#18940 closed defect (fixed)
Simplify __getitem__ for polynomials and deprecate slicing
Reported by:  pbruin  Owned by:  

Priority:  major  Milestone:  sage7.0 
Component:  algebra  Keywords:  
Cc:  Merged in:  
Authors:  Peter Bruin, Jeroen Demeyer  Reviewers:  Ralf Stephan 
Report Upstream:  N/A  Work issues:  
Branch:  a0600bc (Commits, GitHub, GitLab)  Commit:  
Dependencies:  Stopgaps: 
Description (last modified by )
For polynomials, we implement a generic __getitem__()
method, which depends on a new Cython method get_unsafe()
to get single coefficients. Overall, this greatly simplifies the code. Except for truncation pol[:n]
, slicing for polynomials is deprecated, since there is no clear use case and it is not mathematically welldefined what the answer should be (see also comment 39 and further below).
This fix allows us to simplify the method PowerSeries_poly.__getitem__()
. Also, the patch corrects some (previously wrong) slice bounds in LaurentPolynomial_univariate.__getitem__()
.
Change History (64)
comment:1 Changed 7 years ago by
comment:2 followup: ↓ 3 Changed 6 years ago by
For matrices and vectors, this is done using get_unsafe()
which is the implementation of a[i]
for one index i
. Then the slicing is done by a generic __getitem__
function.
comment:3 in reply to: ↑ 2 Changed 6 years ago by
 Dependencies set to #19409
Replying to jdemeyer:
For matrices and vectors, this is done using
get_unsafe()
which is the implementation ofa[i]
for one indexi
. Then the slicing is done by a generic__getitem__
function.
I implemented this suggestion, now testing.
comment:4 Changed 6 years ago by
 Branch set to u/pbruin/18940getitem_step
 Commit set to 97f95822da2ae740aa9a19bcc75f73c216879aeb
 Description modified (diff)
comment:5 Changed 6 years ago by
 Status changed from new to needs_review
comment:6 Changed 6 years ago by
I might review this once the dependency is merged.
comment:7 followup: ↓ 8 Changed 6 years ago by
Just a remark: did you have a look at PySlice_GetIndicesEx? That might be used to avoid slice arithmetic like
+ start, stop, step = n.start, n.stop, n.step
+ if step is None:
+ step = 1
+ if start is None:
+ start = 0
+ elif start < 0:
+ start %= step
+ if stop is None or stop > d:
+ stop = d
comment:8 in reply to: ↑ 7 ; followup: ↓ 9 Changed 6 years ago by
Replying to jdemeyer:
Just a remark: did you have a look at PySlice_GetIndicesEx?
I tried this, but unfortunately it does not return the correct indices for polynomials. For example, it wraps around negative start
values (as in the Python convention that L[1]
with L
a list returns the last item in L
); this is incompatible with Sage's convention that P[n]
returns 0 if P
is a polynomial and n < 0
.
comment:9 in reply to: ↑ 8 ; followup: ↓ 13 Changed 6 years ago by
Replying to pbruin:
Replying to jdemeyer:
Just a remark: did you have a look at PySlice_GetIndicesEx?
I tried this, but unfortunately it does not return the correct indices for polynomials. For example, it wraps around negative
start
values (as in the Python convention thatL[1]
withL
a list returns the last item inL
); this is incompatible with Sage's convention thatP[n]
returns 0 ifP
is a polynomial andn < 0
.
I think this behavior should be documented: I'd add a Warning
block to the documentation to specify that it doesn't (and cannot!) respect Python convention. This may be obvious for Laurent polynomials, but it is not clear for polynomials.
comment:10 Changed 6 years ago by
 Status changed from needs_review to needs_work
comment:11 Changed 6 years ago by
 Commit changed from 97f95822da2ae740aa9a19bcc75f73c216879aeb to e70430a5959274c7163ffc347529a54c9ef03648
comment:12 Changed 6 years ago by
 Commit changed from e70430a5959274c7163ffc347529a54c9ef03648 to 6842d868a0b7c995f7a3d396deacdcdec6913ce6
Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:
6842d86  Trac 18940: forbid step < 0 in Polynomial.__getitem__()

comment:13 in reply to: ↑ 9 Changed 6 years ago by
 Status changed from needs_work to needs_review
Replying to bruno:
Replying to pbruin:
Replying to jdemeyer:
Just a remark: did you have a look at PySlice_GetIndicesEx?
I tried this, but unfortunately it does not return the correct indices for polynomials. For example, it wraps around negative
start
values (as in the Python convention thatL[1]
withL
a list returns the last item inL
); this is incompatible with Sage's convention thatP[n]
returns 0 ifP
is a polynomial andn < 0
.I think this behavior should be documented: I'd add a
Warning
block to the documentation to specify that it doesn't (and cannot!) respect Python convention. This may be obvious for Laurent polynomials, but it is not clear for polynomials.
OK, I added a warning and an example for this.
Also, in the meantime I realised that step <= 0
is (1) not necessarily welldefined, and (2) may cause Sage to crash with the current implementation. I added a few lines to detect this case and raise an IndexError
.
comment:14 Changed 6 years ago by
 Commit changed from 6842d868a0b7c995f7a3d396deacdcdec6913ce6 to 1c56779b3140d12fbe5d0973cdcad0ac9dbb8054
Branch pushed to git repo; I updated commit sha1. New commits:
1c56779  Merge branch 'develop' into ticket/18940getitem_step

comment:15 Changed 6 years ago by
For some classes, you still reimplement __getitem__
for slices. Why is that? Why cannot you always use the generic implementation for slicing?
comment:16 followup: ↓ 31 Changed 6 years ago by
Why not allow step < 0
? Actually, I think that step = 1
might be the most mathematically useful value of step
.
comment:17 followup: ↓ 29 Changed 6 years ago by
 Milestone changed from sage6.8 to sage6.10
 Status changed from needs_review to needs_info
Are you sure that the current behaviour is really what we want?
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2 + x sage: pol[2:] 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2
I would have expected
sage: pol[2:] 9*x^7 + 8*x^6 + 7*x^5 + 6*x^4 + 5*x^3 + 4*x^2 + 3*x + 2
I know that this branch just extends the current behaviour. However, I think we need to step back and define mathematically what should be returned when asking for pol[a:b:c]
. The current definition is not very useful.
I think that list(pol[a:b:c])
should be the same as list(pol)[a:b:c]
, except for different rules when the range is outside of 0 ... pol.degree()
.
comment:18 followup: ↓ 28 Changed 6 years ago by
Peter, what's your usecase for pol[a:b:c]
, i.e. why did you create this ticket?
comment:19 Changed 6 years ago by
For the record, the current definition of pol[a:b]
was introduced with
commit ff63a1c2b5798a777dcef6a4366a43dc60a6bebd Author: William Stein <wstein@gmail.com> Date: Fri Jan 26 02:42:10 2007 0800 A huge amount  starting with writing tons of doctests for power series, pyrex integer_ring, etc. * changed before of polynomial and power series slicing to return objects of the same type, which is much more consistent. * optimized integer constructor * improved random number generation (e.g., ZZ.random_element()) ten times faster and more flexible. * separated out pmem_malloc
comment:20 followup: ↓ 21 Changed 6 years ago by
We currently have
sage: R.<x> = QQ[] sage: p = R.random_element((1, 10)); p 2/3*x^10 + 3/2*x^9  1/2*x^8 + 10*x^7 + x^6 + 3/2*x^5 + 1/3*x^4 + 5/2*x^3  x^2 + 29*x  1 sage: list(p) [1, 29, 1, 5/2, 1/3, 3/2, 1, 10, 1/2, 3/2, 2/3]
So list
does go low to high and the current behavior is consistent with converting to a list.
+1 for supporting negative steps.
comment:21 in reply to: ↑ 20 ; followup: ↓ 30 Changed 6 years ago by
Replying to tscrim:
the current behavior is consistent with converting to a list.
What do you mean? I think that these two outputs should be equal:
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: list(pol[5:]) [0, 0, 0, 0, 0, 5, 6, 7, 8, 9] sage: list(pol)[5:] [5, 6, 7, 8, 9]
comment:22 followup: ↓ 23 Changed 6 years ago by
Ah, I see what you mean. I'm sorry, I didn't read your example closely enough and though you were truncating off the 2 leading terms.
However, list and slice already don't commute because of the python wrap around, so I'm not so sure on how much we should enforce the exact same behavior (granted, an error gets raised in the polynomial case). I personally would find that the degree was lowered more surprising if we still wanted slices to return polynomials as opposed to a list of the coefficients. Yet I don't really have much invested in this.
comment:23 in reply to: ↑ 22 ; followup: ↓ 24 Changed 6 years ago by
Replying to tscrim:
I personally would find that the degree was lowered more surprising
So what's in your opinion the correct answer for
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: list(pol[2::2])
If we cannot agree on a "natural" definition for pol[a:b:c]
, perhaps the best compromise is simply
if isinstance(i, slice): raise NotImplementedError("polynomial slicing is not defined")
comment:24 in reply to: ↑ 23 ; followup: ↓ 25 Changed 6 years ago by
Replying to jdemeyer:
Replying to tscrim:
I personally would find that the degree was lowered more surprising
So what's in your opinion the correct answer for
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: list(pol[2::2])
That depends on if we want a slice of a polynomial returns a polynomial or a simple list of the coefficients. I am (very) slightly in favor of returning a polynomial, so with that [0, 2, 4, 6, 8]
.
If we cannot agree on a "natural" definition for
pol[a:b:c]
, perhaps the best compromise is simplyif isinstance(i, slice): raise NotImplementedError("polynomial slicing is not defined")
This would be okay with me as well, but I don't think my opinion here should weigh very much here because I am mostly ambivalent about this behavior since I don't have a use for it.
comment:25 in reply to: ↑ 24 ; followup: ↓ 26 Changed 6 years ago by
Replying to tscrim:
That depends on if we want a slice of a polynomial returns a polynomial or a simple list of the coefficients. I am (very) slightly in favor of returning a polynomial, so with that
[0, 2, 4, 6, 8]
.
Sorry, I asked a bad question so I cannot interpret your answer. Please try again with this question:
sage: pol = PolynomialRing(QQ, 'x')(range(1,10)) sage: list(pol[3::2])
comment:26 in reply to: ↑ 25 ; followup: ↓ 27 Changed 6 years ago by
Replying to jdemeyer:
Replying to tscrim:
That depends on if we want a slice of a polynomial returns a polynomial or a simple list of the coefficients. I am (very) slightly in favor of returning a polynomial, so with that
[0, 2, 4, 6, 8]
.Sorry, I asked a bad question so I cannot interpret your answer. Please try again with this question:
sage: pol = PolynomialRing(QQ, 'x')(range(1,10)) sage: list(pol[3::2])
I would want this:
sage: list(pol[3::2]) [0, 0, 0, 4, 0, 6, 0, 8]
Actually, my answer above was wrong and shouw be [0, 0, 2, 0, 4, 0, 6, 0, 8]
. [Edit  Some changes due to me not thinking things completely through]
However, when trying to think about how this would work for Laurent polynomials, we currently have this behavior for list
:
sage: R.<x> = LaurentPolynomialRing(QQ) sage: p = ~x + 2 + 3*x + 4*x^2 sage: list(p) [1, 2, 3, 4] sage: p = 3*x + 4*x^2 sage: list(p) [3, 4]
In some ways I would say the iterator for polynomials should be changed to agree with that of Laurent polynomials, in that it starts from the lowest degree. Although I understand there are many (good) reasons why we do not want that behavior. I feel that at some point we just have to choose the least bad of all options and then put up warning messages about our choices.
comment:27 in reply to: ↑ 26 Changed 6 years ago by
Replying to tscrim:
I would want this:
sage: list(pol[3::2]) [0, 0, 0, 4, 0, 6, 0, 8]
Interesting. This is indeed what this branch does, but it doesn't correspond with my intuition about what it should do (I would expect [4, 6, 8]
).
comment:28 in reply to: ↑ 18 ; followup: ↓ 33 Changed 6 years ago by
comment:29 in reply to: ↑ 17 ; followup: ↓ 34 Changed 6 years ago by
Replying to jdemeyer:
Are you sure that the current behaviour is really what we want?
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2 + x sage: pol[2:] 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2
Yes. The fact that polynomials are usually written with decreasing exponents is just a historical convention. I think it is absolutely reasonable that pol[2:]
should return the monomials of degree 2 and higher, whether as a polynomial or as a list. And in fact
sage: list(pol) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
so doing list(pol)[2:]
also extracts the coefficients of the monomials of degree at least 2.
Similarly, in PARI, my experience is that Vecrev
(which does the equivalent of list
above) is more useful than Vec
to extract the list of coefficients of a polynomial.
comment:30 in reply to: ↑ 21 ; followup: ↓ 35 Changed 6 years ago by
Replying to jdemeyer:
Replying to tscrim:
the current behavior is consistent with converting to a list.
What do you mean? I think that these two outputs should be equal:
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: list(pol[5:]) [0, 0, 0, 0, 0, 5, 6, 7, 8, 9] sage: list(pol)[5:] [5, 6, 7, 8, 9]
What would you want the output of pol[5:]
itself to be?
comment:31 in reply to: ↑ 16 Changed 6 years ago by
Replying to jdemeyer:
Why not allow
step < 0
? Actually, I think thatstep = 1
might be the most mathematically useful value ofstep
.
We already have the reverse()
method for that. I can think of two possible results for pol[a:b:1]
, one of which would be the reverse()
of the other. I am afraid allowing step = 1
would only create confusion...
comment:32 Changed 6 years ago by
Mathematically, I can certainly imagine wanting the part of the polynomial between degrees 2 and 5: I might want this:
sage: f 10*x^6 + 1/2*x^5  x^4 + 7*x^3  2*x^2 + x  3 sage: f[2:6] 1/2*x^5  x^4 + 7*x^3  2*x^2
I don't know if I would ever want f[2:6]
to return the polynomial with same coefficients but translated down in degree (i.e., 1/2*x^3  x^2 + 7*x  2
). Computationally, though, if I have that second result, I can easily get back to the first one by multiplying by x^2
. If I have the first, I can get to the second by dividing, but that puts the result in the fraction field, which is probably not what I want. Alternatively, I can also get to the second by creating a polynomial from list(f)[2:6]
, which is not as intuitive, but not too hard.
Surely different mathematicians will have different points of view on this.
I think this is the issue for me: do we want
sage: list(pol[5:]) == list(pol)[5:] True
or do we want
sage: pol[:5] + pol[5:] == pol True
We can't have both, right? I prefer the second. That is, I tend to think of polynomials as graded objects and slicing is returning some of their homogeneous components, and it seems most natural to do this without regrading the result: if a term is in degree 3 in the original polynomial, it should still be in degree 3 after slicing. I would also like this:
sage: pol[0:10:2] + pol[1:10:2] == pol[0:10] True
Adding the even terms to the odd terms should yield the original polynomial.
A related question: should f[2]
return 2
or 2 * x^2
?
When it comes down to it, I'm not sure I care which option is implemented as __getitem__
, but perhaps both should be implemented as methods which are visible via tabcompletion, or as a single method with a keyword which determines which option to use.
comment:33 in reply to: ↑ 28 Changed 6 years ago by
comment:34 in reply to: ↑ 29 ; followup: ↓ 42 Changed 6 years ago by
Replying to pbruin:
Similarly, in PARI, my experience is that
Vecrev
(which does the equivalent oflist
above) is more useful thanVec
to extract the list of coefficients of a polynomial.
I think you are totally misunderstanding me. This is not at all about in which order the monomials are extracted. Obviously, PARI's Vecrev()
is the right thing, I'm not arguing against that.
It's about
9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2
versus
9*x^7 + 8*x^6 + 7*x^5 + 6*x^4 + 5*x^3 + 4*x^2 + 3*x + 2
comment:35 in reply to: ↑ 30 Changed 6 years ago by
Replying to pbruin:
What would you want the output of
pol[5:]
itself to be?
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol[5:] 9*x^4 + 8*x^3 + 7*x^2 + 6*x + 5
(although, I don't really want it to be this, but this is what I would expect)
comment:36 Changed 6 years ago by
What I really want is probably
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol[5:] DeprecationWarning: slicing polynomials is deprecated...
comment:37 Changed 6 years ago by
In the Sage library, there are use cases for pol[:n]
(i.e. truncation) and that's also the only case (I think) where both definitions agree.
comment:38 Changed 6 years ago by
So deprecate slicing unless it's a truncation?
comment:39 Changed 6 years ago by
One more thought. Let's call this degreekeeping slicing:
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol[3::2] 9*x^9 + 7*x^7 + 5*x^5 + 3*x^3
and let's call this degreereducing slicing:
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol[3::2] 9*x^3 + 7*x^2 + 5*x + 3
The implementation of certain divideandconquer algorithms (e.g. Karatsuba multiplication and FFT) can be expressed very easily using degreereducing slicing.
I'm not saying that this is a particularly strong argument for degreereducing slicing, since most likely you'd implement those algorithms using lists instead of polynomials anyway.
comment:40 Changed 6 years ago by
A new idea, let's call it double slicing. The argument to __getitem__
can be a pair of slices i, j
, where the slice i
is used to define the coefficients and the second slice j
is used to define the exponents. It would allow for example:
sage: pol = PolynomialRing(QQ, 'x')(range(10)) sage: pol[:, ::2] 9*x^18 + 8*x^16 + 7*x^14 + 6*x^12 + 5*x^10 + 4*x^8 + 3*x^6 + 2*x^4 + x^2 sage: pol[:5, 100:] 4*x^104 + 3*x^103 + 2*x^102 + x^101
We could define shorthands pol[i]
equivalent to pol[i, i]
for degreekeeping slicing and pol[i,]
equivalent to pol[i, :]
for degreereducing slicing.
This would be very general and cover all possible usecases. The question is: is it overengineered? Do we really need this?
comment:41 followup: ↓ 63 Changed 6 years ago by
I can confirm that the Sage library doesn't use any polynomial slicing except for truncation pol[:n]
. After adding
if start or step is not None: raise NotImplementedError
to the __getitem__
methods, the only doctest failures were with slicing in the actual doctest example.
comment:42 in reply to: ↑ 34 Changed 6 years ago by
Replying to jdemeyer:
Replying to pbruin: I think you are totally misunderstanding me. This is not at all about in which order the monomials are extracted. Obviously, PARI's
Vecrev()
is the right thing, I'm not arguing against that.It's about
9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2versus
9*x^7 + 8*x^6 + 7*x^5 + 6*x^4 + 5*x^3 + 4*x^2 + 3*x + 2
OK, I see. As you can guess from how I interpreted your proposal, for me this is not the expected answer at all.
I would personally be quite happy with just deprecating the start
and stop
parameters and allowing polynomial slicing only for pol[:n]
, i.e. truncation. I think users can easily implement other forms of slicing via lists.
comment:43 Changed 6 years ago by
I think the doubleslicing is probably something we don't really need. So I'm also for deprecating the (nonzero?) start
and step
conditions.
comment:44 Changed 6 years ago by
If we deprecate general slicing, shouldn't we deprecate all kinds of slicing, including pol[:n]
in favour of the truncate()
method?
comment:45 Changed 6 years ago by
 Description modified (diff)
 Status changed from needs_info to needs_work
 Summary changed from Polynomials ignore the step argument in __getitem__ to Simplify __getitem__ for polynomials and deprecate slicing
comment:46 Changed 6 years ago by
 Dependencies #19409 deleted
comment:47 Changed 6 years ago by
 Description modified (diff)
Truncation is actually implemented using slicing, so we cannot easily deprecate that.
I am working on a patch.
comment:48 Changed 6 years ago by
 Branch changed from u/pbruin/18940getitem_step to u/jdemeyer/18940getitem_step
comment:49 Changed 6 years ago by
 Commit changed from 1c56779b3140d12fbe5d0973cdcad0ac9dbb8054 to 94784864094bc84e7eb8c977fd1cf8efc4aa9b6e
Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:
9478486  Deprecate slicing with start index

comment:50 Changed 6 years ago by
 Status changed from needs_work to needs_review
comment:51 Changed 6 years ago by
 Status changed from needs_review to needs_work
Sorry, forgot src/sage/rings/polynomial/padics/polynomial_padic_capped_relative_dense.py
comment:52 Changed 6 years ago by
 Description modified (diff)
comment:53 Changed 6 years ago by
 Commit changed from 94784864094bc84e7eb8c977fd1cf8efc4aa9b6e to 44cca40e3e264811e40e51ae1136019b1f1b713e
comment:54 Changed 6 years ago by
 Status changed from needs_work to needs_review
comment:55 Changed 6 years ago by
FWIW, I think I have personal code that uses (“degreekeeping”) slicing of the form pol[n:]
. I can port it, but I may not be alone...
comment:56 followup: ↓ 57 Changed 6 years ago by
Cool, what's your use case? I'm curious because I couldn't think of any.
comment:57 in reply to: ↑ 56 Changed 6 years ago by
Replying to jdemeyer:
Cool, what's your use case? I'm curious because I couldn't think of any.
Splitting polynomials that actually represent power series expansions into an “approximation” and a “remainder“; removing the constant term of a polynomial; removing loworder terms that are know to be zero mathematically but are computed as intervals containing zero from a residual Φ(pol) where Φ is an operator and pol is a polynomial close to a zero of Φ.
comment:58 followup: ↓ 59 Changed 6 years ago by
@mmezzarobba: what is your opinion on 40?
comment:59 in reply to: ↑ 58 Changed 6 years ago by
Replying to jdemeyer:
@mmezzarobba: what is your opinion on 40?
I don't think I'd need it—any of the two basic forms of slicing (either “degreekeeping” or “degreereducing”) is enough for me. Regardless whether “double slicing” is implemented, I'd prefer pol[i:]
to keep working (with one of the two basic meanings, I don't really care which, though I find the “degreekeeping” version more intuitive with polynomials). It wouldn't be a big effort to change my code to work without slicing in any case.
comment:60 Changed 6 years ago by
 Branch changed from u/jdemeyer/18940getitem_step to u/pbruin/18940getitem_step
 Commit changed from 44cca40e3e264811e40e51ae1136019b1f1b713e to a0600bc0148f3a836cf78f7f81861c150d2c87f0
Fixed a merge conflict.
comment:61 Changed 6 years ago by
 Milestone changed from sage6.10 to sage7.0
 Reviewers set to Ralf Stephan
 Status changed from needs_review to positive_review
This is one of those enhancements I like to do myself. It is also thoroughly done. Since patchbot is happy too, I feel confident to set it positive.
comment:62 Changed 6 years ago by
 Branch changed from u/pbruin/18940getitem_step to a0600bc0148f3a836cf78f7f81861c150d2c87f0
 Resolution set to fixed
 Status changed from positive_review to closed
comment:63 in reply to: ↑ 41 Changed 5 years ago by
 Commit a0600bc0148f3a836cf78f7f81861c150d2c87f0 deleted
Replying to jdemeyer:
I can confirm that the Sage library doesn't use any polynomial slicing except for truncation
pol[:n]
. After addingif start or step is not None: raise NotImplementedErrorto the
__getitem__
methods, the only doctest failures were with slicing in the actual doctest example.
For a use case where deprecating pol[n:]
is annoying, if you have an Eisenstein polynomial pol
and want to compute p/x^n
modulo pol
, it's natural to express it as (pol[n:] >> n) / (pol[:n] // p)
, where the division is taken modulo pol
. Obviously, this is doable with lists.
comment:64 Changed 5 years ago by
Maybe a different solution would be to reintroduce the slicing but with a different interface (not __getitem__
but an explicit method).
Fixing this for all polynomials is somewhat involved because every polynomial class defines its own
__getitem__()
method. The following 14 methods are relevant (line numbers for SageMath 6.8.rc0):