# Walrus fails with \[\] **URL:** \ **Category:** Python Help\ **Created:** [May 8, 2022, 8:15am UTC](https://discuss.python.org/t/walrus-fails-with/15606 "2022-05-08T08:15:14Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![vainaixr](https://avatars.discourse-cdn.com/v4/letter/v/5f8ce5/32.png) [@vainaixr](https://discuss.python.org/u/vainaixr)\ **Post date:** [May 8, 2022, 8:15am UTC](https://discuss.python.org/t/walrus-fails-with/15606/1 "2022-05-08T08:15:14Z")
these are invalid, ```auto ([x] := [1]) ``` gives, ```auto SyntaxError: cannot use assignment expressions with list ``` similarly, ```auto ([x, *_] := [1, 2]) ``` also fails with the same error. What could be the reasoning that they decide to not allow such a syntax? I do not see any problem with them. ---
**Author:** ![sandraC](https://avatars.discourse-cdn.com/v4/letter/s/8491ac/32.png) [@sandraC](https://discuss.python.org/u/sandraC)\ **Post date:** [May 8, 2022, 9:04am UTC](https://discuss.python.org/t/walrus-fails-with/15606/2 "2022-05-08T09:04:23Z")
To me, it is disabled because it may be a bit confusing(and a bit controversial). Will this expression be equal to `[1]`, or `x` ? Just my idea. ---
**Author:** ![vainaixr](https://avatars.discourse-cdn.com/v4/letter/v/5f8ce5/32.png) [@vainaixr](https://discuss.python.org/u/vainaixr)\ **Post date:** [May 8, 2022, 9:42am UTC](https://discuss.python.org/t/walrus-fails-with/15606/3 "2022-05-08T09:42:52Z")
what could be controversial here, let us say we have, 1. ```auto print([[x := [i*2], x] for i in range(1, 5)]) ``` which outputs, ```auto [[[2], [2]], [[4], [4]], [[6], [6]], [[8], [8]]] ``` but if we instead use, 1. ```auto print([[[x] := [i*2], x] for i in range(1, 5)]) ``` then it should output, ```auto [[[2], 2], [[4], 4], [[6], 6], [[8], 8]] ``` but the second one is not valid for some unknown reason. ---
**Author:** ![sandraC](https://avatars.discourse-cdn.com/v4/letter/s/8491ac/32.png) [@sandraC](https://discuss.python.org/u/sandraC)\ **Post date:** [May 8, 2022, 10:06am UTC](https://discuss.python.org/t/walrus-fails-with/15606/4 "2022-05-08T10:06:53Z")
I will try to explain my perspective but, again, it is just my thought. You have : `[x] := [i*2]` and I found 2 different scenarios (with two different results) about what the output may be: ### Scenario 1 It will return `x,` which is equal to `i*2`. ### Scenario 2 It will return what you have typed before the operator(since you haven’t written just ‘x’, but `[x]`): It will return `[x]` which is equal to `[i*2]` ~~ ### Scenario 3 It will return what you have typed after operator(value) : `[i*2]` ~~ Note: Found last scenario ridiculous and decided to remove it. ---
**Author:** ![vainaixr](https://avatars.discourse-cdn.com/v4/letter/v/5f8ce5/32.png) [@vainaixr](https://discuss.python.org/u/vainaixr)\ **Post date:** [May 8, 2022, 10:24am UTC](https://discuss.python.org/t/walrus-fails-with/15606/5 "2022-05-08T10:24:54Z")
I was investigating this a bit more, it turns out that I cannot use walrus with tuple also, so, ```auto print([[(x, y) := (i*2, i*3), x, y] for i in range(1, 5)]) ``` gives, ```auto SyntaxError: cannot use assignment expressions with tuple ``` but the way I want it to work is, it outputs, ```auto [[(2, 3), 2, 3], [(4, 6), 4, 6], [(6, 9), 6, 9], [(8, 12), 8, 12]] ``` I assume, when I write, ```auto [[[x] := [i*2], x] for i in range(1, 5)] ``` then the, `[x] := [i*2]` part should do two things, 1. return `[i*2]` (that is, whatever comes after the walrus operator) 2. set `[x] = [i*2]` and this value of `x` would be used in the second entry of the list in my previous post, that is here, ```auto [[x] := [i*2], x] ^ ``` ---
**Author:** ![rob42](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/rob42/32/8218_2.png) [@rob42](https://discuss.python.org/u/rob42)\ **Post date:** [May 8, 2022, 10:29am UTC](https://discuss.python.org/t/walrus-fails-with/15606/6 "2022-05-08T10:29:39Z")
Remember: the `:=` operator does _not_ do anything that isn’t possible without it, so what would be the code _without_ using the **assignment expression operator**? ---
**Author:** ![steven.daprano](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/steven.daprano/32/1083_2.png) [@steven.daprano](https://discuss.python.org/u/steven.daprano)\ **Post date:** [May 8, 2022, 10:40am UTC](https://discuss.python.org/t/walrus-fails-with/15606/7 "2022-05-08T10:40:16Z")
It is a deliberate design. See the [PEP](https://peps.python.org/pep-0572/#differences-between-assignment-expressions-and-assignment-statements): “Iterable packing and unpacking (both regular or extended forms) are not supported” The reason isn’t given. If you really care, you could dig through the discussions on Python-Ideas and Python-Dev mailing lists, but be warned, there are hundreds of posts. I expect that the reason is one or both of: - too hard to get the parser to understand it; - too confusing to read for the human reader. The walrus operator was hugely controversial, it was so stressful with [so much opposition and negativity](https://www.blog.pythonlibrary.org/2018/07/13/guido-retire-as-bdfl/) that it drove Guido into retiring as the BDFL of the Python language. So it may be that this was just a compromise position, prohibiting a minor corner case that increases complexity without actually being very useful. ---
**Author:** ![vainaixr](https://avatars.discourse-cdn.com/v4/letter/v/5f8ce5/32.png) [@vainaixr](https://discuss.python.org/u/vainaixr)\ **Post date:** [May 8, 2022, 10:58am UTC](https://discuss.python.org/t/walrus-fails-with/15606/8 "2022-05-08T10:58:46Z")
without using the walrus, one would have to recompute, ```auto [[(i*2, i*3), i*2, i*3] for i in range(1, 5)] ``` ```auto [[[i*2], i*2] for i in range(1, 5)] ``` (had to recompute `i*2`, `i*3`) the other way to do this could be, ```auto [[(x := i*2, y := i*3), x, y] for i in range(1, 5)] # but had to repeat walrus ``` ```auto [[[x := i*2], x] for i in range(1, 5)] ``` maybe they do not want to make it more complicated, as for someone reading walrus for the first time, it would be a bit confusing on its own. but if overtime walrus becomes more popular, then I think they will have to reconsider iterable packing and unpacking. ---
**Author:** ![rob42](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/rob42/32/8218_2.png) [@rob42](https://discuss.python.org/u/rob42)\ **Post date:** [May 8, 2022, 11:18am UTC](https://discuss.python.org/t/walrus-fails-with/15606/9 "2022-05-08T11:18:25Z")
When I code, I always keep in mind: _The Zen of Python_ In particular, these points: - Simple is better than complex. - Complex is better than complicated. - Flat is better than nested. - Readability counts. I can see that ‘walrus’ can address some of those points. The biggest issues will be backwards compatibly. There are many systems that are still using Python2.x, and a huge number that have yet to move away from Python 3.6, so I think it’ll be years down the road before we see ‘walrus’ being used in general. ---
**Author:** ![aivarpaalberg](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/aivarpaalberg/32/9457_2.png) [@aivarpaalberg](https://discuss.python.org/u/aivarpaalberg)\ **Post date:** [May 8, 2022, 6:54pm UTC](https://discuss.python.org/t/walrus-fails-with/15606/10 "2022-05-08T18:54:50Z")
> [@vainaijr](#): > > without using the walrus, one would have to recompute, > > ```auto > [[(i*2, i*3), i*2, i*3] for i in range(1, 5)] > > ``` There is no need to recompute: ```auto stream = ((i*2, i*3) for i in range(1, 5)) data = [[[x,y], x,y] for x, y in stream] [[[2, 3], 2, 3], [[4, 6], 4, 6], [[6, 9], 6, 9], [[8, 12], 8, 12]] ``` Based on that one can construct hard to read (but without recompute) one-liner as well: ```auto data = [[[x, y], x, y] for x, y in ((i*2, i*3) for i in range(1, 5))] ``` ---
**Author:** ![thatbirdguythatuknownot](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/thatbirdguythatuknownot/32/7172_2.png) [@thatbirdguythatuknownot](https://discuss.python.org/u/thatbirdguythatuknownot)\ **Post date:** [May 8, 2022, 9:41pm UTC](https://discuss.python.org/t/walrus-fails-with/15606/11 "2022-05-08T21:41:04Z")
> **Scenario 2** > > It will return what you have typed before the operator (since you haven’t written just ‘x’, but `[x]`): > It will return `[x]` which is equal to `[i*2]` That scenario is the simple and consistent way to do it. It’s also how chained assignment works, so shouldn’t it also be how the assignment expression works?