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I used to work for a company where some of the lead architect/developers had mandated on various projects that assertions were not to be used, and they would routinely be removed from code and replaced with exceptions.
I feel they are extremely important in writing correct code. Can anyone suggest how such a mandate could be justified? If so, what's wrong with assertions?
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No, there's nothing wrong with assert as long as you use it as intended. That is, it's supposed to be for catching cases that "can't happen", during debugging, as opposed to normal error handling. Assert: A failure in the program's logic itself.
While assertion is good for catching logic errors and reducing the chances of reaching more serious vulnerability conditions, it can still lead to a denial of service.
Do not use assertions to check the parameters of a public method. An assert is inappropriate because the method guarantees that it will always enforce the argument checks. It must check its arguments whether or not assertions are enabled. Further, the assert construct does not throw an exception of the specified type.
The assert Statement If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a traceback.
We use a modified version of assert, as per JaredPar's comment, that acts like a contract. This version is compiled into the release code so there is a small size overhead, but disabled unless a diagnostics switch is set, such that performance overhead is minimized. Our assert handler in this instance can be set to disabled, silent mode (e.g. log to file), or noisy mode (e.g. display on screen with abort / ignore, where abort throws an exception).
We used automated regression testing as part of our pre-release testing, and asserts are hugely important here as they allow us to find potential internal errors that cannot be picked up at a GUI level, and may not be initially fatal at a user level. With automation, we can run the tests both with and without diagnostics, with little overhead other than the execution time, so we can also determine if the asserts are having any other side effects.
One thing to be careful of with asserts is side effects. For example, you might see something like assert(MyDatabasesIsOk()), which inadvertently corrects errors in the database. This is a bug, as asserts should never change the state of the running application.
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