On 18/10/2016 07:40, Greg Ewing wrote:
Random832 wrote:
For me, it's the fact that:
[[a for a in b] for b in ['uvw', 'xyz']] == [['u', 'v', 'w'], ['x', 'y',
which makes me want to write:
[a for a in b for b in ['uvw', 'xyz']]

You're not alone! Lately I've been becoming convinced that
this is the way we should have done it right back at the
Me too. When I first got to grips with the order of loops in a list comprehension, I found it counter-intuitive and jarring. I still have to remind myself each time. I guess different people have different mental models, and may feel differently. The best way I can think of to explain why I feel this way is:

If the syntax were
    [ x for x in alist for alist in list-of-lists ]
there is a smooth (or rather, monotonic) gradation from the smallest object (x) to the next biggest object (alist) to the biggest object (list-of-lists), which IMHO is easier to follow. Each object is conceptually zero or one steps from its neighbour.
But with the actual syntax
    [ x for alist in list-of-lists for x in alist ]
there is a conceptual hiatus after "x" ("what on earth are alist and list-of-lists, and what have they got do to with x?"). This would be even more marked with more than 2 loops: we jump from the lowest level object to the two highest level objects, and it all seems like a disorienting non-sequitur until the very last loop "joins the dots". You have to identify the "x" at the beginning with the "x" near (but not at!) the end. Instead of (ideally, if not always in practice) reading the expression from left-to-right in one go, your eyes are forced to jump around in order for your brain to assimilate it.

A possible alternative syntax might be to follow more closely the for-loop syntax, e.g.
    [ for alist in list-of-lists: for x in alist: x ]
Here the "conceptual jump" between each object and the next is either 1 or 2, which for me makes this a "second best" option. But at least the "conceptual jump" is bounded (regardless of the number of loops), and this syntax has the advantage of familiarity.

But it's far too late to change it now, sadly.
Indeed. :-( But if I were ruler of the world and could have my own wish-list for Python 4, this (as per the first example) would be on it.

Best wishes
Rob Cliffe
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