The assert Statement

As a somewhat special case for debugging purposes, Python includes the assert statement. It is mostly just syntactic shorthand for a common raise usage pattern, and an assert can be thought of as a conditional raise statement. A statement of the form:

assert <test>, <data>          # The <data> part is optional

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works like the following code:

if __debug__:
    if not <test>:
        raise AssertionError(<data>)

In other words, if the test evaluates to false, Python raises an exception: the data item (if it’s provided) is used as the exception’s constructor argument. Like all exceptions, the AssertionError exception will kill your program if it’s not caught with a try, in which case the data item shows up as part of the error message.

As an added feature, assert statements may be removed from a compiled program’s byte code if the -O Python command-line flag is used, thereby optimizing the program. AssertionError is a built-in exception, and the __debug__ flag is a built-in name that is automatically set to True unless the -O flag is used. Use a command line like python –O main.py to run in optimized mode and disable asserts.