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[Python-Dev] assignment expressions: an alternative proposal

Chris Angelico rosuav at gmail.com
Tue Apr 24 11:10:52 EDT 2018 
On Wed, Apr 25, 2018 at 12:58 AM, Yury Selivanov
<yselivanov.ml at gmail.com> wrote:
> On Tue, Apr 24, 2018 at 10:49 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> [..]
>>>> 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
>
> Since 'diff' and 'g' must be new names according to rule (3), those
> who read the code will notice that both were not previously bound.
> Therefore both are new variables so it can't be a comparison.

That would not be true if this code were in a loop. Or do you have a
different definition of "not previously bound" that is actually a
syntactic feature? For instance:

if (x = 1):
    x = 2

Legal? Not legal?

ChrisA
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