When conventions are followed they do exactly that, actually. And unless the code is a complete trainwreck, it's pretty easy to tell what the return type is, even without explicit annotation in the docstring.
PEP 257, and local conventions in certain projects.
What all comes down to is: if you really have people on your project writing code like the foo/bar/baz example way up above - then you have problems way bigger than static type checks can possibly help you with.
Wouldn't it be something if there existed tooling that enforced this level of discipline and checked its validity before executing any code such that you didn't rely on the entire ecosystem to adhere to the same standards and remove that as a source of ambiguity...
When conventions are followed they do exactly that, actually. And unless the code is a complete trainwreck, it's pretty easy to tell what the return type is, even without explicit annotation in the docstring.