On 28/11/17 23:15, Alon Snir wrote:
On Wed, Nov 29, 2017 at 5:46 AM, Alon Snir <alons...@hotmail.com> wrote:
I would like to mention that the issue of assignment to a target list, is also
relevant to the case of elementwise assignment to a mutable sequence (e.g.
lists and arrays). Here as well, the rhs of the assignment statement states the
number of elements to be assigned. Consequently, the following example will get
into infinite loop:
from itertools import count
A = []
A[:] = count()
Writing "A[:2] = count()" will cause the same result.
Here as well, it is currently safer to use islice if the rhs is a generator or
an iterator. e.g.:
it = count()
A[:] = islice(it,2)
In my opinion, it is be better to devise a solution that could be applied in
both cases. Maybe a new kind of assignment operator that will be dedicated to
this kind of assignment. i.e. elementwise assignment with restriction on the
number of elements to be assigned, based on the length of the lhs object (or
the number of targets in the target list).
ChrisA wrote:
Hmm. The trouble is that slice assignment doesn't have a fixed number of targets. If you say
"x, y = spam", there's a clear indication that 'spam' needs to provide exactly two
values; but "A[:] = spam" could have any number of values, and it'll expand or shrink the
list accordingly.
Rhodri James wrote:
Flatly, no. It is better not to ask for things you don't want in the first
place, in this case the infinite sequence.
Still, don't let me discourage you from working on this. If you can define how
such an assignment would work, or even the length of A[:] as an assignment
target, I'm not going to dismiss it out of hand.
My answer:
The idea is to define an alternative assignment rule, that is to assign exactly as many elements as
the current length of the lhs object (without expanding or shrinking it). Suppose "?=" is
the operator for the alternative assignment rule; A=[None]*2; and "iterator" is any
iterator (finite or infinite). In this case, the following code:
A ?= iterator
would behave like this:
A[:] = islice(iterator, 2) # where 2 is the length of A
And as suggested earlier for the case of assignment to a target list, the
following code:
x, y ?= iterator
would behave like this:
x, y = islice(iterator, 2) # where 2 is the number of targets
You're still getting -1000 from me on this. You want to introduce magic
syntax to make writing sloppy code easier. That's two big barriers you
aren't getting over.
--
Rhodri James *-* Kynesim Ltd
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