The wrap() method of the TextWrapper class (and the module-level wrap method) throws an unhelpful TypeError upon getting a bytes object.

It's not textwrap that's doing it, which is why the error is so unhelpful; the input is assumed to be a str, and the translate method is called on it with a dict argument, which is valid for str.translate, but not for bytes.translate.

You'll get other "unhelpful" error messages for other arguments (e.g. most other built-in types die because they lack an expandtabs method). Is it necessary to provide specific error messages when an API is given a type it never claimed to support? I could see issues with a "check for str" check if someone is implementing their own str-like type that matches the API but gets rejected for not being str.

You're most likely right, so I'll close it. The lesson is most likely to not use weird arguments, but modules such as datetime warn against it, so I was surprised.