04.12.17 13:06, Antoine Pitrou пише:
On Mon, 4 Dec 2017 08:45:55 +0200
Serhiy Storchaka <storch...@gmail.com>
wrote:
04.12.17 01:06, Chris Barker пише:
So: has this already been brought up and rejected?
https://bugs.python.org/issue20632
Am I imagining the performance benefits?
This will add an additional overhead. This will be even slower than
passing the key function, since you will need to look up the __key__
method in every item.
That is a reasonable objection. However, looking up a tp_XXX slot is
very cheap.
But introducing a new slot is not easy. This will increase the size of
the type object, break binary compatibility. According to Stefan
Behnel's researches (https://bugs.python.org/issue31336) the main time
of the creation of a new type is spent on initializing slots. This cost
will pay every Python program, even if it doesn't use the __key__ method.
There are many more common and important methods that don't have the
corresponding slot (__length_hint__, __getstate__, __reduce__, __copy__,
keys).
Yes, it is too special-case. I don't see any advantages in comparison
with defining the __lt__ method.
There are definitely advantages. Sorting calls __lt__ for each
comparison (that is, O(n log n) times) while __key__ would only be
called once per item at the start (that is, O(n) times).
It will call __lt__ for each key comparison (the same O(n log n) times),
but *in addition* it will call __key__ O(n) times. You can get the
benefit only when times of calling __key__ is much smaller than the
difference between times of calling item's __lt__ and key's __lt__, and
maybe only for large lists. But why not just pass the key argument when
you sort the large list?
It will be rather confusing if
different methods of sorting produce inconsistent order.
If __key__ is inconsistent with __lt__, it is the same error as making
__lt__ inconsistent with __gt__.
If __key__ is consistent with __lt__, then we can just use __lt__, and
don't introduce new special methods.
And there could be a decorator that generates all comparison methods
from __key__.
The decorator idea LGTM. But it doesn't need the special __key__ method.
Just pass the key function as a decorator argument.
This idea was proposed more than 3.5 years ago. Is there a PyPI package
that implements it?
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