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.