Remove import of 'ppl' at startup using lazy_import with feature

[mkoeppe]

Currently, sage imports ppl (from pplpy) at startup, via sage.geometry.cone, sage.geometry.integral_points, and sage.geometry.lattice_polytopes.

This should be changed, both

• as preparation for other backends for polyhedral cones (in particular PyNormaliz)
• for the modularization of sagelib (Meta-ticket: Modularize sagelib into separate distributions (pip-installable packages) sagemath-... for Sage 10.x #29705).

In addition we add documentation to the option feature of lazy import and while we are at it, use this option for backend normaliz as well.

CC: @kliem @novoselt @vbraun @w-bruns @jplab @orlitzky @videlec

Component: geometry

Keywords: sd111

Author: Jonathan Kliem, Matthias Koeppe

Branch/Commit: 17c4cdb

Reviewer: Matthias Koeppe, Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/30587

[mkoeppe]

Changed keywords from none to sd111

[kliem]

comment:3

I'm looking at this at the moment.

I think one needs to also take case of Polyhedron_base, because once it is lazily imported, then it immediately imports all depencies, in particular the stuff from ppl.

So if the goal is to have a working sage as good as possible without ppl, then this should be touched as well.

(I tested adding a line from foo import nonsense into backend_ppl.py. Sage is starting all right, but polyhedra are unusable. However, only backend ppl should fail.)

[kliem]

New commits:

7dae2c0	lazy import polyhedra instances
736f7ae	lazy import ppl for cone
c8cb27a	remove import of ppl through lattice polytope

[kliem]

Commit: c8cb27a

[kliem]

Author: Jonathan Kliem

[kliem]

Branch: public/30587

[kliem]

Work Issues: use feature of lazy import

[orlitzky]

comment:7

If the imports would not wind up in an extremely hot path, you might also work around the circular imports with local ones (from foo import whatever within the function that needs whatever). Those don't get run until the function is called, but it does introduce a tiny redundant dictionary lookup the second, third, etc times the function is called.

[sagetrac-git]

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

d8934c0

document feature of lazy import

[sagetrac-git]

Changed commit from c8cb27a to d8934c0

[kliem]

comment:9

So lazy import appears to be a bit slower.

For %timeit polytopes.cube() I went from 411 µs to 417. I don't know if this is acceptable.
So there appears to be a overhead of about 6 µs per call.

---

New commits:

d8934c0

document feature of lazy import

[sagetrac-git]

Changed commit from d8934c0 to 5feb98c