So I do not have extensive experience with mypy but I don't see how it would help.  The entire issue that is that `str` is an instance of  `Iterable[str]` so how is mypy going to catch my error of passing a single string instead of an iterable of strings to a function?

... 

I like the idea of an `AtomicString` but it doesn't really help unless IO and string literals return `AtomicString` instead of `str` (which is just as breaking as changing `str`).

I agree that noisy is broken and that a warning that never becomes an error is probably a bad idea.

While I am firmly in the camp that this is a problem I am not sure if its a problem that should be fixed.  Any solution will break code, there is no way around it. However, I think one possible solution (that has it own set of draw backs) would be to simply not register `str` as subclass of `Iterable`.  This would allow for the following pattern:

```

if isinstance(obj, Iterable):

  # iterable case

else:

  # scalar case
```

Where currently the following is required:

```

if isinstance(obj, Iterable) and not isinstance(obj, (str, bytes)):

  # iterable case

else:

  # scalar case
```

Yes this would break code, but it would break a lot less code than actually changing the behavior of `str`.