[Python-ideas] Technical possibilities for a syntax [was: Reverse assignment operators ...]

[Steven D'Aprano]

On Thu, Nov 17, 2016 at 12:07:34PM +0900, Stephen J. Turnbull wrote:
> Nick Timkovich writes:
> 
>  > I think the best way to remove compound operators would be to go
>  > back in time and hit Dennis Ritchie with a bat at the exact moment
>  > when the notion of them crosses his mind.
> 
> True enough as a concept, but Python didn't have to implement them for
> immutables.  It might have been interesting to allow in-place
> operations only for mutables.

By definition, you can only perform in-place operations on mutables.

But I guess you are talking about the syntax. Should += raise an 
exception if the assignment target is immutable? Problem is, how is the 
Python interpreter supposed to determine whether or not the target is 
immutable or mutable?

There's no isinstance(obj, Immutable) and I doubt there ever will be. 
More importantly, banning immutables would immediately prohibit the 
number one most common use of augmented assignment, and probably the 
largest motivation for the feature:

    x += 1



> Dunno if Guido thought of that at the time, and I suspect that a lot
> of the voices that convinced him to go against his instincts wanted
> them specifically for integers (a lot of people wanted "++" and "--",
> too).  So even if he did think of it, it might not have satisfied the
> proponents that their needs were being addressed.

Indeed. Supporting `alist += [x]` but not `x += 1` would have been 
deeply unsatisfactory.

I think the status quo is actually very clever: augmented assignment is 
broadly equivalent to the expanded assignment:

    obj += foo
    => obj = obj + foo

except that the target obj has the opportunity to optimize it to an 
in-place operation, even if that makes the semantics slightly different 
from the ordinary & operator.



>  > Except Perl.
> 
> Please don't.

I don't think there is any need for that. No harm is done by a little 
light-hearted banter relating to the rivalry between programming 
language communities. In the big picture, a bit of friendly rivalry 
probably does both communities good.

Regardless of what virtues Perl may or may not have, in some ways the 
philosophy behind Perl is dramatically opposed to that of Python. Many 
of us like Python code because it goes against Perl's ideals. We 
discourage many of the things that Perl and its culture encourages: 
terse, long one-liners, heavy use of regexes, heavy use of sigils and 
symbols, "clever" code, and More Than One Way To Do It. There's a 
natural rivalry between Perl and Python.

We should be permitted to criticise other languages, and even dismiss 
them as useless. Programming languages do not have the assumption of 
dignity owed to human beings.

Nick's dismissal of Perl was obviously intended as light-hearted and not 
entirely serious. This is an informal forum, a mailing list, and 
throw-away comments intended as humour shouldn't be held to the same 
standard as serious statements intended as part of a logical 
argument. Had Nick said "Python should not do X, because Perl does it 
and nothing Perl does is of any value at all" then we'd be justified in 
asking him to defend his claim, or to simply dismiss it as obviously 
ridiculous. But he didn't.

I doubt Nick meant that Perl was literally lacking in any redeeming 
features: exaggeration and hyperbole are time-honoured forms of humour. 
His comments weren't disruptive and they most certainly were not 
attacking anyone. It would require the most extraordinarily thin skin to 
take that as an attack against people who use Perl, and even if somebody 
did, well, this is not a support group for people so lacking in 
self-esteem that their self-image is equated to their choice of 
programming language.

(I believe we've already had to ban, or at least warn, one regular to 
this list under the terms of the CoC, because he couldn't distinguish 
ill-founded criticism of Python the language from criticism of the 
people designing and using the language, and he responded with personal 
attacks.)

I'm sorry that I've written more words defending Nick's throw-away two 
word comment than you expended in lightly chastising him. But I didn't 
want to just say that I disagreed with you -- I wanted to give the 
reasons why I support our right to light-heartedly disrespect other 
languages.

(And of course you're right, Python has borrowed regular expression 
syntax and the Cheeseshop from Perl, and possibly more.)



-- 
Steve