Python 2 namespace change? (was Re: [Python-Dev] Changing existing class instances)

[Guido van Rossum]

[me]
> > But I don't think that changing the "from M import v" semantics so
> > that local assignment to v changes the binding of M.v as well is
> > defensible.
> 
> I agree, however, I think that having:
> 
>   from M import v
> 
> causing a name binding that is broken by local
> assigment to v *is* defensible and reasonably
> implementable.
> 
> Changes to 'v' in M (including by reload of M) would be 
> reflected locally unless someone did:
> 
>   v=something
> 
> locally. Local assignment would negate an import, as it
> does now.

Hm, but it still wouldn't have the same semantics as currently, and
that's still a monster hiding under the bed until you're nearly
asleep.  Consider this example:

# in M:
verbose = 1

# in __main__:
from M import verbose

# somewhere else:
M.verbose = 0

Under the current semantics, that would have no effect on verbose in
__main__; but with your semantics it would.  I think that is very hard
to explain; even more so if you say that assigning a different value
to __main__.verbose does not change M.verbose and furthermore breaks
the connection.  This means that if I add

verbose = verbose

to the __main__ code the semantics are different!

I don't understand why you wanted these semantics in the first place.

--Guido van Rossum (home page: http://www.python.org/~guido/)