[Python-Dev] PEP 572: Assignment Expressions

[Chris Angelico]

On Tue, Apr 17, 2018 at 6:26 PM, Chris Jerdonek
<chris.jerdonek at gmail.com> wrote:
> On Tue, Apr 17, 2018 at 12:46 AM, Chris Angelico <rosuav at gmail.com> wrote:
>>
>> Having survived four rounds in the boxing ring at python-ideas, PEP
>> 572 is now ready to enter the arena of python-dev. I'll let the
>> proposal speak for itself. Be aware that the reference implementation
>> currently has a few test failures, which I'm still working on, but to
>> my knowledge nothing will prevent the proposal itself from being
>> successfully implemented.
>
> Very interesting / exciting, thanks!
>
>> Augmented assignment is not supported in expression form::
>>
>>     >>> x +:= 1
>>       File "<stdin>", line 1
>>         x +:= 1
>>             ^
>>     SyntaxError: invalid syntax
>
> Can you include in the PEP a brief rationale for not accepting this
> form? In particular, is the intent never to support it, or is the
> intent to expressly allow adding it at a later date (e.g. after
> getting experience with the simpler form, etc)?

Sure. There are a few reasons; and Imaybe the best place to explain
them is in the rejecteds section.

Augmented assignment is currently all of these:

augassign: ('+=' | '-=' | '*=' | '@=' | '/=' | '%=' | '&=' | '|=' | '^=' |
            '<<=' | '>>=' | '**=' | '//=')

I'm actually not sure whether the augmented-assignment-expression
operators should be "+:=" or ":+=", but either way, it'd be another
thirteen tokens, some of which would be *four* character tokens.
That's an awful lot of extra tokens. There's a lot less value in
chaining on top of a "+=" than a simple assignment, and it doesn't
seem right to have tons of code to support that. (Consider how much
extra code there is to support async functions, and then imagine that
you have a similarly large amount of effort for something that's of
far lesser significance.)

Another reason is that it can be confusing (to a human, not to the
parser) that it would be the result of the augmented assignment, not
the original expression, that would carry through. With regular
assignment (whether it's to a simple name or to a
subscript/attribute), removing the "target :=" part will leave you
with the same value - the value of "x := 1" is 1. With augmented, the
only logical way to do it would be to re-capture the left operand.
Consider:

x = 5
print(x +:= 2)

Logically, this has to set x to 7, then print 7. But in more
complicated situations, it can get confusing more easily than direct
assignment does. Consider, for instance, augmented addition on a list,
or similar.

It could potentially be added later, but I'm not working hard to keep
the option open or anything. Basically it's just "outside the scope of
this document".

ChrisA