David is correct, most PEPs are historical records of design decisions that cease to be updated once completed. The language reference is the normative guide for current behaviour.

The exception is "informational" and "process" PEPs, which cover things that are not covered in the language reference, as well the occasional Standards Track PEP which ends up not being incorporated into the language reference properly (for example, until the import section was added to the language reference in Python 3.3, PEP 302 was the best available reference documentation).