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

[Yury Selivanov]

On Wed, Apr 25, 2018 at 5:58 PM Guido van Rossum <guido at python.org> wrote:
[..]
> It was meant dismissive. With Chris, I am tired of every core dev
starting their own thread about how PEP 572 threatens readability or
doesn't reach the bar for new syntax (etc.). These arguments are entirely
emotional and subjective.

FWIW I started my thread for allowing '=' in expressions to make sure that
we fully explore that path.  I don't like ':=' and I thought that using '='
can make the idea more appealing to myself and others. It didn't, sorry if
it caused any distraction. Although adding a new ':=' operator isn't my main
concern.

I think it's a fact that PEP 572 makes Python more complex.
Teaching/learning Python will inevitably become harder, simply because
there's one more concept to learn.

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.

The PEP lists this example as an improvement:

          [(x, y, x/y) for x in input_data if (y := f(x)) > 0]

I'm an experienced Python developer and I can't read/understand this
expression after one read. I have to read it 2-3 times before I trace where
'y' is set and how it's used.  Yes, an expanded form would be ~4 lines
long, but it would be simple to read and therefore review, maintain, and
update.

Assignment expressions seem to optimize the *writing code* part, while
making *reading* part of the job harder for some of us.  I write a lot of
Python, but I read more code than I write. If the PEP gets accepted I'll
use
the new syntax sparingly, sure.  My main concern, though, is that this PEP
will likely make my job as a code maintainer harder in the end, not easier.

I hope I explained my -1 on the PEP without sounding emotional.

Thank you,
Yury




Yury