Nov. 30, 2017
5:02 p.m.
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