On Monday, August 14, 2017 at 3:30:39 PM UTC+5:30, Ben Finney wrote: > Steven D'Aprano writes: > > > On Sun, 13 Aug 2017 21:06:08 -0700, Rustom Mody wrote: > > > > > Here's a bunch of different ways in which a mapping comprehension > > > could be implemented: > > > > Not in Python they couldn't be > > You began by asking what people would expect syntax to mean. > > Then you expressed surprise that anyone would think a comprehension > would be interpreted by the reader as a single operation. > > The designer of that feature expressed that yes, the intention was that > it be interpreted as a single conceptual operation, not a looping > sequence of operations. > > Your latest counter has been that Python means something special, beyond > what the feature's designer intended, and it's implemented as a looping > sequence of operations. > > So it's normal to point out that Python's implementation is just one way > that it could be implemented, hence the reader can reasonably expect > that it's not the way Python implemented it. > > You were apparently, at the start of this thread, honestly seeking to > know how people interpret the syntax. > > At what point will you accept the feedback: That the comprehension > syntax *does not* necessarily connote a procedural loop, but instead can > quite reasonably be interpreted as its designer intended, a single > conceptual operation.
😃 I thought that this – Steven's OP – was some joke or prank question. As best as I could make out all the initial responders giving some supposed meaning to the various 'comprehensions' seemed to be playing along Since I had nothing cute to say… remained quiet Then in quick succession Terry, Cecil, yourself gave responses that I would give in seriousness if anything, better than what I could come up with So once again I had nothing to add I just put in some history to say that yes, you can take the expansion of comprehensions as loops-with-appends given in the python docs as God's Word For myself, if the python docs contradict the last 100 years or prior art/math history, the latter takes precedence All I want to say is the gratuitous incidental resemblance of for-loops with comprehensions confuses not just beginners but even the python implementers: This leak >>> [x for x in range(5)] [0, 1, 2, 3, 4] >>> x 4 >>> got corrected from python2 to 3 >>> [x for x in range(5)] [0, 1, 2, 3, 4] >>> x Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'x' is not defined However this leak still remains: >>> fl = [(lambda x : x + i) for i in range(5)] >>> [f(2) for f in fl] [6, 6, 6, 6, 6] Compare to haskell: Prelude> let fl = [(\ x -> x + i) | i <- [0..4]] Prelude> [f 2 | f <- fl] [2,3,4,5,6] Prelude> I strongly suspect this bugginess to be related to Greg's original idea of using that for-loop-containing-append as an informal off-the-cuff explanation becoming the formal definition [And I am still unsure whether this OP is a prank trick-question… ] -- https://mail.python.org/mailman/listinfo/python-list
