Class ChainMap

Lib/collections/__init__.py:1037–1165

A ChainMap groups multiple dicts (or other mappings) together to create a single, updateable view. The underlying mappings are stored in a list. That list is public and can be accessed or updated using the *maps* attribute. There is no other state. Lookups search the underly

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class ChainMap(_collections_abc.MutableMapping):
    ''' A ChainMap groups multiple dicts (or other mappings) together
    to create a single, updateable view.

    The underlying mappings are stored in a list.  That list is public and can
    be accessed or updated using the *maps* attribute.  There is no other
    state.

    Lookups search the underlying mappings successively until a key is found.
    In contrast, writes, updates, and deletions only operate on the first
    mapping.

    '''

    def __init__(self, *maps):
        '''Initialize a ChainMap by setting *maps* to the given mappings.
        If no mappings are provided, a single empty dictionary is used.

        '''
        self.maps = list(maps) or [{}]          # always at least one map

    def __missing__(self, key):
        raise KeyError(key)

    def __getitem__(self, key):
        for mapping in self.maps:
            try:
                return mapping[key]             # can't use 'key in mapping' with defaultdict
            except KeyError:
                pass
        return self.__missing__(key)            # support subclasses that define __missing__

    def get(self, key, default=None):
        return self[key] if key in self else default    # needs to make use of __contains__

    def __len__(self):
        return len(set().union(*self.maps))     # reuses stored hash values if possible

    def __iter__(self):
        d = {}
        for mapping in map(dict.fromkeys, reversed(self.maps)):
            d |= mapping                        # reuses stored hash values if possible
        return iter(d)

    def __contains__(self, key):
        for mapping in self.maps:
            if key in mapping:
                return True
        return False

    def __bool__(self):
        return any(self.maps)

    @_recursive_repr()
    def __repr__(self):
        return f'{self.__class__.__name__}({", ".join(map(repr, self.maps))})'

    @classmethod