[Python-Dev] Fix import errors to have data

[Christian Tismer]

Greg Ewing wrote:
>>>Circular imports, to be made consistent, would need
>>>something like a final "commit" after all imports succeeded.
>>
>>Silly idea.  Typically, circular imports aren't planned, they just
>>happen, and (usually) just happen to work.
> 
> 
> I thought Christian meant this was something that would
> be done automatically, not something the user would have
> to do explicitly.
> 
> Maybe it could be as simple as saving a snapshot of
> sys.modules whenever importing of a module is begun,
> and if execution of its body doesn't complete, restoring
> the snapshot?

This is exactly what I thought of.
It seems to be correct, also with nested imports,
since this is a stack-like structure of undoable things.

Example

__main__:
     import A            #1
     #6

A:
     try:
         import B        #2
         #5
     except (ImportError, maybe others):
         # provide surrogate for B

     # other action that fails

B:
     import A            #3
     import C            #4
     # do some stuff that fails

#1: snapshot of sys.modules, A added   [__main__, A]
#2: snapshot of sys.modules, B added   [__main__, A, B]
#3: snapshot of sys.modules, A exists  [__main__, A, B]
#3: snapshot of sys.modules, C added   [__main__, A, B, C]

after failing imports:

#5: restore [__main__, A]
#5: restore [__main__]

So one side effect is that completely valid imports in the
context of a failing import are undone. I guess this is
intended.

A problem are side effects, of course.
It would be nice to have a cleanup construct for
completely or partially imported modules, to undo
any side effects.
How do we remove extension modules which have been loaded
as a side effect?

ciao - chris
-- 
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