Why assert is not largely used?

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?

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.)

Several reasons come to mind...

It is not a primary function

Many programmers, lets not get bogged down by the rationale, disrespect anything which is not a direct participant in the program's penultimate functionality. The assert statement is intended for debugging and testing, and so, a luxury they can ill-afford.

Unit Testing

The assert statement predates the rise and rise of unit-testing. Whilst the assert statement still has its uses, unit-testing is now widely used for constructing a hostile environment with which to bash the crap out of a subroutine and its system. Under these conditions assert statements start to feel like knives in a gunfight.

Improved industry respect for testing

The assert statement serves best as the last line of defence. It rose to lofty and untouchable heights under the C language, when that language ruled the world, as a great way to implement the new-fangled "defensive programming"; it recognises and traps catastrophic disasters in the moment they teeter on the brink. This was before the value of Testing became widely recognised and respected and disasters were substantially more common.

Today, it is unheard of, for any serious commercial software to be released without some form of testing. Testing is taken seriously and has evolved into a massive field. There are Testing professionals and Quality Assurance departments with big checklists and formal sign-offs. Under these conditions programmers tend not to bother with asserts because they have confidence that their code will be subjected to so much tiresome testing that the odds of wacky brink-of-disaster conditions are so remote as to be negligible. That's not to say they're right, but if the blame for lazy programming can be shifted to the QA department, hell why not?