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The cElementTree Element currently returns an incorrect response to sys.getsizeof, as it doesn't account for the extra structure, and for the child pointers array. This patch corrects the computation.

Out of curiosity, why don't the constituents of Element (i.e. the tag, the attributes dict, etc.) count for its size?

__sizeof__ is supposed to account for just the object, not for the size of any object it refers to. If you want to compute total memory consumption, you should get a list of all objects, and then sum up __sizeof__. If some object would also include some of its referents, there would be a danger of accounting some memory twice.

Martin, thanks for the explanation. The patch LGTM, then.
Could it be useful to document this a bit more explicitly in the description of sys.getsizeof? The most intuitive thing to expect from it is to compute the *total* size including contained objects. So this is somewhat surprising:
>>> import sys
>>> sys.getsizeof([1, 2, 3])
96
>>> sys.getsizeof([1, 2, [10] * 500])
96
The last sentence in the doc of sys.getsizeof says: "See recursive sizeof recipe for an example of using getsizeof() recursively to find the size of containers and all their contents.", which can be taken as a hint, but maybe it could be just said straightforwardly. I.e before it, add: "Note that getsizeof returns just the memory occupied by the object itself, not any contained objects it holds references to".

Given how C's sizeof works, it may be debatable whether it's natural to expect that sys.getsizeof should be recursive. If you have a struct with pointers (say, char*) in C, and you do sizeof, the string length isn't considered, either.
But I'm certainly fine with making this more clear.

New changeset 093dec81ea1f by Martin v. Löwis in branch 'default':
Issue #14055: Add __sizeof__ support to _elementtree.
http://hg.python.org/cpython/rev/093dec81ea1f