[Python-Dev] PEP 372 -- Adding an ordered directory to collectionsready for pronouncement (original) (raw)

Guido van Rossum guido at python.org
Mon Mar 2 19:27:16 CET 2009

On Mon, Mar 2, 2009 at 9:39 AM, Raymond Hettinger <python at rcn.com> wrote:

[Antoine Pitrou]

You seem to imply that it is more important for eq to work intuitively between a non-OrderedDict and an OrderedDict, than it is to work intuitively between two OrderedDicts. Yes.  When Armin and I worked through this, it became clear that he had multiple use cases where ordered dicts needed to be used in places that had been originally designed to expect regular dicts. That was also the reason for subclassing dict.  Otherwise, we would have just made a standalone class that defined all the mapping methods. I don't think we going to convince you and that's okay.  We don't have to agree on every design decision.  There were some reasons for either approach and we picked the one that best fit Armin's use cases, that was simplest, that introduced the fewest special rules, and did not create a Liskov violation.  The choice was clearly documented and an alternative was provided for people that needed it.

But you'll have to convince me, and so far I agree with Antoine that doing the comparison without taking the order into account feels really weird. I also think that comparing an odict to a dict with the same items and expecting them to be the same feels wrong. It is not needed to invoke Liskov: Liskov cares about the signature, not about the computed value. There is no rule that says you are not allowed to override odict.eq so that it returns False in cases where Mapping.eq returns True.

I would propose the following formal specification for odict.eq:

def eq(self, other): if not isinstance(other, odict): return NotImplemented # Give other a chance; defaults to False return list(self.items()) == list(other.items())

Obviously an actual implementation can do something more complex instead of the last line, like:

for a, b in zip(self.items(), other.items()): if a != b: return False return True

Outside of your differing judgment on the eq method, are you basically happy with the ordered dict PEP?

I am.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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