[Python-ideas] PEP 572: Statement-Local Name Bindings, take three!

[Chris Angelico]

On Mon, Mar 26, 2018 at 3:34 PM, Guido van Rossum <guido at python.org> wrote:
> On Sun, Mar 25, 2018 at 6:29 PM, Chris Angelico <rosuav at gmail.com> wrote:
>>
>> On Mon, Mar 26, 2018 at 12:24 PM, Guido van Rossum <guido at python.org>
>> wrote:
>> > The scope question is far from easy though. I find it particularly
>> > grating
>> > that an inline assignment occurs in an 'if' statement, its scope is the
>> > entire body of the 'if'. If that body is two pages long, by the end of
>> > it
>> > the reader (or even the writer!) may well have lost track of where it
>> > was
>> > defined and may be confused by the consequence past the end of the body.
>>
>> I think this one can be given to style guides. The useful situations
>> (eg regex match capturing) are sufficiently valuable that the
>> less-useful ones can just come along for the ride, just like "x =
>> lambda: ..." is perfectly valid even though "def" would be preferable.
>
>
> Not so fast. There's a perfectly reasonable alternative to sublocal scopes
> -- just let it assign to a local variable in the containing scope. That's
> the same as what Python does for for-loop variables. Note that for
> comprehensions it still happens to do the right thing (assuming we interpret
> the comprehension's private local scope to be the containing scope).
>
> This alternative has significant advantages in my view -- it doesn't require
> a whole new runtime mechanism to implement it (in CPython you can just
> generate bytecode to store and load locals), and it doesn't require a new
> section in the documentation to explain the new type of variable scope. Also
> it would make Steven happy. :-)

I'm still liking the sublocal system, but making assignment
expressions capable of standing plausibly without them is a Good
Thing.

> Perhaps we could even remove the requirement to parenthesize the new form of
> assignment, so we could say that at the statement level "<var> = <expr>" and
> "<var> := <expr>" just mean the same thing, or "=" is a shorthand for ":=",
> or whatever. In most cases it still makes sense to parenthesize it, since
> the := operator should have the same priority as the regular assignment
> operator, which means that "if x := f() and x != 42:" is broken and should
> be written as "if (x := f()) and x != 42:". But that could be a style guide
> issue. (Also note that I'm not proposing to add "+:=" etc., even though in C
> that's supported.)

That's about where I was thinking of putting it; "test" gets defined
potentially as "NAME := test". It has to right-associate.

At the moment, I'm going to be restricting it to simple names only, so
you can't say "x[1] := 2". That may be changed later.

ChrisA