It was on purpose because TOML has not reached 1.0 yet and breaking people using a module in the stdlib for parsing TOML because TOML itself changed would be an

It was on purpose because TOML has not reached 1.0 yet and breaking people using a module in the stdlib for parsing TOML because TOML itself changed would be an absolute mess. Plus not a single packaging project said it was an issue to either depend on or vendor a TOML parser. Once TOML hits 1.0 I plan to work on getting a TOML parser into Python's stdlib. (disclaimer: I am one of the co-authors of PEP 518 that apparently handled things in way that was "a little absurd" .)