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This patch adds a function named gc.get_stats() which returns a list of dictionaries containing per-generation statistics:
>>> import pprint, gc
>>> pprint.pprint(gc.get_stats())
[{'collected': 0, 'collections': 12, 'uncollectable': 0},
 {'collected': 0, 'collections': 1, 'uncollectable': 0},
 {'collected': 0, 'collections': 0, 'uncollectable': 0}]

What are the possible performance implications of the statistics?

You mean negative implications? None :-)

Why dictionaries and not struct sequences?

Oh, I understand. GC callbacks receive a dict.
For avoid confusion with gc.DEBUG_STATS and to conform with callbacks argument name, may be better to name this function as gc.get_infos([generation])?

I.e. gc.get_info([generation]). Returns the info for specified generation or total if the parameter omitted.

> Why dictionaries and not struct sequences?
Because it's much simpler like that.
> For avoid confusion with gc.DEBUG_STATS and to conform with callbacks
> argument name, may be better to name this function as
> gc.get_infos([generation])?
Well, this is really about statistics, not general information.
Furthermore, it is not related to the callbacks functionality.
As for the generation parameter, I don't think it's useful: the result
list is only 3 elements long.

New changeset 43d87cdf9457 by Antoine Pitrou in branch 'default':
Issue #16351: New function gc.get_stats() returns per-generation collection statistics.
http://hg.python.org/cpython/rev/43d87cdf9457 

Now committed.

> Because it's much simpler like that.
Well, I wrote a patch with structure sequences, it is really much more expansive.
I have some comments.
1. You can allocate list of NUM_GENERATIONS elements and then use PyList_SET_ITEM(result, i, stat). This is 4 lines shorter.
2. And may be return the tuple? get_count() and get_threshold() return tuples.
3. You forgot to add get_stats() to the module docstring.

New changeset 17bd04fbf3d3 by R David Murray in branch 'default':
whatsnew for gc.get_stats, plus doc tweaks.
http://hg.python.org/cpython/rev/17bd04fbf3d3