File PACKAGING-NOTES of Package python3

Notes for packagers of Python3
==============================

0. Faster build turnaround
--------------------------

By default, python builds with profile-guided optimization. This needs
an additional run of the test suite and it is generally slow.
PGO build takes around 50 minutes.

For development, use "--without profileopt" option to disable PGO. This
shortens the build time to ~5 minutes including test suite.

1. import_failed.map
----------------------

This is a mechanism installed as part of python3-base, that places shim modules
on python's path (through a generated zzzz-import-failed-hooks.pth file, so that
it is imported as much at the end as makes sense; and an _import_failed subdir
of /usr/lib/pythonX.Y). Then when the user tries to import a module that is part
of a subpackage, the ImportError will contain a helpful message telling them
which missing subpackage to install.

This can sometimes cause problems on non-standard configurations, if the pth
gets included too early (for instance if you are using a script to include all
pths by hand in some strange order). Just something to look out for.