# dokumentasi Django 2.2.9.dev20191202104323

# Kode sumber untuk django.utils.datastructures

    import copy
    from collections import OrderedDict
    from collections.abc import Mapping

    class OrderedSet:
        """
        A set which keeps the ordering of the inserted items.
        Currently backs onto OrderedDict.
        """

        def __init__(self, iterable=None):
            self.dict = OrderedDict.fromkeys(iterable or ())

        def add(self, item):
            self.dict[item] = None

        def remove(self, item):
            del self.dict[item]

        def discard(self, item):
            try:
                self.remove(item)
            except KeyError:
                pass

        def __iter__(self):
            return iter(self.dict)

        def __contains__(self, item):
            return item in self.dict

        def __bool__(self):
            return bool(self.dict)

        def __len__(self):
            return len(self.dict)

    class MultiValueDictKeyError(KeyError):
        pass

    class MultiValueDict(dict):
        """
        A subclass of dictionary customized to handle multiple values for the
        same key.

        >>> d = MultiValueDict({'name': ['Adrian', 'Simon'], 'position': ['Developer']})
        >>> d['name']
        'Simon'
        >>> d.getlist('name')
        ['Adrian', 'Simon']
        >>> d.getlist('doesnotexist')
        []
        >>> d.getlist('doesnotexist', ['Adrian', 'Simon'])
        ['Adrian', 'Simon']
        >>> d.get('lastname', 'nonexistent')
        'nonexistent'
        >>> d.setlist('lastname', ['Holovaty', 'Willison'])

        This class exists to solve the irritating problem raised by cgi.parse_qs,
        which returns a list for every key, even though most Web forms submit
        single name-value pairs.
        """
        def __init__(self, key_to_list_mapping=()):
            super().__init__(key_to_list_mapping)

        def __repr__(self):
            return "<%s: %s>" % (self.__class__.__name__, super().__repr__())

        def __getitem__(self, key):
            """
            Return the last data value for this key, or [] if it's an empty list;
            raise KeyError if not found.
            """
            try:
                list_ = super().__getitem__(key)
            except KeyError:
                raise MultiValueDictKeyError(key)
            try:
                return list_[-1]
            except IndexError:
                return []

        def __setitem__(self, key, value):
            super().__setitem__(key, [value])

        def __copy__(self):
            return self.__class__([
                (k, v[:])
                for k, v in self.lists()
            ])

        def __deepcopy__(self, memo):
            result = self.__class__()
            memo[id(self)] = result
            for key, value in dict.items(self):
                dict.__setitem__(result, copy.deepcopy(key, memo),
                                 copy.deepcopy(value, memo))
            return result

        def __getstate__(self):
            return {**self.__dict__, '_data': {k: self._getlist(k) for k in self}}

        def __setstate__(self, obj_dict):
            data = obj_dict.pop('_data', {})
            for k, v in data.items():
                self.setlist(k, v)
            self.__dict__.update(obj_dict)

        def get(self, key, default=None):
            """
            Return the last data value for the passed key. If key doesn't exist
            or value is an empty list, return `default`.
            """
            try:
                val = self[key]
            except KeyError:
                return default
            if val == []:
                return default
            return val

        def _getlist(self, key, default=None, force_list=False):
            """
            Return a list of values for the key.

            Used internally to manipulate values list. If force_list is True,
            return a new copy of values.
            """
            try:
                values = super().__getitem__(key)
            except KeyError:
                if default is None:
                    return []
                return default
            else:
                if force_list:
                    values = list(values) if values is not None else None
                return values

        def getlist(self, key, default=None):
            """
            Return the list of values for the key. If key doesn't exist, return a
            default value.
            """
            return self._getlist(key, default, force_list=True)

        def setlist(self, key, list_):
            super().__setitem__(key, list_)

        def setdefault(self, key, default=None):
            if key not in self:
                self[key] = default
                # Do not return default here because __setitem__() may store
                # another value -- QueryDict.__setitem__() does. Look it up.
            return self[key]

        def setlistdefault(self, key, default_list=None):
            if key not in self:
                if default_list is None:
                    default_list = []
                self.setlist(key, default_list)
                # Do not return default_list here because setlist() may store
                # another value -- QueryDict.setlist() does. Look it up.
            return self._getlist(key)

        def appendlist(self, key, value):
            """Append an item to the internal list associated with key."""
            self.setlistdefault(key).append(value)

        def items(self):
            """
            Yield (key, value) pairs, where value is the last item in the list
            associated with the key.
            """
            for key in self:
                yield key, self[key]

        def lists(self):
            """Yield (key, list) pairs."""
            return iter(super().items())

        def values(self):
            """Yield the last value on every key list."""
            for key in self:
                yield self[key]

        def copy(self):
            """Return a shallow copy of this object."""
            return copy.copy(self)

        def update(self, *args, **kwargs):
            """Extend rather than replace existing key lists."""
            if len(args) > 1:
                raise TypeError("update expected at most 1 argument, got %d" % len(args))
            if args:
                other_dict = args[0]
                if isinstance(other_dict, MultiValueDict):
                    for key, value_list in other_dict.lists():
                        self.setlistdefault(key).extend(value_list)
                else:
                    try:
                        for key, value in other_dict.items():
                            self.setlistdefault(key).append(value)
                    except TypeError:
                        raise ValueError("MultiValueDict.update() takes either a MultiValueDict or dictionary")
            for key, value in kwargs.items():
                self.setlistdefault(key).append(value)

        def dict(self):
            """Return current object as a dict with singular values."""
            return {key: self[key] for key in self}

    class ImmutableList(tuple):
        """
        A tuple-like object that raises useful errors when it is asked to mutate.

        Example::

            >>> a = ImmutableList(range(5), warning="You cannot mutate this.")
            >>> a[3] = '4'
            Traceback (most recent call last):
                ...
            AttributeError: You cannot mutate this.
        """

        def __new__(cls, *args, warning='ImmutableList object is immutable.', **kwargs):
            self = tuple.__new__(cls, *args, **kwargs)
            self.warning = warning
            return self

        def complain(self, *wargs, **kwargs):
            if isinstance(self.warning, Exception):
                raise self.warning
            else:
                raise AttributeError(self.warning)

        # All list mutation functions complain.
        __delitem__ = complain
        __delslice__ = complain
        __iadd__ = complain
        __imul__ = complain
        __setitem__ = complain
        __setslice__ = complain
        append = complain
        extend = complain
        insert = complain
        pop = complain
        remove = complain
        sort = complain
        reverse = complain

    class DictWrapper(dict):
        """
        Wrap accesses to a dictionary so that certain values (those starting with
        the specified prefix) are passed through a function before being returned.
        The prefix is removed before looking up the real value.

        Used by the SQL construction code to ensure that values are correctly
        quoted before being used.
        """
        def __init__(self, data, func, prefix):
            super().__init__(data)
            self.func = func
            self.prefix = prefix

        def __getitem__(self, key):
            """
            Retrieve the real value after stripping the prefix string (if
            present). If the prefix is present, pass the value through self.func
            before returning, otherwise return the raw value.
            """
            use_func = key.startswith(self.prefix)
            if use_func:
                key = key[len(self.prefix):]
            value = super().__getitem__(key)
            if use_func:
                return self.func(value)
            return value

    def _destruct_iterable_mapping_values(data):
        for i, elem in enumerate(data):
            if len(elem) != 2:
                raise ValueError(
                    'dictionary update sequence element #{} has '
                    'length {}; 2 is required.'.format(i, len(elem))
                )
            if not isinstance(elem[0], str):
                raise ValueError('Element key %r invalid, only strings are allowed' % elem[0])
            yield tuple(elem)

    class CaseInsensitiveMapping(Mapping):
        """
        Mapping allowing case-insensitive key lookups. Original case of keys is
        preserved for iteration and string representation.

        Example::

            >>> ci_map = CaseInsensitiveMapping({'name': 'Jane'})
            >>> ci_map['Name']
            Jane
            >>> ci_map['NAME']
            Jane
            >>> ci_map['name']
            Jane
            >>> ci_map  # original case preserved
            {'name': 'Jane'}
        """

        def __init__(self, data):
            if not isinstance(data, Mapping):
                data = {k: v for k, v in _destruct_iterable_mapping_values(data)}
            self._store = {k.lower(): (k, v) for k, v in data.items()}

        def __getitem__(self, key):
            return self._store[key.lower()][1]

        def __len__(self):
            return len(self._store)

        def __eq__(self, other):
            return isinstance(other, Mapping) and {
                k.lower(): v for k, v in self.items()
            } == {
                k.lower(): v for k, v in other.items()
            }

        def __iter__(self):
            return (original_key for original_key, value in self._store.values())

        def __repr__(self):
            return repr({key: value for key, value in self._store.values()})

        def copy(self):
            return self