[Python-Dev] PEP 448 review

[Neil Girdhar]

http://bugs.python.org/issue2292

On Mon, Mar 2, 2015 at 3:17 PM, Victor Stinner <victor.stinner at gmail.com>
wrote:

> Where is the patch?
>
> Victor
>
> Le lundi 2 mars 2015, Neil Girdhar <mistersheik at gmail.com> a écrit :
>
> Hi everyone,
>>
>> The patch is ready for review now, and I should have time this week to
>> make changes and respond to comments.
>>
>> Best,
>>
>> Neil
>>
>> On Wed, Feb 25, 2015 at 2:42 PM, Guido van Rossum <guido at python.org>
>> wrote:
>>
>>> I'm back, I've re-read the PEP, and I've re-read the long thread with
>>> "(no subject)".
>>>
>>> I think Georg Brandl nailed it:
>>>
>>> """
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *I like the "sequence and dict flattening" part of the PEP, mostly
>>> because itis consistent and should be easy to understand, but the
>>> comprehension syntaxenhancements seem to be bad for readability and
>>> "comprehending" what the codedoes.The call syntax part is a mixed bag on
>>> the one hand it is nice to be consistent with the extended possibilities in
>>> literals (flattening), but on the other hand there would be small but
>>> annoying inconsistencies anyways (e.g. the duplicate kwarg case above).*
>>> """
>>>
>>> Greg Ewing followed up explaining that the inconsistency between dict
>>> flattening and call syntax is inherent in the pre-existing different rules
>>> for dicts vs. keyword args: {'a':1, 'a':2} results in {'a':2}, while f(a=1,
>>> a=2) is an error. (This form is a SyntaxError; the dynamic case f(a=1,
>>> **{'a': 1}) is a TypeError.)
>>>
>>> For me, allowing f(*a, *b) and f(**d, **e) and all the other
>>> combinations for function calls proposed by the PEP is an easy +1 -- it's a
>>> straightforward extension of the existing pattern, and anybody who knows
>>> what f(x, *a) does will understand f(x, *a, y, *b). Guessing what f(**d,
>>> **e) means shouldn't be hard either. Understanding the edge case for
>>> duplicate keys with f(**d, **e) is a little harder, but the error messages
>>> are pretty clear, and it is not a new edge case.
>>>
>>> The sequence and dict flattening syntax proposals are also clean and
>>> logical -- we already have *-unpacking on the receiving side, so allowing
>>> *x in tuple expressions reads pretty naturally (and the similarity with *a
>>> in argument lists certainly helps). From here, having [a, *x, b, *y] is
>>> also natural, and then the extension to other displays is natural: {a, *x,
>>> b, *y} and {a:1, **d, b:2, **e}. This, too, gets a +1 from me.
>>>
>>> So that leaves comprehensions. IIRC, during the development of the patch
>>> we realized that f(*x for x in xs) is sufficiently ambiguous that we
>>> decided to disallow it -- note that f(x for x in xs) is already somewhat of
>>> a special case because an argument can only be a "bare" generator
>>> expression if it is the only argument. The same reasoning doesn't apply (in
>>> that form) to list, set and dict comprehensions -- while f(x for x in xs)
>>> is identical in meaning to f((x for x in xs)), [x for x in xs] is NOT the
>>> same as [(x for x in xs)] (that's a list of one element, and the element is
>>> a generator expression).
>>>
>>> The basic premise of this part of the proposal is that if you have a few
>>> iterables, the new proposal (without comprehensions) lets you create a list
>>> or generator expression that iterates over all of them, essentially
>>> flattening them:
>>>
>>>     >>> xs = [1, 2, 3]
>>>     >>> ys = ['abc', 'def']
>>>     >>> zs = [99]
>>>     >>> [*xs, *ys, *zs]
>>>     [1, 2, 3, 'abc', 'def', 99]
>>>     >>>
>>>
>>> But now suppose you have a list of iterables:
>>>
>>>     >>> xss = [[1, 2, 3], ['abc', 'def'], [99]]
>>>     >>> [*xss[0], *xss[1], *xss[2]]
>>>     [1, 2, 3, 'abc', 'def', 99]
>>>     >>>
>>>
>>> Wouldn't it be nice if you could write the latter using a comprehension?
>>>
>>>     >>> xss = [[1, 2, 3], ['abc', 'def'], [99]]
>>>     >>> [*xs for xs in xss]
>>>     [1, 2, 3, 'abc', 'def', 99]
>>>     >>>
>>>
>>> This is somewhat seductive, and the following is even nicer: the *xs
>>> position may be an expression, e.g.:
>>>
>>>     >>> xss = [[1, 2, 3], ['abc', 'def'], [99]]
>>>     >>> [*xs[:2] for xs in xss]
>>>     [1, 2, 'abc', 'def', 99]
>>>     >>>
>>>
>>> On the other hand, I had to explore the possibilities here by
>>> experimenting in the interpreter, and I discovered some odd edge cases
>>> (e.g. you can parenthesize the starred expression, but that seems a
>>> syntactic accident).
>>>
>>> All in all I am personally +0 on the comprehension part of the PEP, and
>>> I like that it provides a way to "flatten" a sequence of sequences, but I
>>> think very few people in the thread have supported this part. Therefore I
>>> would like to ask Neil to update the PEP and the patch to take out the
>>> comprehension part, so that the two "easy wins" can make it into Python 3.5
>>> (basically, I am accepting two-thirds of the PEP :-). There is some time
>>> yet until alpha 2.
>>>
>>> I would also like code reviewers (Benjamin?) to start reviewing the
>>> patch <http://bugs.python.org/issue2292>, taking into account that the
>>> comprehension part needs to be removed.
>>>
>>> --
>>> --Guido van Rossum (python.org/~guido)
>>>
>>>
>>
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