[issue13667] __contains__ method behavior
New submission from João Bernardo jbv...@gmail.com: Hi, I'm working on a class which implements the __contains__ method but the way I would like it to work is by generating an object that will be evaluated later. It'll return a custom object instead of True/False class C: def __contains__(self, x): return I will evaluate this thing later... Don't bother now but when I do: 1 in C() True It seems to evaluate the answer with bool! Reading the docs (http://docs.python.org/py3k/reference/expressions.html#membership-test-details) It says: `x in y` is true if and only if `y.__contains__(x)` is true. It looks like the docs doesn't match the code and the code is trying to mimic the behavior of lists/tuples where x in y is the same as any(x is e or x == e for e in y) and always yield True or False. There is a reason why it is that way? Thanks! -- assignee: docs@python components: Documentation, Interpreter Core messages: 150283 nosy: JBernardo, docs@python priority: normal severity: normal status: open title: __contains__ method behavior type: behavior versions: Python 3.2, Python 3.3 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Georg Brandl ge...@python.org added the comment: an object is true is a short way of saying bool(obj) is True. So the docs match the behavior. Returning the actual object instead of True/False from the in operator is a feature request. -- assignee: docs@python - nosy: +georg.brandl type: behavior - enhancement versions: -Python 3.2 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
João Bernardo jbv...@gmail.com added the comment: @Georg Brandl Oh sorry, now I see... true != True But still, why is that the default behavior? Shouldn't it use whatever the method returns? -- type: enhancement - behavior versions: +Python 3.2 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Changes by João Bernardo jbv...@gmail.com: -- type: behavior - enhancement ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Georg Brandl ge...@python.org added the comment: Well, usually what you want *is* a boolean indicating whether the element is in the collection or not. Being able to overload in, mostly for metaprogramming purposes, is a request that probably nobody thought of when implementing in. -- nosy: +benjamin.peterson, pitrou -docs@python versions: -Python 3.2 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Benjamin Peterson benja...@python.org added the comment: I think the idea has some merit. I think it should be well vetted on python-ideas, though. One thing that will certianly weigh against it is that implementation would not be trivial. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
João Bernardo jbv...@gmail.com added the comment: I see that every other comparison operator (, , =, =, ==, !=) except for `is` work the way I expect and is able to return anything. e.g. numpy.arange(5) 3 array([ True, True, True, False, False], dtype=bool) I didn't checked the code (and probably I'm talking nonsense), but seems like the `in` operator has an extra call to `PyObject_IsTrue` that maybe could be dropped? Of course it can break code relying on `x in y` being True/False but it would only happen on customized classes. Another option that won't break code is to add a different method to handle these cases. Something like __contains_non_bool__, but that'd be a big api change. -- components: -Documentation ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Benjamin Peterson benja...@python.org added the comment: 2011/12/28 João Bernardo rep...@bugs.python.org: João Bernardo jbv...@gmail.com added the comment: I see that every other comparison operator (, , =, =, ==, !=) except for `is` work the way I expect and is able to return anything. e.g. numpy.arange(5) 3 array([ True, True, True, False, False], dtype=bool) I didn't checked the code (and probably I'm talking nonsense), but seems like the `in` operator has an extra call to `PyObject_IsTrue` that maybe could be dropped? I'm not sure what you're referring to, but I doubt that would do the job. Of course it can break code relying on `x in y` being True/False but it would only happen on customized classes. Another option that won't break code is to add a different method to handle these cases. Something like __contains_non_bool__, but that'd be a big api change. And completely hideous. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
João Bernardo jbv...@gmail.com added the comment:
Using my poor grep abilities I found that on Objects/typeobject.c
(I replaced some declarations/error checking from the code with ...)
static int
slot_sq_contains(PyObject *self, PyObject *value) {
...
func = lookup_maybe(self, __contains__, contains_str);
if (func != NULL) {
...
res = PyObject_Call(func, args, NULL);
...
if (res != NULL) {
result = PyObject_IsTrue(res);
Py_DECREF(res);
}
}
else if (! PyErr_Occurred()) {
/* Possible results: -1 and 1 */
result = (int)_PySequence_IterSearch(self, value,
PY_ITERSEARCH_CONTAINS);
}
}
I don't know if I'm in the right place, but the function returns `int` and
evaluates the result to 1 or 0 if __contains__ is found.
I also don't know what SQSLOT means, but unlike the other operators (which are
defined as TPSLOT), `slot_sq_contains` is a function returning int while
`slot_tp_richcompare` returns PyObject *.
Why is that defined that way?
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[issue13667] __contains__ method behavior
Georg Brandl ge...@python.org added the comment: It's defined that way because it's a slot returning a bool, so it doesn't need to return anything except for 0 or 1. Changing this to return a PyObject would mean that every extension module (i.e. module written in C) that defines a custom __contains__ would need to be adapted. That is the non-trivial implementation that Benjamin was talking about. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Raymond Hettinger raymond.hettin...@gmail.com added the comment: -1 on this proposal. It has everyone paying a price for a questionable feature that would benefit very few. -- nosy: +rhettinger ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Alex Gaynor alex.gay...@gmail.com added the comment: For what it's worth I proposed this on -ideas a while ago, the sticking points were what does `not in` do (no one had an answer anyone was happy with for this), and do we need a way to override it from the other perspective (e.g. if I want to do `SpecialObj() in [1, 2, 3]`, is there a way to do that?). -- nosy: +alex ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
João Bernardo jbv...@gmail.com added the comment: The problem with `not in` is because it must evaluate the result. It's not just another operator like == and !=. Looks like we're suffering from premature optimization and now it would break a lot of code to make it good. For my application, I created a different method to generate the object (Not as good as I wanted, but there's no option right now): `MyClass().has(1)` instead of `1 in MyClass()` So, if no one comes up with a better idea, this issue should be closed. Thanks -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue13667] __contains__ method behavior
Changes by Benjamin Peterson benja...@python.org: -- resolution: - rejected status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue13667 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
