Re: Local variable definition in Python list comprehension
On Thu, Sep 1, 2022 at 9:16 AM Chris Angelico wrote: > On Fri, 2 Sept 2022 at 02:10, James Tsai wrote: > > > > Hello, > > > > 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]. > > > > Is it worthwhile to add a new feature like this in Python? If so, how > can I propose this to PEP? > > Not everything has to be a one-liner. > So true! I like list comprehensions and generator expressions, but sometimes I end up regretting their use when there's a bug, and I have to convert one to a for loop + list.append in order to debug. -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
在 2022年9月2日星期五 UTC+2 00:17:23， 写道：
> On 02Sep2022 07:01, Chris Angelico wrote:
> >On Fri, 2 Sept 2022 at 06:55, James Tsai wrote:
> >> 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.
> True, but a comprehension can be more expressive than a less
> "functional" expression (series of statements).
>
> James, can you provide (a) a real world example where you needed to
> write a series of statements or loops and (b) a corresponding example of
> how you would have preferred to have written that code, possibly
> inventing some syntax or misusing ":=" as if it workeed they way you'd
> like it to work?
>
> Cheers,
> Cameron Simpson
Yeah, I think list comprehension is particularly useful to construct a deeply
nested list/dict. For example, I am now using Plotly to visualize a cellular
network including several base stations and users. Here is the function I have
written:
def plot_network(area, base_stations, users):
bs_poses = np.array([bs.pos for bs in base_stations])
ue_poses = np.array([ue.pos for ue in users])
fig = px.scatter(x=bs_poses[:, 0], y=bs_poses[:, 1])
fig.add_scatter(x=ue_poses[:, 0], y=ue_poses[:, 1])
fig.update_layout(
xaxis=dict(range=[0, area[0]], nticks=5),
yaxis=dict(range=[0, area[1]], nticks=5),
shapes=[dict(
type="circle",
fillcolor="PaleTurquoise",
x0=x-r, y0=y-r, x1=x+r, y1=y+r,
hovertext=f"({x:.2f}, {y:.2f})",
opacity=0.3
) for bs in base_stations for x, y in [bs.pos]
for r in [bs.cell_radius]],
)
return fig
Simply put, I want to scatter the BSs and users, and additionally I want to
draw a big circle around each BS to represent its cell coverage. I can choose
to write 'x0=bs.pos[0]-bs.cell_radius, y0=...' instead, but it becomes less
concise, and if x, y, or r is the return value of a function instead of a
property, it becomes more computationally expensive to repeat calling the
function as well. I also can create the list of 'shapes' by appending to a
list, like
shapes = []
for bs in base_stations:
x, y = bs.pos
r = bs.cell_radius
shapes.append(dict(...))
fig.update_layout(
xaxis=dict(range=[0, area[0]], nticks=5),
yaxis=dict(range=[0, area[1]], nticks=5),
shapes=shapes
)
But in my opinion this is much less concise. I think it looks better to create
the list within the nested structure. So I totally agree that list
comprehension adds much expressiveness in Python. I only wonder whether it is a
good idea to introduce a specific syntax for local variable assignment in list
comprehensions, instead of using "for r in [bs.cell_radius]".
I am also surprised to know that the assignment operator ":=" in a list
comprehension will assign a variable outside of the scope of the comprehension.
I think it does not make sense since a list comprehension without a ":=" will
never change name bindings outside itself.
--
https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
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 wrote: > On Fri, 2 Sept 2022 at 06:55, James Tsai wrote: > > > > 在 2022年9月1日星期四 UTC+2 18:34:36， 写道： > > > On 9/1/22, James Tsai 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 > -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
On 02Sep2022 07:01, Chris Angelico wrote: >On Fri, 2 Sept 2022 at 06:55, James Tsai wrote: >> 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. True, but a comprehension can be more expressive than a less "functional" expression (series of statements). James, can you provide (a) a real world example where you needed to write a series of statements or loops and (b) a corresponding example of how you would have preferred to have written that code, possibly inventing some syntax or misusing ":=" as if it workeed they way you'd like it to work? Cheers, Cameron Simpson -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
On 2022-09-01 13:33:16 -0700, James Tsai wrote: > 在 2022年9月1日星期四 UTC+2 18:34:36， 写道： > > On 9/1/22, James Tsai 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] In that case [(x, x**2) for x in range(10)] seems to be somewhat more readable. I would also say that [(x, x**2) for x in range(10) if x + x**2 < 80] doesn't really seem worse than any of the variants with y in it. (Yes, I get that your real duplicated expression is probably a bit more complex than `x**2`, but by the time a temporary variable really improves readability it's probably a good time to split that across multiple lines, too.) hp -- _ | Peter J. Holzer| Story must make more sense than reality. |_|_) || | | | h...@hjp.at |-- Charles Stross, "Creative writing __/ | http://www.hjp.at/ | challenge!" signature.asc Description: PGP signature -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
On Fri, 2 Sept 2022 at 06:55, James Tsai wrote: > > 在 2022年9月1日星期四 UTC+2 18:34:36， 写道： > > On 9/1/22, James Tsai 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
Re: Local variable definition in Python list comprehension
在 2022年9月1日星期四 UTC+2 18:16:03， 写道： > On Fri, 2 Sept 2022 at 02:10, James Tsai wrote: > > > > Hello, > > > > 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]. > > > > Is it worthwhile to add a new feature like this in Python? If so, how can I > > propose this to PEP? > Not everything has to be a one-liner. > > ChrisA 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. -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
在 2022年9月1日星期四 UTC+2 18:34:36， 写道： > On 9/1/22, James Tsai 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. -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
在 2022年9月1日星期四 UTC+2 16:15:17， 写道： > James Tsai writes: > > > 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]. > x and y are, to a first approximation, new local variables defined in a > list comprehension. I think you need to restate what it is you want. > > Is it worthwhile to add a new feature like this in Python? If so, how > > can I propose this to PEP? > To make any sort of case you'd need to give an example that does not > have a clearer way to write it already. Your working version is, to me, > clearer that the ones you want to be able to write. > > -- > Ben. By local variable definition I mean binding a variable to a single value, so it doesn't include giving an iterable that a variable can take values iteratively, e.g. 'for x in range(10)'. Does it not worth introducing a specific syntax to do this, instead of creating a new list ad hoc to define the variable like 'for y in [1]'? -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
On 9/1/22, James Tsai 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)] -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
On Fri, 2 Sept 2022 at 02:10, James Tsai wrote: > > Hello, > > 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]. > > Is it worthwhile to add a new feature like this in Python? If so, how can I > propose this to PEP? Not everything has to be a one-liner. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Local variable definition in Python list comprehension
James Tsai writes: > 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]. x and y are, to a first approximation, new local variables defined in a list comprehension. I think you need to restate what it is you want. > Is it worthwhile to add a new feature like this in Python? If so, how > can I propose this to PEP? To make any sort of case you'd need to give an example that does not have a clearer way to write it already. Your working version is, to me, clearer that the ones you want to be able to write. -- Ben. -- https://mail.python.org/mailman/listinfo/python-list
Local variable definition in Python list comprehension
Hello, 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]. Is it worthwhile to add a new feature like this in Python? If so, how can I propose this to PEP? -- https://mail.python.org/mailman/listinfo/python-list
