Joshua Bronson added the comment:
Come to think of it, to be exact, rather than extending Reversible,
OrderedMapping could extend a narrower interface, something like
collections.abc.Ordered, along with extending Mapping. (Reversible implies
Ordered, but Ordered does not imply Reversible: a singly-linked list is Ordered
but not Reversible, for example.)
I guess we don't currently have Ordered because it wouldn't necessarily add any
abstractmethods beyond what Iterable already provides. But it could provide a
generic, concrete __eq__ implementation, something like:
class Ordered(Iterable):
def __eq__(self, other):
if not isinstance(other, Ordered):
return NotImplemented
return all(i == j for (i, j) in zip(self, other))
which is not something that any existing collections.abc Iterable currently
provides.
Even if an Ordered interface that's totally empty were available in the
standard library, then Iterables across diparate codebases could opt into
extending it as a standard signal that they iterate over their elements in a
well-defined order, and could participate in other polymorphic code. Currently
we have no such standard signal, do we?
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