Question

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?

Answer 1

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.)