Currently the PEP specifies
It is up to the tool to decide how to handle [a
pyproject.tomlfile without the
build-systemtable]. Potential options are:
- Act as if the
pyproject.tomlfile does not exist.
- Assume the table exists with the default values specified above.
IMO this has potential for unnecessarily confusion. In practice most would simply follow pip’s behaviour (1. if the table is missing entirely; 2. if the table exists but misses values). Why not specify that?