| msg341160 - (view) |
Author: wim glenn (wim.glenn) * |
Date: 2019-04-30 18:48 |
| Could cases like these be made to work? *Should* cases like these be made to work? L = [0, 1, 2] L[::2], *rest = "abcdef" # ValueError: attempt to assign sequence of size 1 to extended slice of size 2 a, L[::2] = "abc" # ValueError: too many values to unpack (expected 2) The list slice knows exactly how many slots need to be filled, so I can't immediately think of any obvious ambiguity. Maybe there are some implementation complications with supporting e.g. generators on the RHS (because RHS must be evaluated before LHS - https://docs.python.org/3/reference/expressions.html#evaluation-order). |
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| msg341183 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) *  |
Date: 2019-05-01 09:23 |
| For the second case, you can use a, *L[::2] = "abc" For the first case this does not work, because an assignment can have only one starred expression. Making the first case to work as you expected is breaking change. Currently L[:], *rest = 'abcdef' sets L to ['a'] and rest to ['b', 'c', 'd', 'e', 'f']. Consistent implementing of your idea would set L to ['a', 'b', 'c'] and rest to ['d', 'e', 'f'] (because len(L[:]) == 3 before assignment). What is your use case? Why do you need such syntax? |
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| msg341242 - (view) |
Author: Windson Yang (Windson Yang) * |
Date: 2019-05-02 01:22 |
| In your first case, *any positive index except 2 will work*, For example: L = [0, 1, 2] L[::1], *rest = "abcdef" # L became ['a'] or L[::3], *rest = "abcdef" # L became ['a', 1, 2] I found iff when you change the length of L to 1(L[::3]) or didn't change L at all (L[::1], L[::]), this expression will work. But I'm not sure that is what you expected. |
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| msg342389 - (view) |
Author: wim glenn (wim.glenn) * |
Date: 2019-05-13 21:33 |
| Serhiy, `a, *L[::2] = "abc"` as an alternative is interesting, thanks. The other example `L[:], *rest = 'abcdef'` is less interesting because L[:] can be arbitrary size. When noticing this, I had tried to consume a generator into every other position in a list by using: L[::2], *extra = g Though there are obvious workarounds (e.g. using `itertools.islice`), it surprised me that CPython did not "do what I mean" out of the box. However, since creating the issue, it was brought to my attention that trying to handle this assignment may result in potential ambiguity, for example: L = [0, 1, 2] L[::2], *rest = "ab", "c", "d" There is no obvious choice for result here. So, perhaps this issue should just be closed. |
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