This ticket introduces a new Python package sage_conf.

• sage-the-distribution will generate this Python package at ./configure time and install it at build time before starting to build/install sagelib using src/setup.py.
• Distributions will generate and install their own sage_conf by a method of their choice before starting to build/install sagelib.

A console_script allows to query individual variable values from the shell, or output all variables in .env format:

$ local/bin/sage-config MAXIMA
/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/bin/maxima
$ local/bin/sage-config
VERSION=9.1.beta0
MAXIMA=/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/bin/maxima
SAGE_LOCAL=/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local
SAGE_ROOT=/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring

The module and the script are used as follows:

1. It provides configuration information to sagelib at installation time (src/setup.py).

   Ultimately we want to be able to install sagelib with pip. To support installation by pip from PyPI, from an URL, etc., we cannot expect to configure the build like we do now, by writing the configuration to the file src/sage-env-config. Moreover, if pip is running in --isolated mode, also no environment variables are passed. Then the only information flow is through arguments to pip build and through the installed Python packages. By installing sage_conf first, we make the configuration available to the pip install of sagelib.

2. It provides configuration information to the docbuild, in particular about the install locations of documentation of external packages.

3. It provides configuration information to the runtime of sagelib, making sagelib more independent from the environment variables set by src/bin/sage-env (local/bin/sage-env).

a. By providing SAGE_LOCAL as one the configuration variables, it removes assumptions regarding install locations of sagelib relative to $SAGE_LOCAL. This enables the following:

• Installing an experimental version of sagelib in other install locations, such as in a user site packages directory.

• Making sagelib available in a user's venv, as in the following example:
  Without this ticket:

  $ sage -python -m venv --system-site-packages ~/personal-sage-venv/
  $ source ~/personal-sage-venv/bin/activate
  (personal-sage-venv) $ python
  >>> import sage.env
  >>> sage.env.SAGE_LOCAL
  '/Users/mkoeppe/personal-sage-venv'                                     # wrong
  >>> import sage.all
  RuntimeError: You must get the file local/bin/sage-maxima.lisp
  

  With this ticket:

  >>> sage.env.SAGE_LOCAL
  '/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local'
  >>> import sage.all
  >>> sage.all.maxima('1')
  1
  

• Add configure option --with-sage-venv=SAGE_VENV to create venv there instead of in SAGE_LOCAL #29013/spkg-configure.m4 for python3 #27824: Using the system python3 via spkg-configure.m4
  b. For a broader context: see Meta-ticket: Split sage-env into 5 to clean up sage configuration #21707 Task: Split sage-env into 5

CC: @kiwifb @antonio-rojas @isuruf @embray @infinity0 @timokau @jdemeyer @dimpase @jhpalmieri @vbraun

Component: build

Author: Matthias Koeppe

Branch: af18a01

Reviewer: Dima Pasechnik, Erik Bray

Issue created by migration from https://trac.sagemath.org/ticket/29038