[Python-Dev] assignment expressions: an alternative proposal

[Steven D'Aprano]

On Tue, Apr 24, 2018 at 10:29:11PM +0100, Anthony Flury via Python-Dev wrote:

> If Python is going to do assignment expressions we shouldn't overload 
> parens in my opinion - we should have a separate operator - doing this 
> avoids needing to exclude rebinding, and makes such expressions 
> considerably more useful.

Exactly!

Yury's suggestion to prohibit rebinding existing names is throwing away 
the baby with the bathwater. Using assignment-expression to over-write 
an existing variable is only a error when it is a mistake, done by 
accident.

To satisfy the compiler, we have to invent new and unique names for what 
is conceptually the same variable being reused. Instead of simplifying 
your code, it will make it more complex.

Despite his statement that there's only one assignment, there are 
actually two: assignment that allows rebinding, and assignment that 
sometimes doesn't. The semantic differences between Yury's = and = are 
actually *greater* than the differences between = and :=


-- 
Steve