gh-37399: Developer Guide: Document practices for data files
    
Topics here:
- ext_data is legacy location only, see #33037
- separate repos for large data files
- importlib_resources

Follow-ups:
- use of Features

<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #37399
Reported by: Matthias Köppe
Reviewer(s): gmou3, Gonzalo Tornaría, Matthias Köppe, Sebastian Oehms

Author: Release Manager

src/doc/en/developer/coding_basics.rst

@@ -89,9 +89,9 @@ In particular,
 Files and directory structure
 =============================
 
-Roughly, the Sage directory tree is layout like this. Note that we use
-``SAGE_ROOT`` in the following as a shortcut for the (arbitrary) name
-of the directory containing the Sage sources:
+Roughly, the Sage directory tree is laid out like this. Note that we
+use ``SAGE_ROOT`` in the following as a shortcut for the name of the
+directory containing the Sage sources:
 
 .. CODE-BLOCK:: text
 
@@ -104,7 +104,7 @@ of the directory containing the Sage sources:
             setup.py
             ...
             sage/            # Sage library
-                ext_data/    # extra Sage resources (formerly src/ext)
+                ext_data/    # extra Sage resources (legacy)
             bin/             # the scripts in local/bin that are tracked
         upstream/            # tarballs of upstream sources
         local/               # installed binaries
@@ -149,15 +149,36 @@ Adding new top-level packages below :mod:`sage` should be done
 sparingly.  It is often better to create subpackages of existing
 packages.
 
-Non-Python Sage source code and supporting files can be included in one
-of the following places:
+Non-Python Sage source code and small supporting files can be
+included in one of the following places:
 
 - In the directory of the Python code that uses that file.  When the
   Sage library is installed, the file will be installed in the same
-  location as the Python code. For example,
-  ``SAGE_ROOT/src/sage/interfaces/maxima.py`` needs to use the file
-  ``SAGE_ROOT/src/sage/interfaces/maxima.lisp`` at runtime, so it refers
-  to it as ::
+  location as the Python code. This is referred to as "package data".
+
+  The preferred way to access the data from Python is using the
+  `importlib.resources API
+  <https://importlib-resources.readthedocs.io/en/latest/using.html>`_,
+  in particular the function :func:`importlib.resources.files`.
+  Using it, you can:
+
+  - open a resource for text reading: ``fd = files(package).joinpath(resource).open('rt')``
+  - open a resource for binary reading: ``fd = files(package).joinpath(resource).open('rb')``
+  - read a resource as text: ``text = files(package).joinpath(resource).read_text()``
+  - read a resource as bytes: ``bytes = files(package).joinpath(resource).read_bytes()``
+  - open an xz-compressed resource for text reading: ``fd = lzma.open(files(package).joinpath(resource).open('rb'), 'rt')``
+  - open an xz-compressed resource for binary reading: ``fd = lzma.open(files(package).joinpath(resource).open('rb'), 'rb')``
+
+  If the file needs to be used outside of Python, then the
+  preferred way is using the context manager
+  :func:`importlib.resources.as_file`. It should be imported in the
+  same way as shown above.
+
+- Older code in the Sage library accesses
+  the package data in more direct ways. For example,
+  ``SAGE_ROOT/src/sage/interfaces/maxima.py`` uses the file
+  ``SAGE_ROOT/src/sage/interfaces/maxima.lisp`` at runtime, so it
+  refers to it as::
 
     os.path.join(os.path.dirname(__file__), 'sage-maxima.lisp')
 
@@ -169,11 +190,39 @@ of the following places:
     from sage.env import SAGE_EXTCODE
     file = os.path.join(SAGE_EXTCODE, 'directory', 'file')
 
-In both cases, the files must be listed (explicitly or via wildcards) in
+  This practice is deprecated, see :issue:`33037`.
+
+In all cases, the files must be listed (explicitly or via wildcards) in
 the section ``options.package_data`` of the file
 ``SAGE_ROOT/pkgs/sagemath-standard/setup.cfg.m4`` (or the corresponding
 file of another distribution).
 
+Large data files should not be added to the Sage source tree. Instead, it
+is proposed to do the following:
+
+- create a separate git repository and upload them there [2]_,
+
+- add metadata to the repository that make it a pip-installable
+  package (distribution package), as explained for example in the
+  `Python Packaging User Guide
+  <https://packaging.python.org/en/latest/tutorials/packaging-projects/>`_,
+
+- `upload it to PyPI
+  <https://packaging.python.org/en/latest/tutorials/packaging-projects/#uploading-the-distribution-archives>`_,
+
+- create metadata in ``SAGE_ROOT/build/pkgs`` that make your new
+  pip-installable package known to Sage; see :ref:`chapter-packaging`.
+
+For guiding examples of external repositories that host large data
+files, see https://github.com/sagemath/conway-polynomials, and
+https://github.com/gmou3/matroid-database.
+
+.. [2]
+
+  It is also suggested that the files are compressed, e.g., through
+  the command ``xz -e``. They can then be read via a command such as
+  ``lzma.open(file, 'rt')``.
+
 
 Learn by copy/paste
 ===================

src/doc/en/developer/coding_in_python.rst

@@ -7,6 +7,7 @@ Coding in Python for Sage
 This chapter discusses some issues with, and advice for, coding in
 Sage.
 
+.. _section-python-language-standard:
 
 Python language standard
 ========================