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In the below snippet, SubError will propagate with __context__ set to None, instead of MainError. Maybe this isn't a bug?
 class MainError(Exception): pass
 class SubError(Exception): pass
 def main():
 try:
 raise MainError()
 except MainError:
 yield
 coro = main()
 coro.send(None)
 coro.throw(SubError())

This is due to the fact that Python 3 added the ability to define only __eq__ and get a free __ne__ defined. If my memory serves me right, functools.total_ordering was added in 3.2 and then backported to 2.x - where the relationship with __eq__ and __ne__ is not present. total_ordering doesn't do anything to touch __ne__ as it expects Python itself to do so (which it doesn't in 2.x).

Oops, that was *completely* the wrong issue. I apologize for the noise.

I now don't think this is a bug.
In the above example, SubError is instantiated outside of `main` generator. It's also thrown *into* `main` from the *outside* scope. And __context__ should be set for exceptions that were originated by the code that was handling other exceptions.
Closing this issue with 'not a bug'.

I disagree with the stated reason for closing this, because in general, implicit context chaining doesn't care about where the exception was instantiated, only where it was raised. For example:
-----
err = ValueError()
try:
 raise KeyError
except Exception:
 raise err
-----
Prints:
-----
Traceback (most recent call last):
 File "/tmp/bar.py", line 3, in <module>
 raise KeyError
KeyError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/tmp/bar.py", line 5, in <module>
 raise err
ValueError
-----
I would expect 'gen.throw(OBJ)' to be equivalent to doing 'raise OBJ' inside the generator, and raise does set __context__.