[Python-Dev] Fix import errors to have data (original) (raw)

Christian Tismer tismer at stackless.com
Mon Aug 2 11:01:26 CEST 2004

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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