Yesterday, I was reviewing some code for our projects and in a PR I saw something roughly similar to this:

    assert hasattr(SomeObject, 'some_attribute')
except AssertionError:

That didn't strike me as a good idea to rely on assert because when Python is launched using the PYTHONOPTIMIZE flag, which you can activate with the eponymous environment variable or with -O or -OO, all assertions are stripped from the code.

To my surprise, a lot of people are dismissing -O and -OO saying that no one uses those flags in production and that a code containing asserts is fine.

PYTHONOPTIMIZE has three possible values: 0, 1 (-O) or 2 (-OO). 0 is the default, nothing happens.

For 1 this is what happens:

  • asserts are stripped
  • the generated bytecode files are using the .pyo extension instead of .pyc
  • sys.flags.optimize is set to 1
  • __debug__ is set to False

And for 2:

  • everything 1 does
  • doctsrings are stripped.

To my knowledge, one legacy reason to run -O was to produce a more efficient bytecode, but I was told that this is not true anymore.

Another behavior that has changed is related to pdb: you could not run some step-by-step debugging when PYTHONOPTIMIZE was activated.

Last, the pyo vs pyc thing should go away one day, according to PEP 488

So what does that leaves us ? is there any good reason to use those flags ?

Some applications leverage the __debug__ flag to offer two running modes. One with more debug information or a different behavior when an error is encoutered.

That's the case for pyglet, according to their doc.

Some companies are also using the -OO mode to slighlty reduce the memory footprint of running apps. It seems to be the case at YouTube.

And it's entirely possible that Python itself in the future, adds some new optimizations behind that flag.

So yeah, even if you don't use yourself those options, it's good practice to make sure that your python code is tested with all possible values for PYTHONOPTIMIZE.

It's easy enough, just run your tests with -O and -OO and without, and make sure your code does not depend on doctsrings or assertions.

If you have to depend on one of them, make sure your code gracefully handles the optimize modes or raises an early error explaining why you are not compatible with them.

Thanks to Brett Cannon, Michael Foord and others for their feedback on Twitter on this.