I have tried several times (a couple of decades ago) to introduce DBC in Python, using one of the many available libraries (for an overview: see https://lab.abilian.com/Tech/Python/DbC%20in%20Python/ ) but, like you, wasn't convinced.
I believe your argument on the performance penalty is right, and as a corollary, this implies that contracts are mostly useful if associated with a formal proof system. Contracts in production are probably a bad idea, in most cases.
The usefulness (or lack thereof) of contracts was famously discussed in 1997 in this paper: https://www.irisa.fr/pampa/EPEE/Ariane5.html (with additional comments here: https://www.irisa.fr/pampa/EPEE/Ariane5-comments.html ).
I have tried several times (a couple of decades ago) to introduce DBC in Python, using one of the many available libraries (for an overview: see https://lab.abilian.com/Tech/Python/DbC%20in%20Python/ ) but, like you, wasn't convinced.
I believe your argument on the performance penalty is right, and as a corollary, this implies that contracts are mostly useful if associated with a formal proof system. Contracts in production are probably a bad idea, in most cases.