On Wed, 25 Apr 2018 18:52:34 -0700
Łukasz Langa <luk...@langa.pl> wrote:
> > On 25 Apr, 2018, at 5:20 PM, Chris Angelico <ros...@gmail.com> wrote:
> >
> > On Thu, Apr 26, 2018 at 10:11 AM, Yury Selivanov
> > <yselivanov...@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.
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