In some code we have the first 5-10 lines are basically in the shape of : assert isinstance(arg1, type) assert isinstance(arg2, SubObjectClass) And so on. Mainly because we deal in code that deals a lot with traditional types (as in signed 8 bit, Strange-Endian 32bit in 16bit words , and so on) which makes code turn nasty in pretty much any language.

Further down these are cast into more native types, and then walk out into public API's.

And there once more you need to do somewhat strict data-validation.

For these cases, typing rules and annotations will be a boon.

And do note, that there is more common a case of human error or refactoring error that causes problem. Someone sending in a default [] array in an object, and somewhere else it's an expected Boolean.