[Python-Dev] PEP 448 review

[Victor Stinner]

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
> <javascript:_e(%7B%7D,'cvml','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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