ThreadLocal state in PythonInterpreter

https://hg.python.org/jython/rev/c392bcd63a50 introduced a thread local 
to PythonInterpreter that holds the locals used when executing code. As 
far as I can tell, this is only engaged when creating a JSR-223 script 
engine, but then unconditionally affects the script engine.

I understand why I might want multple engines with their own namespace, 
but why would I want any single engine in which the local variables 
automagically change to a different dictionary according to which 
(platform) thread looks at them?

In the circumstances where I might want do this, why do we also suppress 
the creation of a __main__ module to represent the current namespace? 
(https://hg.python.org/jython/rev/c392bcd63a50#l3.107) This difference 
from a normal interpreter seems to be an annoyance for some users 
(https://bugs.jython.org/issue2846).

It all looks deliberate, and is quite an old change, but what on earth 
is the motivation?

Jeff

-- 
Jeff Allen