On 31/07/2018 16:52, Chris Angelico wrote:
On Wed, Aug 1, 2018 at 1:28 AM, MRAB <pyt...@mrabarnett.plus.com> wrote:
On 2018-07-31 08:40, Robin Becker wrote:
A bitbucket user complains that python 3.6.6 with -Wall -b prints warnings
.............
The warning looks wrong to be.
In Python 2, u'a' and b'a' would be treated as the same key, but in Python 3
they are distinct and can co-exist.
Something for Python's bug tracker, I think!
It's a warning specifically requested by the -b option. The two keys
in question have the same hash, which means they have to be compared
directly; they will compare unequal, but because of the -b flag, the
comparison triggers a warning. If that warning is spurious, *don't use
the -b option*.
ChrisA
I looked at the language documentation for 3.7 mappings
These represent finite sets of objects indexed by nearly arbitrary values. The
only types of values not acceptable as keys are values containing lists or
dictionaries or other mutable types that are compared by value rather than by
object identity, the reason being that the efficient implementation of
dictionaries requires a key’s hash value to remain constant. Numeric types used
for keys obey the normal rules for numeric comparison: if two numbers compare
equal (e.g., 1 and 1.0) then they can be used interchangeably to index the same
dictionary entry.
it says explicitly that numeric keys will use numeric comparison, but no mention is made of strings/bytes etc etc and there's an
implication that object identity is used rather than comparison. In python 3.x b'a' is not the same as 'a' either the
documentation is lacking some reference to strings/bytes or the warning is wrong. Using the excuse that normal comparison is being
used seems a bit feeble. It would clearly improve speed if object identity were to be used, but I suppose that's not the case for
other reasons.
--
Robin Becker
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