Currently we don't test builds done outside the source tree, neither on GitHub nor the buildbots. [1]  As a result, such builds get broken occasionally.  I've certainly broken then a couple times, inadvertently.  It would be helpful if we tested out-of-tree builds on GitHub, or at least on a stable buildbot.

FTR, to do an out-of-tree build locally and test it:

mkdir ../build
cd ../build
../cpython/configure
make
./python -m test


[1] Actually I recently added test.test_tools.test_freeze.TestFreeze.test_freeze_simple_script which happens to do an out-of-tree build incidentally.  However, we should be testing out-of-tree builds explicitly.

FTR, I used to run all Windows builds out of tree on Azure Pipelines, but too many tests broke so I reverted it: https://github.com/python/cpython/blob/456e27ac0ac6bc1cfd6da0191bd7802d8667457b/.azure-pipelines/windows-steps.yml#L7-L10

Setting $env:Py_OutDir before building is the easiest way to change it. You can also pass "/p:Py_OutDir=<path>" to build.bat, but you'll run into quoting issues almost immediately.

I always do out of tree builds _(I find people who build anything in tree to be very weird - it makes a mess)_.

I intend to look into making sure a Linux buildbot runs this way but don't let that stop anyone else from doing so.

Good news: I got a test case for OOT build with read-only sources

Bad news: 15 test cases are failing and testing takes much longer because pyc files are missing.

15 re-run tests:
    test__xxsubinterpreters test_ast test_bdb test_capi test_doctest
    test_import test_importlib test_interpreters test_pickle
    test_pydoc test_runpy test_support test_threading test_trace
    test_unicodedata

We have to live with read-only OOT builds and writable tests for now.

Nice trick with that RO bind mount.  tests... yes we do have some writable source tree messes in there to be dealt with.

FWIW, it is also impossible to build the docs OOT without cluttering the source tree.

    $ cd ../build
    $ make -C ../cpython.git/Doc venv
    $ make -C ../cpython.git/Doc html  # <= modifies Doc/build in the source tree

New changeset cee07b162843694e8166ad8715162d4d5886b50f by Christian Heimes in branch 'main':
bpo-45695: Test out-of-tree builds on GHA (GH-29904)
https://github.com/python/cpython/commit/cee07b162843694e8166ad8715162d4d5886b50f