How to exit when assertion fails in python?

Question

I would like to use assertions, so that my script exists, when assertions fail.

For that I wrote an own function, which stops the script

def assertExit(mlambda, errorMessage):
    res = mlambda()
    if res != True:
        sys.exit(errorMessage)

assertExit((lambda: False), "Should fail")

Is there a way to do that with pythons native assertions?

assert False # should exit the script

Native assertions will raise an exception if they fail, which will terminate the application unless you catch it. — SwiftsNamesake
– SwiftsNamesake, Commented Aug 8, 2017 at 7:33
Simply not catching AssertionErrors should do the trick… why do you need an even more direct exit? — deceze
– deceze ♦, Commented Aug 8, 2017 at 7:33
you are right, failed assertions do stop the script already. I was confused, because the logs order was wrong, and in the logs, after assertion exception, some output was logged — Skip
– Skip, Commented Aug 8, 2017 at 7:53

deepbrook · Accepted Answer · 2020-05-24 17:15:33Z

5

As has been pointed out, an unhandled AssertionError, as thrown by assert, should already stop your script.

assert always fails if the condition being tested doesn't evaluate to True. That is as long as you do not run python with optimizations enabled (-O flag), in which case it will not throw an AssertionError (as assert statements are skipped in optimized mode).

So

def assert_exit(condition, err_message):
    try:
        assert condition
    except AssertionError:
        sys.exit(err_message)

Should be what you want, if you absolutely want to call sys.exit() and use assert; However this

assert condition

will stop your script just as fine and provide a stacktrace, which saves you the trouble of entering a custom error message each time you call assert_exit, and points you directly to the offending party.

edited May 24, 2020 at 17:15

answered Aug 8, 2017 at 7:40

deepbrook

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