[Python-Dev] PEP 572: Assignment Expressions

[Nick Coghlan]

On 25 April 2018 at 03:01, Tim Peters <tim.peters at gmail.com> wrote:
> assignment statement buried deep inside expressions.  But in
> conventional use, "binding" is restricted to identifiers, which vastly
> simplifies the mental model for "the worst" that can happen.

[snip]

> But, in the absence of Guido chiming in, it's really up to you.  A few
> people have expressed positive feelings about changing the name to
> "binding expressions", and none opposed it (that I saw), but the
> sample size is too small to claim that "proves" anything.

I go back and forth, as I suspect even if we call them "name binding
expressions" in the language reference (which I think would be a good
idea), they'll often be referred to colloquially as "assignment
expressions". That's probably OK though - list displays and dict
displays are regularly referred to as literals, and the meaning
remains clear, even though the use of literal is technically
inaccurate.

I also think it would be good for the PEP to spell out the following
semantic invariant for code running in a regular namespace:

    _rhs = expr
    assert (target := _rhs) is _rhs and target is _rhs

It's the restriction to single names as targets that makes it possible
to impose such a strong assertion about what the syntax means.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia