When I attempt to build CPython from a subfolder (as we do in Fedora) make regen-all dies with:

  python3.8 -m Parser.pgen ../../Grammar/Grammar \
  	../../Grammar/Tokens \
  	../../Include/graminit.h.new \
  	../../Python/graminit.c.new
  /usr/bin/python3.8: No module named Parser.pgen

To reproduce, run:

  $ rm -rf build && mkdir -p build/mybuild && (cd build/mybuild && ../../configure && make regen-all)

This is probably a regression, as it works in the 3.7 branch.

Setting PYTHON_FOR_REGEN="python3" (python3.7 on my system) doesn't make a difference:

  python3 -m Parser.pgen ../../Grammar/Grammar \
  	../../Grammar/Tokens \
  	../../Include/graminit.h.new \
  	../../Python/graminit.c.new
  /usr/bin/python3: No module named Parser.pgen


build/mybuild/Parser exists but it is empty directory.

Setting PYTHON_FOR_REGEN="PYTHOINPATH=<path_to_sources> python3" workarounds the issue.

On branch 3.7 (3076a3e0d1), build/mybuild/Parser is also empty, but the make-regen populates it.

I'll bisect.

regen-keyword is recent: bpo-36143.

regen-grammar has been modified to use Parser.pgen in bpo-35808.

1f24a719e7be5e49b876a5dc7daf21d01ee69faa

New changeset 06d04e77ca36133e82bf6c363c09ba82e07a9c75 by Victor Stinner in branch 'master':
bpo-36733: Fix PYTHONPATH for make regen-add (GH-12969)
https://github.com/python/cpython/commit/06d04e77ca36133e82bf6c363c09ba82e07a9c75

Thanks for your bug report Miro Hrončok. The fix will be part of the next Python 3.8 alpha release:

"3.8.0 alpha 4: Monday, 2019-04-29"
https://www.python.org/dev/peps/pep-0569/