[dragonwriter]

> I warned people in a programming forum that Python ordering of objects by insertion time was a implementation detail, because it's not guaranteed by any PEP

PEPs do not provide a spec for Python, they neither cover the initial base language before the PEP process started, nor were all subsequent language changes made through PEPs. The closest thing Python has to a cross-implementation standard is the Python Language Reference for a particular version, treating as excluded anything explicitly noted as a CPython implementation detail. Dictionaries being insertion-ordered went from a CPython implementation detail in 3.6 to guaranteed language feature in 3.7+.