What are acceptable use-cases for python's `assert` statement?

python assert
Gattster picture Gattster · Jan 26, 2010 · Viewed 7.8k times · Source

I often use python's assert statement to check user input and fail-fast if we're in a corrupt state. I'm aware that assert gets removed when python with the -o(optimized) flag. I personally don't run any of my apps in optimized mode, but it feels like I should stay away from assert just in-case.

It feels much cleaner to write

assert filename.endswith('.jpg')

than

if not filename.endswith('.jpg'):
    raise RuntimeError

Is this a valid use case for assert? If not, what would a valid use-case for python's assert statement be?

Answer

rob picture rob · Jan 26, 2010

Assertions should be used for expressing invariants, or preconditions.
In your example, you are using them for checking unexpected input - and that's a completely different class of exceptions.

Depending on the requirements, it may be perfectly OK to raise an exception on wrong input, and stop the application; however the code should always be tailored for expressiveness, and raising an AssertionError is not that explicit.
Much better would be to raise your own exception, or a ValueError.

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