assert-is-only-a-development-check.md

Assert Is Only A Development Check

The assert keyword is used in Python to write a statement that will check some assertion and raise an error if it isn't met. This is only meant to be used as a check during development because it can be easily optimized out of the code.

stuff = None

assert stuff, "We need to have some stuff to proceed"

print(f"We have {stuff or 'something'}!")

If I execute this code with python, it will raise on that second line of code.

❯ python assert_example.py
Traceback (most recent call last):
  File "/Users/lastword/dev/jbranchaud/py-vmt/assert_example.py", line 3, in <module>
    assert stuff, "We need to have some stuff to proceed"
           ^^^^^
AssertionError: We need to have some stuff to proceed

This assert statement will be stripped out of the compiled bytecode if the -O (capital o) flag is used. Notice how running the same file with that flag does not lead to an AssertionError.

❯ python -O assert_example.py
We have something!

If I want to make sanity checks for situations that would be caused by a bug in the code, an assert statement can be a good candidate. However, if I am making runtime checks like validating user input, then an if statement and raising something like a ValueError is better.