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I found that Python's assert statement is a good way to catch situations that should never happen. And it can be removed by Python optimization when the code is trusted to be correct.
It seems to be a perfect mechanism to run Python applications in debug mode. But looking at several Python projects like django, twisted and zope, the assert is almost never used. So, why does this happen?
Why are asserts statements not frequently used in the Python community?
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You can use an assert to check if your logical assumption is correct. You can also use assert statements to check if the control flow is correct or not. For example, if you have a function that returns a value, you may want to put an assert statement. However, you may get a 'non-reachable' code error.
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.
Python's assert statement is a debugging aid, not a mechanism for handling run-time errors. The goal of using assertions is to let developers find the likely root cause of a bug more quickly. An assertion error should never be raised unless there's a bug in your program.
Yes it is a very good practice to assert your assumptions. Read Design By Contract. assert can be used to verify pre conditions, invariants and post conditions during integration and testing phases. This helps to catch the errors while in development and testing phases.
I guess the main reason for assert not being used more often is that nobody uses Python's "optimized" mode.
Asserts are a great tool to detect programming mistakes, to guard yourself from unexpected situations, but all this error checking comes with a cost. In compiled languages such as C/C++, this does not really matter, since asserts are only enabled in debug builds, and completely removed from release builds.
In Python, on the other hand, there is no strict distinction between debug and release mode. The interpreter features an "optimization flag" (-O), but currently this does not actually optimize the byte code, but only removes asserts.
Therefore, most Python users just ignore the -O flag and run their scripts in "normal mode", which is kind of the debug mode since asserts are enabled and __debug__ is True, but is considered "production ready".
Maybe it would be wiser to switch the logic, i.e., "optimize" by default and only enable asserts in an explicit debug mode(*), but I guess this would confuse a lot of users and I doubt we will ever see such a change.
((*) This is for example how the Java VM does it, by featuring a -ea (enable assertions) switch.)
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