[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Antoine Pitrou]

On Wed, 25 Apr 2018 18:52:34 -0700
Łukasz Langa <lukasz at langa.pl> wrote:
> > On 25 Apr, 2018, at 5:20 PM, Chris Angelico <rosuav at gmail.com> wrote:
> > 
> > On Thu, Apr 26, 2018 at 10:11 AM, Yury Selivanov
> > <yselivanov.ml at gmail.com> wrote:  
> >> Just yesterday this snippet was used on python-dev to show how great the
> >> new syntax is:
> >> 
> >>          my_func(arg, buffer=(buf := [None]*get_size()), size=len(buf))
> >> 
> >> To my eye this is an anti-pattern.  One line of code was saved, but the
> >> other line becomes less readable.  The fact that 'buf' can be used after
> >> that line means that it will be harder for a reader to trace the origin of
> >> the variable, as a top-level "buf = " statement would be more visible.  
> > 
> > Making 'buf' more visible is ONLY a virtue if it's going to be used
> > elsewhere. Otherwise, the name 'buf' is an implementation detail of
> > the fact that this function wants both a buffer and a size.  
> 
> You're claiming that `:=` is nicer in this situation because it's less
> prominent than regular assignment and thus doesn't suggest that the name
> stays visible later.
> 
> But as others said, `:=` *does* make the name visible later until the
> enclosing scope ends.  In fact, a large part of its appeal is that you
> can use the result later (as in the `re.search()` example).  Will it be
> visible enough to the reaser in those cases then?
> 
> There seems to be a conflict there.

Not only, but seeing `:=` hints that something *special* is going on
(some inner expression is being bound to a name). So now we have to be
extra careful when reading and reviewing code written that people who
like using that syntactical feature.

I also wonder how long it will be before someone writes:

    def f(arg):
        global _lazy_value
        if predicate(arg) and (_lazy_value := frobnicate()) > arg:
            ...

(or something similar with "nonlocal")

Regards

Antoine.