[Python-Dev] misbehaving __contains__

[Guido van Rossum]

The issue is that the sq_contains slot is defined as returning an int,
while the tp_richcompare slot is defined as returning a PyObject *.

Your first opportunity for changing this will be Py3k. Please submit a patch!

On Jan 22, 2008 11:26 AM, tomer filiba <tomerfiliba at gmail.com> wrote:
> 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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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)