[Python-Dev] assignment expressions: an alternative proposal

[Ethan Furman]

On 04/24/2018 08:19 AM, Yury Selivanov wrote:

> Yes, because I'm trying to think about this from a pragmatic side of
> things. My question to myself: "what syntax could I use that would
> prevent me from making '=' vs '==' mistake when I code?"  To me, the
> answer is that I usually want to compare local variables.

I think we need to disambiguate between typo-typos and thinko-typos.  I suspect the vast majority of the '=' bugs are 
not due to the programmer /thinking/ the wrong operation, but of their hands/keyboards not /entering/ the right symbols; 
having a legal operator ("==") degrade into another legal operator ("=") that looks similar but means incredibly 
different things is a trap that we should not add to Python.

You might say that we have the same problems with ">=", "<=", and "!=".  We don't with "!=" because neither "!" nor "=" 
can stand alone and would fail.  We only have it partially with "<=" and ">=" because missing the angle bracket results 
in failure, but missing the "=" results in a working statement -- but that statement is still the same type of operation 
and is easier to debug when boundary cases fail.

> When I compare to variables from outer scopes they *usually* are on
> the *right* side of '=='.

You mean something like

   if 2 == x:

?  I never write code like that, and I haven't seen it, either.

--
~Ethan~