[Python-Dev] assignment expressions: an alternative proposal

[Steven D'Aprano]

On Tue, Apr 24, 2018 at 11:03:35AM -0400, Yury Selivanov wrote:

> My point was that when you see lots of '=' and ':=' used at the
> statement level, one might try to write "if x = 1" instead of "if x :=
> 1" -- boom, we have an unexpected SyntaxError for some users.

That's a *good* thing. They will then learn not to write x = 1 as an 
expression.

Also, if I write lots of x := 1 binding-expressions as statements, my 
code is bad and deserves to fail code-review. But why would I write the 
extra colon (one character, two key-presses) to use

    x := 1

as a statement, when x = 1 will work? That's a sure sign that I don't 
know what I'm doing. (Or that I desperately wish I was writing Pascal.)

I don't think we need worry about this. The sort of code that is filled 
with binding-expressions used as statements will almost certainly be so 
un-Pythonic and ugly in many other ways, that this won't make any 
difference.


> In my opinion adding *any* assignment expression syntax to Python
> *will* create this sort of issues.  PEP 572 isn't free of them, my
> proposal isn't free of them.  My proposal doesn't add a new ':='
> operator at the cost of slightly complicating rules around '='.  PEP
> 572 avoids complicating '=', but adds an entirely new form of
> assignment.

Indeed. That is true: either way, we introduce complexity into the 
language. (But that will allow us to reduce complexity in *our* code.) 
Given that increasing complexity is inevitable regardless of whether we 
choose PEP 572 or your suggestion, it is better to choose the option 
which keeps binding-expressions and assignment statements separate, 
since they are two different concepts.



-- 
Steve