': ', '.join(('gzip', 'deflate')),
        'Accept': '*/*',
        'Connection': 'keep-alive',
    })

參考

• collections.abc — Abstract Base Classes for Containers
• Understanding getitem method
• python 的 iterator