[Python-ideas] a sorting protocol dunder method?
steve at pearwood.info
Mon Dec 4 06:41:40 EST 2017
On Sun, Dec 03, 2017 at 10:48:18PM -0800, Carl Meyer wrote:
> I think this is an interesting idea, and I don't believe that either
> performance or "sortable vs comparable" are very relevant.
Performance is always relevant -- while performance shouldn't be the
sole deciding factor, it should be a factor.
And since the entire use-case for this is sorting versus comparison
operators, I'm having trouble understanding why you think that sorting
versus comparison operators is irrelevant.
> I doubt there is much performance to gain here,
I doubt there is any performance gain -- rather a performance hit is far
> and I think the default sort order for
> a class must continue to match its comparison behavior.
This proposal changes that: if a class defines __key__ but not __lt__,
then the default sort behaviour will be different from the comparison
If it defines both, it isn't clear which will be used for sorting.
Should __lt__ take priority, or __key__? Whichever we choose, somebody
is going to be upset and confused by the choice.
> Most often when I define equality and comparison dunder methods for a
> custom class, I'm effectively just deferring the comparison to some
> field or tuple of fields of the object. E.g.
> from functools import total_ordering
> class BankAccount:
> def __init__(self, balance):
> self.balance = balance
This example shows exactly the confusion of concepts I'm talking about.
Why should bank accounts be sorted by their balance, instead of by their
account number, or account name, or date they were opened?
Why should BankAccounts sort differently according to their balance, a
quantity which can change after every transaction? What happens when
somebody decides that bank accounts default sorting should be by their
account name rather than the ever-changing balance?
I'm not saying that it never makes sense to sort a bunch of accounts
according to their balance. I'm saying that functionality is not part of
the account themselves. If it belongs anywhere, it belongs in the
collection of accounts. And even that is dubious: I believe that where
it really belongs is in the report generator that needs to sort the
collection of accounts.
> It'd be nice to be able to eliminate an import
That's an argument that applies to *literally* everything in the
standard library. Should we make everything a built-in?
The prerequisites for eliminating the need for an import should be a
*lot* higher than just "it would be nice".
I disagree with the design of this: it is putting the decision of how to
sort uncomparable objects in the wrong place, in the object itself
rather than in the report that wants to sort them.
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