What are the python builtin __exit__ argument types?

exc_type is the exception’s class. exc_val is the exception instance. exc_tb is a traceback object, of which there is a reference in types.TracebackType.

In general it should be the case that

• type(exc_val) is exc_type
• exc_val.__traceback__ is exc_tb

Note that __exit__ is still invoked when there was no exception raised by the code under a context manager, and the args will be (None, None, None) so all three arguments should be annotated optional.

Then a correct annotation for it should look something like this:

def __exit__(self, exctype: Optional[Type[BaseException]],
             excinst: Optional[BaseException],
             exctb: Optional[TracebackType]) -> bool: ...

You might wonder why this API has three arguments when two of them can be trivially determined from the exception instance itself. But it wasn’t always that way, in older versions of Python you could raise strings as exceptions, and the exception’s __traceback__ attribute wasn’t there until Python 2.5. And you can still raise old-style classes as exceptions in Python 2.7 (!)