Le jeudi 27 Janvier 2005 21:29, Steven Bethard a écrit :
So None being smaller than anything (except itself) is hard-coded into Python's compare routine. My suspicion is that even if/when objects of different types are no longer comparable by default (as has been suggested for Python 3.0), None will still compare as smaller than anything...
Well, here's python doesn't seem to confirm what you're saying :
a = "10" b = 10 a > b
True
b > a
False
id(a)
1077467584
id(b)
134536516
Actually, this code doesn't contradict what I said at all. I said that None (and only None) was special-cased.
It really looks like the addresses are compared when objects are of different types if there is no __cmp__ or __lt__ user made specification to compare objects of different types.
Use the source Luke! ;) Download it from CVS and take a look. The end of default_3way_compare:
static int
default_3way_compare(PyObject *v, PyObject *w)
{
...
/* None is smaller than anything */
if (v == Py_None)
return -1;
if (w == Py_None)
return 1; /* different type: compare type names; numbers are smaller */
if (PyNumber_Check(v))
vname = "";
else
vname = v->ob_type->tp_name;
if (PyNumber_Check(w))
wname = "";
else
wname = w->ob_type->tp_name;
c = strcmp(vname, wname);
if (c < 0)
return -1;
if (c > 0)
return 1;
/* Same type name, or (more likely) incomparable numeric types */
return ((Py_uintptr_t)(v->ob_type) < (
Py_uintptr_t)(w->ob_type)) ? -1 : 1;
}So it looks like Python uses the type name. Testing this:
py> A = type('A', (object,), {})
py> Z = type('Z', (object,), {})
py> lst = [str('a'), dict(b=2), tuple(), Z(), A()]
py> [type(o).__name__ for o in lst]
['str', 'dict', 'tuple', 'Z', 'A']
py> sorted(type(o).__name__ for o in lst)
['A', 'Z', 'dict', 'str', 'tuple']
py> sorted(lst)
[<__main__.A object at 0x011E29D0>, <__main__.Z object at 0x011E29F0>, {'b': 2}, 'a', ()]
Yup. Looks about right. (Note that the source code also special-cases numbers...)
So, the order is consistent, but arbitrary, just as Fredrik Lundh pointed out in the documentation.
Steve -- http://mail.python.org/mailman/listinfo/python-list
