On 7/19/22 16:43, Dan Stromberg wrote:

When porting code from python2 -> python3, one of the biggest hurdles is getting all the things that used to be just str, converted to sometimes-str-sometimes-bytes.

python3 -b warns about comparisons and likely-unsafe conversions.

If -b is no longer alerting about these, what does that really mean?

Another way to phrase the question:

What kind of str/bytes problems does -b not warn about?

-- ~Ethan~

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