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A data structure to impose a total order on a collection of objects

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A data structure to impose a total order on a collection of objects.


ordering requires Python 3.6+. If the type hints were removed, it would probably work in earlier versions, as well.


An ordering is represented by an Ordering object. The order may be different to the usual order between objects, or even between objects that are not normally comparable.

>>> from ordering import Ordering
>>> order = Ordering([2, 0, 1])

We can then compare the objects in our ordering, to check if the first object comes before the second in the order.

>>>, 1) # 1 after 0
>>>, 2) # 2 before 0

Our ordering is mutable, and we can add elements on either end, or immediately before or after an existing element.

>>> order.insert_end(3) # Add 3 on the end
>>>, 3) # 3 after 1

>>> order.insert_after(0, 4) # Insert 4 immediately after 0
>>>, 4) # 4 after 0
>>>, 4) # 4 before 1

Orderings are iterable, yielding their items in order.

>>> list(order)
[2, 0, 4, 1, 3]

And can be used as a key in sorting functions, to sort with respect to that order.

>>> sorted(range(3), key=order)
[2, 0, 1]

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