On 29.11.2017 9:08, Steven D'Aprano wrote:
On Tue, Nov 28, 2017 at 12:31:06PM -0800, Raymond Hettinger wrote:
I also cc python-dev to see if anybody here is strongly in favor or against
this inclusion.
Put me down for a strong -1. The proposal would occasionally save a
few keystokes but comes at the expense of giving Python a more Perlish
look and a more arcane feel.
I think that's an unfair characterisation of the benefits of the PEP.
It's not just "a few keystrokes".
Ironically, the equivalent in Perl is // which Python has used for
truncating division since version 2.4 or so. So if we're in danger of
looking "Perlish", that ship has sailed a long time ago.
Perl is hardly the only language with null-coalescing operators -- we
might better describe ?? as being familiar to C#, PHP, Swift and Dart.
That's two mature, well-known languages and two up-and-coming languages.
My experience with these operators in C# says:
* They do save "more than a few keystrokes". Even more importantly, they
allow to avoid double evaluation or the need for a temporary variable
workaround that are inherent in "<expr> if <expr> else <alternative>"
* (An alternative solution for the latter problem would be an
assignment expression, another regularly rejected proposal.)
* They make it temptingly easy and implicit to ignore errors.
* They are alien to Python's standard semantics on search failure which
is to raise an exception rather than return None
[...]
timeout ?? local_timeout ?? global_timeout
As opposed to the status quo:
timeout if timeout is not None else (local_timeout if local_timeout is not
None else global_timeout)
Or shorter, but even harder to understand:
(global_timeout if local_timeout is None else local_timeout) if timeout is
None else timeout
I'd much prefer to teach the version with ?? -- it has a simple
explanation: "the first of the three given values which isn't None". The
?? itself needs to be memorized, but that's no different from any other
operator. The first time I saw ** I was perplexed and couldn't imagine
what it meaned.
Here ?? doesn't merely save a few keystrokes, it significantly reduces
the length and complexity of the expression and entirely cuts out the
duplication of names.
If you can teach
timeout or local_timeout or global_timeout
then you ought to be able to teach ??, as it is simpler: it only
compares to None, and avoids needing to explain or justify Python's
truthiness model.
'foo' in (None ?? ['foo', 'bar'])
If you can understand
'foo' in (False or ['foo', 'bar'])
then surely you can understand the version with ??.
requested_quantity ?? default_quantity * price
Again:
(default_quantity if requested_quantity is None else requested_quantity) *
price
I'd much prefer to read, write and teach the version with ?? over the
status quo.
--
Regards,
Ivan
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