[Python-Dev] recursive closures - reference leak
Kristján Valur Jónsson
kristjan at ccpgames.com
Tue Dec 8 13:45:52 CET 2009
Consider this code:
Ok, this is a bit contrived, but there are other valid reasons for calling a closure recursively.
The problem with this is that once you have called factorial() once, you end up with a recursive cycle. "factorial" has become a cell object, referencing the "helper" function, which again refers to the outer cell object. This requires "gc" to clean up. Also, it is entirely non-obvious. the problem becomes worse if the inner function also refers to some large, temporary variable, since it will get caught up in the reference loop.
A workaround is this. Replace the final line with:
helper = None
This clears the cell before function exit. Note that it is not possible to "del" it, you get a syntax error for that.
Now, the reason I'm posting this here, and not to python-lang, is this: Is there a way to implement this differently so that this pattern doesn't result in a cycle?
Possible ideas would be:
1) clear "helper" on function exit. Not good if the internal function is exported, though.
2) Use have 'helper' store a weakref to the cell object
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