return eats exception

You asked about the Python developers' reasoning. I can't speak for them, but no other behavior makes sense. A function can either return a value, or it can raise an exception; it can't do both. The purpose of a "finally" clause is to provide cleanup code that is "guaranteed" to be run, regardless of exceptions. By putting a return statement in a finally clause, you have declared that you want to return a value, no matter what, regardless of exceptions. If Python behaved as you are asking and raised the exception, it would be breaking the contract of the "finally" clause (because it would fail to return the value you told it to return).

The exception disappears when you use return inside a finally clause. .. Is that documented anywhere?

It is:

If finally is present, it specifies a ‘cleanup’ handler. The try clause is executed, including any except and else clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The finally clause is executed. If there is a saved exception, it is re-raised at the end of the finally clause. If the finally clause raises another exception or executes a return or break statement, the saved exception is lost.