On 10/29/2012 06:53 AM, Chris Angelico wrote:
Can you provide links to these notes? I'm looking at
cpython/Include/sliceobject.h that has this comment:
/*
A slice object containing start, stop, and step data members (the
names are from range). After much talk with Guido, it was decided to
let these be any arbitrary python type. Py_None stands for omitted values.
*/
Also, the code for slice objects in CPython works with Py_ssize_t (a
signed quantity of the same length as size_t), which will allow at
least 2**31 for an index. I would guess that your crashes were nothing
to do with 20 million elements and slices.
ChrisA
Let's look at the source code rather than the web notes -- the source
must be the true answer anyhow.
I downloaded the source code for python 3.3.0, as the tbz;
In the directory "Python-3.3.0/Python", look at Python-ast.c, line 2089
& ff.
Clearly a slice is malloced for a slice_ty type.
It has four elements: kind, lower, upper, and step.
So, tracing it back to the struct definition...
"Include/Python-ast.h" has "typedef struct _slice *slice_ty;"
And, here's the answer!:
enum _slice_kind {Slice_kind=1, ExtSlice_kind=2, Index_kind=3};
struct _slice {
enum _slice_kind kind;
union {
struct {
expr_ty lower;
expr_ty upper;
expr_ty step;
} Slice;
struct {
asdl_seq *dims;
} ExtSlice;
struct {
expr_ty value;
} Index;
} v;
};
So, slice() does indeed have arbitrary python types included in it;
contrary to what I read elsewhere.
expr_ty is a pointer to an arbitrary expression, so the actual structure
is 4 pointers, at 32 bits each = 16 bytes.
The size of the structure itself, given in an earlier post, is 20 bytes
-- which means one more pointer is involved, perhaps the one pointing to
the slice structure itself.
Hmm...!
An empty tuple gives sys.getsizeof( () ) = 24.
But, I would expect a tuple to be merely a list of object pointers;
hence I would expect 4 bytes for len(), and then a head pointer 4 bytes,
and then a pointer for each object.
3 objects gives 12 bytes, + 8 = 16 bytes.
Then we need one more pointer so Python knows where the struct is...
So a Tuple of 3 objects ought to fit nicely into 20 bytes; the same size
as slice() --
but it's 24, even when empty...
And 36 when initialized...
What are the extra 16 bytes for?
All I see is:
typedef struct { object** whatever } PyTupleObject;
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