RetroSearch Browse

Tue May 25 12:05:48 EDT 2004 · https://mail.python.org/pipermail/python-dev/2004-May/044957.html

[Gustavo Niemeyer]
> I belive that either my question was misinterpreted, or I'm
> missing something.

Probably both, but who cares <wink>.

> I'm not suggesting to limit 'a==a' to return True/False only. I'm
> suggesting to introduce a shortcut in the *list* comparison (more
> specifically, in list_richcompare), since this is what is
> currently being done, but with a more expensive logic.

What does "currently" mean to you?  2.3.3 and HEAD differ here.

> Going through the function, in a case where v is w and op is
> either Py_EQ or Py_NE, we have

These cases aren't interesting on HEAD.  Nobody calls list_richcompare
directly.  If something calls PyObject_RichCompareBool() with Py_EQ or
Py_NE, then on HEAD the result is returned instantly, because of this code
at the start of PyObject_RichCompareBool():

	/* Quick result when objects are the same.
	   Guarantees that identity implies equality. */
	if (v == w) {
		if (op == Py_EQ)
			return 1;
		else if (op == Py_NE)
			return 0;
	}

That code doesn't care whether v and w are lists or ints or Unicode strings
or floats (etc).  2.3.3 doesn't have this code.

> 1) Both objects are lists, go on.

On HEAD, we don't get into list_richcompare.

> 2) Both objects have the same size, go on.
> 3) Searching for the first index where items are different, using
>    PyObject_RichCompareBool(), won't find anything. Go on.

If v and w were created via, e.g.,

    v = []
    v.append(v)
    w = v

then step #3 is a lot more complicated in 2.3.3 (because we recurse into
list_richcompare repeatedly then, and a bunch of gone-on-HEAD machinery
tries to notice that and guess the right result).

> 4) No more items to compare. Check sizes.
> 5) Sizes match. Check comparison operator.
> 6) Return either Py_True or Py_False.
>
> This whole logic might be replaced by simply:
>
>     if (v == w && (op == Py_EQ || op == Py_NE)) {
>         res = (op == Py_EQ ? Py_True : Py_False);
>         Py_INCREF(res);
>         return res;
>     }

On HEAD, it has been replaced, but for all object types with one blow.

Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://mail.python.org/pipermail/python-dev/2004-May/044957.html below:

[Python-Dev] cmp(x,x)