On 10/29/2012 06:49 PM, Chris Kaynor wrote:
Every Python object requires two pieces of data, both of which are
pointer-sized (one is a pointer, one is an int the size of a pointer).
These are: a pointer to the object's type, and the object's reference
count. A tuple actually does not need a head pointer: the head pointer
is merely an offset from the tuple's pointer. It merely has a ref
count, type, an item count, and pointers to its contents. A slice has
the same type pointer and reference count, then three pointers to the
start, stop, and step objects. This means a slice object should be the
same size as a two-item tuple: the tuple needs a count, while that is
fixed at 3 for a slice (though some items may be unset). NOTE: The
above is taken from reading the source code for Python 2.6. For some
odd reason, I am getting that an empty tuple consists of 6
pointer-sized objects (48 bytes on x64), rather than the expected 3
pointer-sized (24 bytes on x64). Slices are showing up as the expected
5 pointer-sized (40 bytes on x64), and tuples grow at the expected 1
pointer (8 bytes on x64) per item. I imagine I am missing something,
but cannot figure out what that would be.
All I see is:
typedef struct { object** whatever } PyTupleObject;
It's fairly straight forward in 3.2.0. I debugged the code with GDB and
watched.
Perhaps it is the same in 2.6 ?
In addition to those items you mention, of which the reference count is
not even *inside* the struct -- there is additional debugging
information not mentioned. Built in objects contain a "line number", a
"column number", and a "context" pointer. These each require a full
word of storage.
Also, built in types appear to have a "kind" field which indicates the
object "type" but is not a pointer. That suggests two "object" type
indicators, a generic pointer (probably pointing to "builtin"? somewhere
outside the struct) and a specific one (an enum) inside the "C" struct.
Inside the tuple struct, I count 4 undocumented words of information.
Over all, there is a length, the list of pointers, a "kind", "line",
"col" and "context"; making 6 pieces in total.
Although your comment says the head pointer is not required; I found in
3.3.0 that it is a true head pointer; The Tuple() function on line 2069
of Python-ast.c, (3.3 version) -- is passed in a pointer called *elts.
That pointer is copied into the Tuple struct.
How ironic, slices don't have debugging info, that's the main reason
they are smaller.
When I do slice(3,0,2), suprisingly "Slice()" is NOT called.
But when I do a[1:2:3] it *IS* called.
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