[Python-Dev] assignment expressions: an alternative proposal

[Yury Selivanov]

On Tue, Apr 24, 2018 at 10:07 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>> "=" is always an assignment.
>> "==" is always an equality check.
>
> That's not the distinction I meant, I meant the difficulty of
> explaining the discrepancies in this list:
>
> a = 1 # Assignment
> (a = 1) # Also assignment
>
> a, b = 1, 2 # Tuple assignment
> (a, b = 1, 2) # SyntaxError. Why?
>
> ...
> Whereas if binding expressions use a different symbol, the question is
> far less likely to arise, and if it does come up, then the answer is
> the same as the one for def statements vs lambda expressions: because
> one is a statement, and the other is an expression.

A lot of other questions arise though.  PEP 572 proposes:

a = 1  # assignment
a := 1  # also assignment
(a := 1)  # also assignment
(a = 1)  # error, why?

It's also difficult to explain which one to use when.  The net result
is that code will be littered with both at random places.  That will
decrease the readability of Python code at least for some users who
have similar taste to myself.

With '=' in expressions, the code will look uniform.  There will be a
simple rule to put parens around assignments in expression and use
simple names.  After one or two descriptive SyntaxError users will
learn how this syntax works (like people learn everything in coding).

This all is very subjective.

Yury