[Python-Dev] assignment expressions: an alternative proposal (original) (raw)

Ethan Furman ethan at stoneleaf.us
Tue Apr 24 11:51:22 EDT 2018

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

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