[Python-ideas] PEP 572: Statement-Local Name Bindings, take three!

[Rhodri James]

On 26/03/18 02:24, Guido van Rossum wrote:
> I gotta say I'm warming up to := in preference over 'as',*if*  we're going
> to do this at all (not a foregone conclusion at all).

I have the usual objection to glyphs (hard to look up or get help on), 
but ":=" raises two issues all of its own.

* On the plus side, it looks like some kind of assignment.  People 
reading through the code will not be overly surprised to find it results 
in a name binding.

* On the minus side, it doesn't quite look like an assignment statement. 
  While my crystal ball is cloudy, I can well imagine beginners becoming 
very confused over which symbol to use in which circumstance, and a lot 
of swearing when:

   x := f()
   if (y = g(x)) is not None:
     h(y)

results in syntax errors.

I'm inclined to think you want assignment expressions to look unlike 
assignment statements to avoid this sort of confusion.

-- 
Rhodri James *-* Kynesim Ltd