I came across this issue whilst trying to stage a core16 snap into a core18. It seems that, when mixing these cores, your build environment (during snapcraft build) is overwritten by the unpacking of the incoming stage snap. This obviously breaks build packages that you have. E.g. this yaml https://www.irccloud.com/pastebin/mxelP3CN/ gives errors like: ``` Fatal Python error: Py_Initialize: Unable to get the locale encoding ImportError: No module named 'encodings' ``` ``` fatal: unable to find remote helper for 'https ``` Even if you put this stage snap to the bottom of the build process, you will end up with an error like: ``` Failed to stage: Parts 'aws-cli' and 'docker' have the following files, but with different contents: etc/mime.types lib/x86_64-linux-gnu/libexpat.so.1 usr/bin/pdb3 usr/bin/py3clean usr/bin/py3compile usr/bin/pydoc3 usr/bin/pygettext3 usr/bin/python3 usr/bin/python3m usr/bin/run-mailcap usr/lib/valgrind/python3.supp usr/lib/x86_64-linux-gnu/libexpatw.so.1 usr/lib/x86_64-linux-gnu/libffi.so.6.0.4 usr/lib/x86_64-linux-gnu/libmpdec.so.2.4.2 usr/lib/x86_64-linux-gnu/libsqlite3.so.0.8.6 usr/share/doc/libexpat1/changelog.Debian.gz usr/share/doc/libexpat1/copyright usr/share/doc/libffi6/changelog.Debian.gz usr/share/doc/libmpdec2/changelog.Debian.gz usr/share/doc/libpython3-stdlib/README.Debian usr/share/doc/libpython3-stdlib/changelog.Debian.gz usr/share/doc/libsqlite3-0/changelog.Debian.gz usr/share/doc/mime-support/changelog.gz usr/share/doc/python3-minimal/README.Debian usr/share/doc/python3-minimal/changelog.Debian.gz usr/share/man/man1/pdb3.1.gz usr/share/man/man1/pydoc3.1.gz usr/share/man/man1/pygettext3.1.gz usr/share/man/man1/python3.1.gz usr/share/man/man1/python3m.1.gz usr/share/man/man1/run-mailcap.1.gz usr/share/pixmaps/python3.xpm usr/share/python3/debian_defaults usr/share/python3/debpython/version.py usr/share/python3/py3versions.py Snapcraft offers some capabilities to solve this by use of the following keywords: - `filesets` - `stage` - `snap` - `organize` To learn more about these part keywords, run `snapcraft help plugins`. ``` I propose that snapcraft gives a warning when pulling in the snap to be staged if the cores mismatch.