I think the documentation is fairly clear. The -b option only checks *comparisons*, not any other operations. And only equality and inequality comparisons at that. https://docs.python.org/3/using/cmdline.html#cmdoption-b Although the documentation doesn't mentioned it, -b also checks for calling str() on a bytes object. Other operations always fail: * Order comparisons (less than, greater than) between string and bytes are always an error in Python3 * Likewise for concatenation b'' + '' * And other string methods, e.g. 'hello'.index(b'e') raises. Most operations on mixed str/bytes already fail. The only troublesome cases are the calls to str() and == and != comparisons, and they are checked by -b. What else is there to check? Not checked: you can mix bytes and strings in the `and` and `or` operators, same as you can mix any combination of types: 'hello' or b'goodbye' or [2.5, 3.6] # Legal. Also not checked: identity comparison with `is`. A bytes object and a string object will never be identical. Neither of those should be checked. As for what that means with regard to your application, that depends on how extensive your testing is. -b only checks comparisons which are actually reached at runtime, not comparisons that aren't reached: [steve ~]$ python3 -b -c "print(('a' == 'a') if True else ('a' == b'a'))" True -- Steve