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Wed Apr 25 20:20:40 EDT 2018 · https://mail.python.org/pipermail/python-dev/2018-April/153067.html

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. Should you
want to expand this out over more lines, you could do this:

template = [None]
buf = template*get_size()
length = len(buf)
my_func(arg, buffer=buf, size=length)

What are the names 'template' and 'length' achieving? Why should they
claim your attention? They are useless relics of a done-and-dusted
calculation, being retained for no reason. They do not deserve
top-level placement.

The form as given above is starting to get a bit noisy, but I strongly
disagree that 'buf' deserves to be a stand-alone name. It is as
valueless as 'template' is.

ChrisA

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