[Python-Dev] misbehaving contains (original) (raw)

Chuck Mason (Visual Concepts) cmason at vcentertainment.com
Tue Jan 22 20:35:12 CET 2008

My guess is that results from the bottom of cmp_outcome (ceval.c),

The default case handles the PyCmp_EQ case via PyObject_RichCompare, which might not return Py_True or Py_False.

But 'in' is handled inside the switch and is subject to the final statements:

v = res ? Py_True : Py_False;
Py_INCREF(v);
return v

Though, IANAPydev.

C

-----Original Message----- From: python-dev-bounces+cmason=vcentertainment.com at python.org [mailto:python-dev-bounces+cmason=vcentertainment.com at python.org] On Behalf Of tomer filiba Sent: Tuesday, January 22, 2008 11:27 AM To: python-dev at python.org Subject: [Python-Dev] misbehaving contains

i'm using python to create expression objects, for more intuitive usage as predicates, for instance: x = (Arg(0) > 17) & (Arg(1).foo == "bar") instead of x = And(Gt(Arg(0), 17), Eq(GetAttr(Arg(1), "foo"), "bar"))

so now i can use x.eval(18, "spam") and get the result of the expression.

i'm doing it by overriding all the operators to return expression objects, instead of evaluating the result immediately. it works fine, but i encountered a problem with making contains behave so.

it turns out contains coerces the return value into a bool. this might seem logical at first, but is not consistent with the result of the language.

consider the following code:

class Foo(object): ... def contains(self, key): ... return 17 ... def eq(self, other): ... return 19 ...

f=Foo() f == 8 19 8 in f True

as you can see, eq does not coerces the result to bool, so why should contains do that?

i've looked at PySequence_Contains in objects/abstract.c, but i can't quite understand where the coercion is made. is because the function is typed as int? if so, i believe it should either be changed to PyObject, or have cmp_outcome (in ceval.c) not use this API directly, and rather return the result as it is returned from contains.

i see no reason it should behave differently than eq and the rest of comparison operators.

-tomer

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