# http://code.activestate.com/recipes/577413-topological-sort/
#
# The MIT License (MIT)
#
# Copyright (c) ActiveState.com
#
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from pprint import pformat

try:
    # Python 3
    from functools import reduce
except:
    pass


def toposort2(data):
    if len(data) == 0:
        return

    for k, v in data.items():
        v.discard(k) # Ignore self dependencies

    # add items that are listed as dependencies but not as dependents to data
    extra_items_in_deps = reduce(set.union, data.values()) - set(data.keys())
    data.update(dict([(item,set()) for item in extra_items_in_deps]))

    while True:
        ordered = set(item for item,dep in data.items() if len(dep) == 0)
        if len(ordered) == 0:
            break
        yield sorted(ordered, key=lambda x:repr(x))
        data = dict([(item, (dep - ordered)) for item,dep in data.items()
                                                        if item not in ordered])

    assert not data, "A cyclic dependency exists amongst\n%s" % pformat(data)