[Python-ideas] A comprehension scope issue in PEP 572

Tim Peters tim.peters at gmail.com
Sun May 6 22:49:39 EDT 2018

>> In a different thread I noted that I sometimes want to write code like
>> this:
>> ...
>>    while any(n % (thisp := p) == 0 for p in small_primes):
>>        n //= thisp
>> ...

[Ryan Gonzalez <rymg19 at gmail.com>]
> Couldn't you just do:
> def first(it):
>    return next(it, None)
> while (item := first(p for p in small_primes if n % p == 0)):
>    # ...

In the "different thread" I mentioned above, I already noted that kind
of spelling.

I'm not at a loss to think of many ways to spell it ;-)  The point of
this thread was intended to be about the semantics of binding
expressions in comprehensions.  For that purpose, the PEP noting that

    total = 0
    progressive_sums = [total := total + value for value in data]

fails too is equally relevant.  Of course there are many possible ways
to rewrite that too that would work.  That doesn't change that the
failing attempts "look like they should work", but don't, but could if
the semantics of ":=" were defined differently inside
magically-created anonymous lexically nested functions

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