import collections
import itertools


# from jaraco.collections 3.5.1
class DictStack(list, collections.abc.Mapping):
    """
    A stack of dictionaries that behaves as a view on those dictionaries,
    giving preference to the last.

    >>> stack = DictStack([dict(a=1, c=2), dict(b=2, a=2)])
    >>> stack['a']
    2
    >>> stack['b']
    2
    >>> stack['c']
    2
    >>> len(stack)
    3
    >>> stack.push(dict(a=3))
    >>> stack['a']
    3
    >>> set(stack.keys()) == set(['a', 'b', 'c'])
    True
    >>> set(stack.items()) == set([('a', 3), ('b', 2), ('c', 2)])
    True
    >>> dict(**stack) == dict(stack) == dict(a=3, c=2, b=2)
    True
    >>> d = stack.pop()
    >>> stack['a']
    2
    >>> d = stack.pop()
    >>> stack['a']
    1
    >>> stack.get('b', None)
    >>> 'c' in stack
    True
    """

    def __iter__(self):
        dicts = list.__iter__(self)
        return iter(set(itertools.chain.from_iterable(c.keys() for c in dicts)))

    def __getitem__(self, key):
        for scope in reversed(tuple(list.__iter__(self))):
            if key in scope:
                return scope[key]
        raise KeyError(key)

    push = list.append

    def __contains__(self, other):
        return collections.abc.Mapping.__contains__(self, other)

    def __len__(self):
        return len(list(iter(self)))