On Fri, 2 Sept 2022 at 06:55, James Tsai <jamestzt...@gmail.com> wrote: > > 在 2022年9月1日星期四 UTC+2 18:34:36，<ery...@gmail.com> 写道： > > On 9/1/22, James Tsai <james...@gmail.com> wrote: > > > > > > I find it very useful if I am allowed to define new local variables in a > > > list comprehension. For example, I wish to have something like > > > [(x, y) for x in range(10) for y := x ** 2 if x + y < 80], or > > > [(x, y) for x in range(10) with y := x ** 2 if x + y < 80]. > > > > > > For now this functionality can be achieved by writing > > > [(x, y) for x in range(10) for y in [x ** 2] if x + y < 80]. > > You can assign a local variable in the `if` expression. For example: > > > > >>> [(x, y) for x in range(10) if x + (y := x**2) < 30] > > [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16)] > > Yeah this works great but like [(x, y) for x in range(10) for y in [x**2]] I > written before, is kind of a hack. And if initially I do not need an "if" > condition in the list comprehension, this becomes less convenient. I still > can write > >>> [(x, y) for x in range(10) if (y := x**2) or True] > > But I wonder if Python could have a specific syntax to support this. >
But why would you need to assign to y in that example? If you're using it more than once, you can use :=, and if you aren't, you don't need to. But do be aware that := does not create a comprehension-local name binding, but a nonlocal instead. > No but very often when I have written a neat list/dict/set comprehension, I > find it very necessary > to define local variable(s) to make it more clear and concise. Otherwise I > have to break it down > to several incrementally indented lines of for loops, if statements, and > variable assignments, > which I think look less nice. Well, if it's outgrown a list comp, write it on multiple lines. Like I said, not everything has to be a one-liner. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
