What does Python optimization (-O or PYTHONOPTIMIZE) do?

Question

The docs only say that Python interpreter performs "basic optimizations", without going into any detail. Obviously, it's implementation dependent, but is there any way to get a feel for what type of things could be optimized, and how much run-time savings it could generate?

Is there any downside to using -O?

The only thing I know is that -O disables assert, but presumably one shouldn't use assert for things that could still go wrong in production.

Answer 1

In Python 2.7, -O has the following effect:

• the byte code extension changes to .pyo
• sys.flags.optimize gets set to 1
• __debug__ is False
• asserts don't get executed

In addition -OO has the following effect:

• sys.flags.optimize gets set to 2
• doc strings are not available

To verify the effect for a different release of CPython, grep the source code for Py_OptimizeFlag.

Link to official documentation: https://docs.python.org/2.7/tutorial/modules.html#compiled-python-files