The __debug__ constant is really weird - any block of code guarded with `if __debug__:` will be entirely omitted from the bytecode under PYTHONOPTIMIZE=1. This and `assert` are the only two examples of real “conditional compilation” in Python. This is also the reason why you cannot assign to __debug__: doing so would make it possible to invalidate the compiler’s assumption about `if __debug__:` statements.

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plant-ian 4 hours ago | parent | next [-]

I honestly have never even heard of this constant and I feel like I've been using python for a pretty long time. Although maybe my memory for some things just gets garbage collected if I don't use it enough. Does it actually get used that often in real world code? Seems like it might be kind of risky.

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UqWBcuFx6NV4r 2 hours ago | parent | next [-]

Ditto. I’ve certainly never used it and can’t recall seeing it in any codebases I’ve worked on or looked at. Sounds interesting though!

I’ve of course certainly heard of, seen, and used `assert`, but more often than not, outside of pytest, I see its use way more in potential footgun scenarios—I doubt that many people know that assertions can be silenced, and that they’d probably be better off raising exceptions in many cases where they’re using `assert`.

	▲	nneonneo 2 hours ago \| parent [-]
		I made a CTF problem where `assert` was used as a critical safety check - and where "accidentally" running the program under -O (for speed!) resulted in a security vulnerability. A large fraction of the people who attempted the problem seemingly missed this bug. I would not be surprised in the least if that pattern existed in the wild. In fact, it's quite common to see this in C/C++ codebases too: people will use assert() to check a security-relevant property, and then disable those checks in their release builds "because it can't happen".

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rcxdude 3 hours ago | parent | prev | next [-]

I feel like it's the kind of thing you might wind up caring about if you're micro-optimizing your python, but in my experience that's a losing game and you're better served rewriting it in another language than bothering with trying to speed up the execution of the raw python code (it's not that you can't optimize python code, but only in broader strokes. If you are looking at the bytecode you're in too deep and every time I've seen it tried the code has been ported shortly afterwards).

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Vexs 4 hours ago | parent | prev [-]

I see asserts used in production code as part of flow control way too frequently, so I assume the majority of python users aren't aware of the -O flag, much less this behavior- which I too haven't ever heard of. Of recently, I've noticed claude is a big fan of asserts too.

	▲	UqWBcuFx6NV4r 2 hours ago \| parent [-]
		Yep. I’ve had to tell Claude to basically not use assert. Thankfully it’s very complaint in this one area.

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Numerlor 3 hours ago | parent | prev [-]

If 0. Etc. are also compiled out, at compile time __debug__ is simply False or True and the existing optimization paths take care of it.

Assigning to __debug__ wouldn't do anything to the compiler as it never actually reads the variable, so assignment would just cause weirdness from other use