Slightly pedantic correction: This is a performance optimization in CPython. I wouldn't be surprised if other implementations have something similar, but to my best knowledge, this behaviour isn't part of the standard.
The standard is documented in PEPs[1]. CPython is the flagship implementation, and many PEPs discuss it directly, but if you're looking to write another Python, that's where you'd go.
To the best of my knowledge, Python isn't one such language with a standard, is it?