He's talking about CPython, the default interpreter/compiler for Python in many systems. CPython internally uses 1-byte representation if the maximum Unicode co

He's talking about CPython, the default interpreter/compiler for Python in many systems. CPython internally uses 1-byte representation if the maximum Unicode code point of its chars is 127 or less (therefore ASCII), 2-byte representation for values less than 32767 (thus utf-16, but only the first half-page), and for the rest it uses UTF-32, which means all lookups can be done in O(1). Assuming UTF-8 encoding for source files is not that strange, as it is getting more and more the default or preferred encoding for string-based I/O.