On 9/27/07, Fernando Perez wrote: > ... > In [12]: a = N.arange(10) > > In [13]: sys.getrefcount(a) > Out[13]: 2 > > Here the refcount is 2, as expected: the original array and the name > 'a' pointing to it. > > Now, a naked, unassigned slice of the original array has a refcount of 1: > > In [14]: sys.getrefcount(a[::2]) > Out[14]: 1 >
Whoa! This looks wrong to me. The slice of some larger object (which in turn has a reference to it) should not have a reference count of 1. (-: Ok, maybe it should be 1.00001 but not 1! ;-) The mutable larger object in which it is contained certainly *references* parts of itself. No? > And yet, modifying it in-place has an effect on the original object > (which we have called 'a'): > > In [15]: a[::2].fill(999) > > In [16]: a > Out[16]: array([999, 1, 999, 3, 999, 5, 999, 7, 999, 9]) > Uggh! Awful. :-( > > The slice 'a[::2]' has a refcount of 1, so it appears 'safe' from a > purely refcounting argument. But it actually shares memory with the > original N.arange(10) object, so it can overwrite it. > As far as I am concerned "sharing memory" is equivalent to a reference. > Note that I'm only pointing this out as a word of caution that there > are cases where refcounting arguments can be a bit delicate. The > basic idea of reusing in-place objects as an optimization is a very > good one for cases where you know that your underlying objects simply > don't expose any interface for antics like the above. > I think your points are well taken. There certainly seems to be some confusion and a lot written about this subject on the web. For example from the Python docs: http://docs.python.org/ext/refcountsInPython.html ----- Nobody ``owns'' an object; however, you can own a reference to an object. An object's reference count is now defined as the number of owned references to it. The owner of a reference is responsible for calling Py_DECREF() when the reference is no longer needed. Ownership of a reference can be transferred. ------- And it defines the concept of a "borrowed" reference, etc., etc. Regards, Bill Page. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-devel@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sage-devel URLs: http://sage.scipy.org/sage/ and http://modular.math.washington.edu/sage/ -~----------~----~----~----~------~----~------~--~---
