make gens and orbits of PermutationGroup immutable

[mantepse]

We don't want the following to happen:

sage: G = PermutationGroup([ [(3,4)], [(1,3)] ])
sage: O = G.orbits(); O
[[1, 3, 4], [2]]
sage: O[0] = 1
sage: G.orbits()
[1, [2]]

sage: G = PermutationGroup([[2,1]]); G
Permutation Group with generators [(1,2)]
sage: g = G.gens()
sage: g[0] = 1
sage: G.gens()
[1]

Therefore, we change the return type of gens and orbits to be a tuple of tuples. We keep the _repr_ of PermutationGroup_generic, because the output is more compact.

Note that gens returns a tuple for all other groups I checked.

CC: @tscrim

Component: group theory

Author: Martin Rubey

Branch/Commit: 90005e7

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33824

[mantepse]

Branch: u/mantepse/make_orbits_of_permutationgroup_immutable

[mantepse]

New commits:

3f65072

make PermutationGroup_generic.orbits return a tuple of tuples, for immutability

[mantepse]

Commit: 3f65072

[mantepse]

Author: Martin Rubey

[mantepse]

comment:3

Here is another problem, of the same kind:

sage: G = PermutationGroup([[2,1]]); G
Permutation Group with generators [(1,2)]
sage: g = G.gens()
sage: g[0] = 1
sage: G.gens()
[1]

I checked a few examples of groups, the all return a tuple.

[sagetrac-git]

Changed commit from 3f65072 to da2c2e5

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

da2c2e5

make PermutationGroup_generic.gens return a tuple, to ensure immutability

[tscrim]

comment:7

A while-we-are-at-it thing would be to replace the $ with a back tick `. If you decide to do this or not, you can set a positive review on my behalf once the patchbot is green.

[tscrim]

Reviewer: Travis Scrimshaw

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

d5adcb0

replace $...$ with `...`

[sagetrac-git]

Changed commit from da2c2e5 to d5adcb0

[mantepse]

comment:9

Done, I also looked at the doc which comes out fine.

[sagetrac-git]

Changed commit from d5adcb0 to 90005e7

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

90005e7

remove unused import

[tscrim]

comment:11

Thank you. LGTM.

[vbraun]

Changed branch from u/mantepse/make_orbits_of_permutationgroup_immutable to 90005e7