CPython supports two levels of optimization. You can enable them with -O and -OO flags. -O sets __debug__ to False and removes all assert statements from the program. -OO do the same and also discards docstrings. A regular version of a script is cached to .pyc file while an optimized one is cached to .pyo. However, since Python 3.5 .pyo is no more a thing, .opt-1.pyc and .opt-2.pyc are introduced by PEP 488 instead.