[Python-Dev] Fix import errors to have data

[Christian Tismer]

Guido van Rossum wrote:

>>It's going to be difficult to ensure that nothing ever gets a
>>reference to a broken module, because of circular imports.  Suppose A
>>imports B, which imports A. If A's initialisation subsequently fails,
>>then even if A is removed from sys.modules, B still contains a
>>reference to the broken A.

Well, circular imports first appeared to me like the reverse
problem of circular references. The latter was only solvable
by a garbage collector. Well, it's simpler after all.

Circular imports, to be made consistent, would need
something like a final "commit" after all imports succeeded.
I think of a simple, generator-like "undoable assignment"
to sys.modules, like ICON's "<-" operator. When a top-level
import starts, the module gets into sys.modules, but this
is also recorded as not finished. Further imports can happen,
until this is all done. Then, the pending undos are cancelled,
this is the "commit".
If any uncaught exception occours, the latent undo is executed
for all imports started in this context, and we end up with
an exception and (hopefully) almost no side effects.
Maybe this could be user-extensible, as on-failure-undo actions.

> Yeah, but I don't mind; my patch is still a big improvement in other
> cases and doesn't really make things any worse in the above case (B
> has a reference to a broken A, just like without my patch).

It is definately an improvement, much more at what people
would expect, anyway.

> Did anybody look at my patch?  Shall I check it in?

Yes I did.

+1

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