I +1 this at surface level; Both Haskell list comprehensions and Scala for comprehensions have variable assignment in them, even between iterating and this is often very useful. Perhaps syntax can be generalised as:
[expr_using_x_and_y for i in is x = expr_using_i for j in is y = expr_using_j_and_x] This demonstrates the scope of each assignment; available in main result and then every clause that follows it. Sorry to op who will receive twice, forgot reply to all On 15 Feb 2018 7:03 am, "fhsxfhsx" <fhsxf...@126.com> wrote: > As far as I can see, a comprehension like > alist = [f(x) for x in range(10)] > is better than a for-loop > for x in range(10): > alist.append(f(x)) > because the previous one shows every element of the list explicitly so > that we don't need to handle `append` mentally. > > But when it comes to something like > [f(x) + g(f(x)) for x in range(10)] > you find you have to sacrifice some readableness if you don't want two > f(x) which might slow down your code. > > Someone may argue that one can write > [y + g(y) for y in [f(x) for x in range(10)]] > but it's not as clear as to show what `y` is in a subsequent clause, not > to say there'll be another temporary list built in the process. > We can even replace every comprehension with map and filter, but that > would face the same problems. > > In a word, what I'm arguing is that we need a way to assign temporary > variables in a comprehension. > In my opinion, code like > [y + g(y) for x in range(10) **some syntax for `y=f(x)` here**] > is more natural than any solution we now have. > And that's why I pro the new syntax, it's clear, explicit and readable, > and is nothing beyond the functionality of the present comprehensions so > it's not complicated. > > And I hope the discussion could focus more on whether we should allow > assigning temporary variables in comprehensions rather than how to solve > the specific example I mentioned above. > > > > > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > >
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