[Python-ideas] [Python-Dev] What's the status of PEP 505: None-aware operators?

Ivan Pozdeev vano at mail.mipt.ru
Thu Nov 30 12:02:08 EST 2017

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.


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