The only reasonable way to accurately represent "anything that exposes a buffer memoryview can read" as a type check is to write an appropriately duck-typed ABC. You can't enumerate all the types that the binary codecs accept as input, because that list of types isn't finite (unlike the output types, which are far more tightly constrained).

I'd also be fine with Serhiy's suggestion of a private "non Unicode codec" set that is maintained by hand and checked *before* the codec operations in the codec methods - that then just becomes an internal implementation detail to improve the efficiency of the output type checks where we have the additional info needed to save the interpreter some work.