Section for composite maps

[soehms]

Currently, a section method for the class FormalCompositeMap is not implemented.
This ticket supplies a section in cases, where all factors of the composite map have section
implemented.

Examples which will be improved are:

sage: P.<x> = QQ[]
sage: P.coerce_map_from(ZZ).section() is None
True
sage: k = GF(47)
sage: R.<x> = PolynomialRing(k)
sage: R.coerce_map_from(ZZ).section() is None
True

Component: categories

Keywords: section map

Author: Sebastian Oehms

Branch/Commit: bb25f16

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/27081

[soehms]

Branch: u/soehms/section_of_composite_map_27081

[soehms]

Commit: 3fd303e

[soehms]

New commits:

3fd303e

27081: initial version

[embray]

comment:3

Ticket retargeted after milestone closed (if you don't believe this ticket is appropriate for the Sage 8.8 release please retarget manually)

[embray]

comment:4

Moving tickets from the Sage 8.8 milestone that have been actively worked on in the last six months to the next release milestone (optimistically).

[tscrim]

comment:5

Sorry for letting my review of this take so long. Once these changes are done, then it is a positive review:

-        Compute a section map from sections of the factors of self if they have been implemented
+        Compute a section map from sections of the factors of
+        ``self`` if they have been implemented.

I would write this with short-circuiting and the reversal already built-in:

        sections = []
        for m in reversed(self):
            try:
                sec = m.section()
            except TypeError:
                return None
            if sec is None:
                return None
            sections.append(m)

        from sage.categories.homset import Hom
        from sage.categories.sets_with_partial_maps import SetsWithPartialMaps
        H = Hom(self.codomain(), self.domain(), category=SetsWithPartialMaps())
        return FormalCompositeMap(H, sections)

Capitalize the beginning of the .. NOTE:: in polynomial_element.pyx.

[tscrim]

Reviewer: Travis Scrimshaw

[sagetrac-git]

Changed commit from 3fd303e to bb25f16

[sagetrac-git]

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

c049ce9	Merge branch 'u/soehms/section_of_composite_map_27081' of git://trac.sagemath.org/sage into section_of_composite_map_27081
bb25f16	27081: corrections according to review

[soehms]

comment:7

thanks, Travis! I'll be waiting for the patchbot!

[tscrim]

comment:8

Thank you.

[vbraun]

Changed branch from u/soehms/section_of_composite_map_27081 to bb25f16