[bvrmn]

From typing perspective there is no sense to have Container[T].join() -> str for any T.

[still_grokking]

That's obviously the wrong signature.

The semantically correct one is "Container[T : Printable].join(): String"; T must implement the Printable concept, but Python lacks concepts (or type-classes as these are alternatively called).

But this all is irrelevant as this is anyway not the signature of `str.join()` in Python. It's `str.join(Iterable[str]) -> str` there. With sane design and syntax it would just become `Iterable[str].join(str)`.

[mystifyingpoi]

I agree, but from convenience perspective there is a lot of sense. Java Streams is actually a good example of a "correct" design, but not very convenient.

[bvrmn]

That's my point. Python has convenient and good type design with str.join ignored by other languages.

For example I'm lost which abstract class to inherit in Scala to obtain mkString for my custom container.