Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-27 Thread Alexander Belopolsky via Python-Dev
On Mon, Feb 26, 2018 at 7:51 PM, Guido van Rossum wrote: .. > The reason is that for people who are not Python experts there's no obvious > reason why `for VAR = EXPR` should mean one thing and `for VAR in EXPR` > should mean another. This would be particularly surprising for

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Guido van Rossum
On Mon, Feb 26, 2018 at 4:30 PM, Rob Cliffe via Python-Dev < python-dev@python.org> wrote: > On 26/02/2018 19:08, Guido van Rossum wrote: > > I would like to remind all wannabe language designers that grammar design > is not just solving puzzles. It's also about keeping the overall feel of > the

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Rob Cliffe via Python-Dev
On 26/02/2018 19:08, Guido van Rossum wrote: I would like to remind all wannabe language designers that grammar design is not just solving puzzles. It's also about keeping the overall feel of the language readable. I'm getting the idea that none of the proposals discussed so far (whether new

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Nick Coghlan
On 27 February 2018 at 05:08, Guido van Rossum wrote: > I would like to remind all wannabe language designers that grammar design > is not just solving puzzles. It's also about keeping the overall feel of > the language readable. I'm getting the idea that none of the proposals

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Guido van Rossum
I would like to remind all wannabe language designers that grammar design is not just solving puzzles. It's also about keeping the overall feel of the language readable. I'm getting the idea that none of the proposals discussed so far (whether new syntax or clever use of existing syntax) satisfy

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Rob Cliffe via Python-Dev
On 22/02/2018 19:04, Serhiy Storchaka wrote: Yet one discussion about reusing common subexpressions in comprehensions took place last week on the Python-ideas maillist (see topic "Temporary variables in comprehensions" [1]). The problem is that in comprehension like `[f(x) + g(f(x)) for x in

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Stephen J. Turnbull
Michel Desmoulin writes: > Le 25/02/2018 à 14:11, Nikolaus Rath a écrit : >>> result = [ (f(x) as y) + g(y) for x in range(10)] > Honestly I find this version the most readable while the double for > loop is completely weird to me, despite doing python for a living > for years. I find

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Chris Angelico
On Mon, Feb 26, 2018 at 8:00 PM, Nick Coghlan wrote: > On 26 February 2018 at 01:08, Chris Angelico wrote: >> >> Speaking as a C programmer who's quite happy to write code like "while >> ((var = func()) != sentinel)", I wouldn't object to this coming up in

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-26 Thread Nick Coghlan
On 26 February 2018 at 01:08, Chris Angelico wrote: > Speaking as a C programmer who's quite happy to write code like "while > ((var = func()) != sentinel)", I wouldn't object to this coming up in > Python; the "as name" syntax has the huge advantage over C's syntax in > that

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Guido van Rossum
On Sun, Feb 25, 2018 at 6:36 AM, Serhiy Storchaka wrote: > 23.02.18 19:30, Guido van Rossum пише: > >> I'm not saying anything new here, but since you asked specifically for my >> opinion: I don't care for the idiom; it's never occurred to me before, and >> it smells of

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Chris Angelico
On Mon, Feb 26, 2018 at 12:11 AM, Nikolaus Rath wrote: > On Feb 25 2018, Chris Angelico wrote: >> On Sun, Feb 25, 2018 at 11:02 PM, Nikolaus Rath wrote: >>> On Feb 22 2018, Serhiy Storchaka wrote: 1. Inner

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Serhiy Storchaka
23.02.18 19:30, Guido van Rossum пише: I'm not saying anything new here, but since you asked specifically for my opinion: I don't care for the idiom; it's never occurred to me before, and it smells of cleverness. If I saw it in a code review I would probably ask for a regular for-loop to make

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Michel Desmoulin
Le 25/02/2018 à 14:11, Nikolaus Rath a écrit : > On Feb 25 2018, Chris Angelico wrote: >> On Sun, Feb 25, 2018 at 11:02 PM, Nikolaus Rath wrote: >>> On Feb 22 2018, Serhiy Storchaka wrote: 1. Inner generator expression:

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Nikolaus Rath
On Feb 25 2018, Chris Angelico wrote: > On Sun, Feb 25, 2018 at 11:02 PM, Nikolaus Rath wrote: >> On Feb 22 2018, Serhiy Storchaka wrote: >>> 1. Inner generator expression: >>> >>> result = [y + g(y) for y in (f(x) for x in

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Chris Angelico
On Sun, Feb 25, 2018 at 11:02 PM, Nikolaus Rath wrote: > On Feb 22 2018, Serhiy Storchaka wrote: >> 1. Inner generator expression: >> >> result = [y + g(y) for y in (f(x) for x in range(10))] >> > [...] >> >> And maybe there are other ways. > > I think

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-25 Thread Nikolaus Rath
On Feb 22 2018, Serhiy Storchaka wrote: > 1. Inner generator expression: > > result = [y + g(y) for y in (f(x) for x in range(10))] > [...] > > And maybe there are other ways. I think the syntax recently brough up by Nick is still the most beautiful: result = [

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-24 Thread Mike Miller
I'm not sure, I found the "with f(x) as y" form previously mentioned the most readable despite it being a new use case, while not needing new keywords. -Mike On 2018-02-23 22:07, David Mertz wrote: FWIW, the nested loop over a single item is already in the language for 15 years or something.

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread David Mertz
FWIW, the nested loop over a single item is already in the language for 15 years or something. It's not that ugly, certainly not enough to need a new 'let' or 'where' keyword that basically does exactly the same thing with 3 fewer characters. On Feb 23, 2018 10:04 PM, David Mertz

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Nick Coghlan
On 24 February 2018 at 06:50, Stefan Behnel wrote: > But in general, yes, changing a list iterable into a tuple is an > improvement as tuples are more efficient to allocate. Haven't tried it in > CPython (*), but it might make a slight difference for very short > iterables,

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread David Mertz
On Feb 23, 2018 9:26 PM, "Steven D'Aprano" wrote: Given a potentially expensive DRY violation like: [(function(x), function(x)+1) for x in sequence] there are at least five ways to solve it. A 6th way is to wrap the expensive function in @lru_cache() to make it

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Steven D'Aprano
On Fri, Feb 23, 2018 at 11:23:04AM -0800, Chris Barker wrote: > But I still think the original: > > [g(y) for x in range(5) for y in [f(x)]] > > Is always going to be confusing to read. Though I do agree that it's not > too bad when you unpack it into for loops: > > In [89]: for x in range(5):

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Stephen J. Turnbull
Chris Barker writes: > But I still think the original: > > [g(y) for x in range(5) for y in [f(x)]] > > Is always going to be confusing to read. But the point I was making with "def f(x=[0]):" was this: you have a situation where your desired semantics is "value of some type"[1], but the

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Stefan Behnel
Chris Barker schrieb am 23.02.2018 um 20:23: > BTW, would it be even a tiny bit more efficient to use a tuple in the inner > loop? > > [g(y) for x in range(5) for y in (f(x),)] Serhiy's optimisation does not use a loop at all anymore and folds it into a direct assignment "y=f(x)" instead. But

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Chris Barker
On Fri, Feb 23, 2018 at 10:45 AM, Guido van Rossum wrote: > There are useful things you can only do with comprehensions if the second > for-loop can use the variable in the first for-loop. E.g. > > [(i, j) for i in range(10) for j in range(i)] > indeed -- and that is fairly

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Guido van Rossum
There are useful things you can only do with comprehensions if the second for-loop can use the variable in the first for-loop. E.g. [(i, j) for i in range(10) for j in range(i)] On Fri, Feb 23, 2018 at 10:16 AM, Chris Barker wrote: > On Fri, Feb 23, 2018 at 9:51 AM,

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Chris Barker
On Fri, Feb 23, 2018 at 9:51 AM, Guido van Rossum wrote: > As to the validity or legality of this code, it's both, and working as > intended. > > A list comprehension of the form > > [STUFF for VAR1 in SEQ1 for VAR2 in SEQ2 for VAR3 in SEQ3] > > should be seen (informally)

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Guido van Rossum
As to the validity or legality of this code, it's both, and working as intended. A list comprehension of the form [STUFF for VAR1 in SEQ1 for VAR2 in SEQ2 for VAR3 in SEQ3] should be seen (informally) as for VAR1 in SEQ1: for VAR2 in SEQ2: for VAR3 in SEQ3:

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Chris Barker - NOAA Federal
> Is it similar enough to > >def f(x=[0]): No, not at all — it’s a very different use case. When I first saw this on the original thread, I needed to stare at it a good while, and then whip up some code to experiment with it to know what it did. And not because I don’t know what a single

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Guido van Rossum
On Thu, Feb 22, 2018 at 11:04 AM, Serhiy Storchaka wrote: > Yet one discussion about reusing common subexpressions in comprehensions > took place last week on the Python-ideas maillist (see topic "Temporary > variables in comprehensions" [1]). The problem is that in

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Paul Moore
On 23 February 2018 at 09:12, Stefan Behnel wrote: > Stephen J. Turnbull schrieb am 23.02.2018 um 03:31: >> Barry Warsaw writes: >> > rather than having to pause to reason about what that 1-element >> > list-like syntax actually means, and 2) will this encourage even >> >

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Serhiy Storchaka
22.02.18 23:33, Barry Warsaw пише: On Feb 22, 2018, at 11:04, Serhiy Storchaka wrote: Stephan Houben proposed an idiom which looks similar to new hypothetic syntax: result = [y + g(y) for x in range(10) for y in [f(x)]] `for y in [expr]` in a comprehension means

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-23 Thread Stefan Behnel
Stephen J. Turnbull schrieb am 23.02.2018 um 03:31: > Barry Warsaw writes: > > rather than having to pause to reason about what that 1-element > > list-like syntax actually means, and 2) will this encourage even > > more complicated comprehensions that are less readable than just > > expanding

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-22 Thread Stephen J. Turnbull
Barry Warsaw writes: > My questions are 1) will this become idiomatic enough to be able to > understand at a glance what is going on, Is it similar enough to def f(x=[0]): which is sometimes seen as a way to produce a mutable default value for function arguments, to be "idiomatic"? >

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-22 Thread Barry Warsaw
On Feb 22, 2018, at 11:04, Serhiy Storchaka wrote: > > Stephan Houben proposed an idiom which looks similar to new hypothetic syntax: > >result = [y + g(y) for x in range(10) for y in [f(x)]] > > `for y in [expr]` in a comprehension means just assigning expr to y. I

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-22 Thread Ethan Furman
On 02/22/2018 11:54 AM, Joao S. O. Bueno wrote: > On 22 February 2018 at 16:04, Serhiy Storchaka wrote: >> Stephan Houben proposed an idiom which looks similar to new hypothetic >> syntax: >> >> result = [y + g(y) for x in range(10) for y in [f(x)]] > > This thing has bitten me in the past

Re: [Python-Dev] The `for y in [x]` idiom in comprehensions

2018-02-22 Thread Joao S. O. Bueno
This thing has bitten me in the past - At the time I put together the "stackfull" package - if allows stuff like: from stackfull import push, pop ... [push(f(x)) + g(pop()) for x in range(10)] It is painfully simple in its workings: it creates a plain old list in the fame f_locals and uses