Local variable definition in Python list comprehension

[Avi Gross]

Dumb question. Your y is purely a function of x. So create an f(x) where
you want your y. It probably can even be anonymous inline. I mean your
return values of (x, y) would be (x, f(x)) ...

On Thu, Sep 1, 2022, 5:04 PM Chris Angelico <rosuav at gmail.com> wrote:

> On Fri, 2 Sept 2022 at 06:55, James Tsai <jamestztsai at gmail.com> wrote:
> >
> > 在 2022年9月1日星期四 UTC+2 18:34:36，<ery... at gmail.com> 写道：
> > > On 9/1/22, James Tsai <james... at 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
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