bug in change_ring for polynomials

[xcaruso]

The method change_ring is not always working when we pass in a morphism.

sage: K = GF(5^2)
sage: S.<x> = K[]
sage: P = S.random_element()
sage: P.change_ring(K.frobenius_endomorphism())
Traceback (most recent call last):
...
TypeError: 

The current code is weird: it calls the method hom with a morphism as first argument (instead of the list the images of generators). Instead, I propose to fall back to map_coefficients.

CC: @roed314

Component: commutative algebra

Author: Xavier Caruso

Branch/Commit: e9840e4

Reviewer: Marc Mezzarobba, Markus Wageringel

Issue created by migration from https://trac.sagemath.org/ticket/28541

[xcaruso]

Branch: u/caruso/change_ring

[xcaruso]

Author: Xavier Caruso

[xcaruso]

Changed branch from u/caruso/change_ring to none

[xcaruso]

Commit: 2ab843c

[xcaruso]

Branch: u/caruso/change_ring

[sagetrac-git]

Changed commit from 2ab843c to c615bb6

[sagetrac-git]

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

c615bb6

fallback to map_coefficients

[sagetrac-git]

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

fe1d9da

add doctest

[sagetrac-git]

Changed commit from c615bb6 to fe1d9da

[mezzarobba]

Reviewer: Marc Mezzarobba

[mezzarobba]

comment:6

lgtm!

[vbraun]

comment:7

Merge conflict

[sagetrac-git]

Changed commit from fe1d9da to e9840e4

[sagetrac-git]

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

e9840e4

Merge branch 'u/caruso/change_ring' of git://trac.sagemath.org/sage into change_ring

[mwageringel]

Changed reviewer from Marc Mezzarobba to Marc Mezzarobba, Markus Wageringel

[mwageringel]

comment:10

This looks good to me. The merge has not changed anything, so I am setting this to positive.

[vbraun]

Changed branch from u/caruso/change_ring to e9840e4