Why does the go Ethereum code reject odd number of digit hex numbers?

Question

This is a question specific to the go-ethereum implementation. It may be present in other implementations, but I haven't looked yet.

In go-ethereum/common/bytes.go, the function isHex returns false if an input has an odd number of digits, and I was wondering why:

func isHex(str string) bool {
    if len(str)%2 != 0 {
        return false
    }
    for _, c := range []byte(str) {
        if !isHexCharacter(c) {
            return false
        }
    }
    return true
}

Answer 1

In the context of Ethereum, hex is a set of bytes encoded in base 16. Since a byte is actually 256 values, it takes 2 hex characters to make up a byte. So if there are only 3 hex characters, you wouldn't know which pairs of characters made up the individual bytes.