[Python-Dev] cmp(x,x) (original) (raw)

Tim Peters tim.one at comcast.net
Tue May 25 12:05:48 EDT 2004

[Gustavo Niemeyer]

I belive that either my question was misinterpreted, or I'm missing something.

Probably both, but who cares .

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 listrichcompare), 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 PyEQ or PyNE, 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 PyObjectRichCompareBool(), 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 PyTrue or PyFalse.

This whole logic might be replaced by simply: if (v == w && (op == PyEQ || op == PyNE)) { res = (op == PyEQ ? PyTrue : PyFalse); PyINCREF(res); return res; }

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

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