Amend PEP 518 to explicitly specify tool behaviour when pyproject.toml with missing/partial build-tools definition

[brettcannon]

@pf_moore

largely because we don’t (certainly didn’t at the time) know for certain whether pip’s behaviour is correct, and want to leave the option open for experimentation.

Do note that the PEP is marked as experimental so technically no one has to follow the PEP.

@pradyunsg

I think this is right but can’t check to verify right now .

Since no one is worried enough to verify then I think it’s reasonable to assume that this is how pip operates.

Proposal

Tools should not require the existence of the [build-system] table. A pyproject.toml file may be used to store configuration details other than build-related data and thus lack a [build-system] table legitimately. If the file exists but is lacking the [build-system] table then the default values as specified above should be used. If the table is specified but is missing required fields then the tool should consider it an error.

What do people think of that as the change to that section of the PEP?