Category of submodules of sage.modules.free_module.FreeModule_ambient_pid

[mkoeppe]

sage: (QQ^2).submodule([[1,1]])
Vector space of degree 2 and dimension 1 over Rational Field
Basis matrix:
[1 1]
sage: _.category()
Category of finite dimensional vector spaces with basis over (number fields and quotient fields and metric spaces)

Subobjects is missing, so the standard methods ambient, lift, retract are missing.

This is in contrast to CombinatorialFreeModule.

(There is a class-specific method ambient_module though.)

Also there is an apparent conflict of the element method lift with the one defined in free_module_element.pyx; not resolved here.

CC: @tscrim @louisng114

Component: linear algebra

Author: Matthias Koeppe

Branch/Commit: 62f28a3

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33431

[mkoeppe]

Branch: u/mkoeppe/category_of_submodules_of_sage_modules_free_module_freemodule_ambient_pid

[sagetrac-git]

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

88c67d9

FreeModule_submodule_with_basis_pid.{lift,retract}: New

[sagetrac-git]

Commit: 88c67d9

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Description changed:

--- 
+++ 
@@ -15,3 +15,4 @@
 
 (There is a class-specific method `ambient_module` though.)
 
+Also there is an apparent conflict of the element method `lift` with the one defined in `free_module_element.pyx`.

[mkoeppe]

Description changed:

--- 
+++ 
@@ -15,4 +15,4 @@
 
 (There is a class-specific method `ambient_module` though.)
 
-Also there is an apparent conflict of the element method `lift` with the one defined in `free_module_element.pyx`.
+Also there is an apparent conflict of the element method `lift` with the one defined in `free_module_element.pyx`; not resolved here.

[sagetrac-git]

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

edec50e	src/sage/modules/free_module.py, src/sage/modules/fg_pid/fgp_morphism.py: Use ModulesWithBasis instead of FreeModules, which is only an alias defined in sage.categories.all
8230736	FreeModule_submodule_with_basis_pid: Fix up category

[sagetrac-git]

Changed commit from 88c67d9 to 8230736

[tscrim]

comment:9

LGTM other than catching Exception seems quite broad and might hide bugs. Could we make that more refined?

[mkoeppe]

comment:10

I generally agree (and specifically it did hide a bug, just fixed in 8230736!) but this is copied directly from Module_free_ambient.__init__ and I don't have special insights to what it should be tightened.

[tscrim]

comment:11

I would do the standard ones: ValueError, TypeError, AttributeError, and maybe NotImplementedError. If it is raising something outside of these (such as an assert) then it almost certainly should be another issue to address.

[sagetrac-git]

Changed commit from 8230736 to 2929a9e

[sagetrac-git]

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

2929a9e

src/sage/modules/free_module.py: Make 'except Exception' more specific

[mkoeppe]

comment:13

OK. I've made this change (in both places)

[mkoeppe]

comment:14

+        if category is None:
+            from sage.categories.modules_with_basis import ModulesWithBasis
+            category = ModulesWithBasis(R.category()).FiniteDimensional()
+            try:
+                if R.is_finite() or len(basis) == 0:
+                    category = category.Enumerated().Finite()
+            except (ValueError, TypeError, AttributeError, NotImplementedError):
+                pass
+        category = category.Subobjects()

I guess I should probably change this to category &= ModulesWithBasis(R.category()).Subobjects()?