[Python-Dev] assignment expressions: an alternative proposal

[Paul Moore]

On 24 April 2018 at 14:46, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 24 April 2018 at 23:38, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>> I propose to use the following syntax for assignment expressions:
>>
>>     ( NAME = expr )
>>
>> I know that it was proposed before and this idea was rejected, because
>> accidentally using '=' in place of '==' is a pain point in
>> C/C++/JavaScript.
>>
>> That said, I believe we can still use this syntax as long as we impose
>> the following three restrictions on it:
>>
>> 1. Only NAME token is allowed as a single target.
>>
>> 2. Parenthesis are required.
>>
>> 3. Most importantly: it is *not* allowed to mask names in the current
>> local scope.
>
> While I agree this would be unambiguous to a computer, I think for
> most humans it would be experienced as a confusing set of arcane and
> arbitrary rules about what "=" means in Python.

Also, there's the ambiguity and potential for misreading in the
opposite direction (accidentally *reading* = as == even though it
isn't):

if (diff = x - x_base) and (g = gcd(diff, n)) > 1:
     return g

My immediate reading of this is as an equality comparison between diff
and x - x_base (which would send me futilely looking for a definition
of diff) and an equality comparison of g and gcd(diff, n)... wait, why
am I doing (equality comparison) > 1? Oh, hang on... At this point,
any hope of me quickly understanding what this code does is lost.

Paul