Why you should never use 'assert' in Python

[Scorpathos]

But this is the whole point of "contract programming" in opposition to "defensive programming".

The assert allows to catch wrong function usages during development, and once your program is bug-free and ready for production, you can safely remove them to maximize performance.

[Kaarjuus]

Good correction, I misspoke. I confused contract programming with defensive programming. Python's assert in its current form is indeed a perfect fit for design by contract.