[Python-Dev] Re: Sets: elt in dict, lst.include

[Ka-Ping Yee]

On Wed, 31 Jan 2001, Skip Montanaro wrote:
> Ping> x is sequence-like if it provides __getitem__() but not keys()
> 
> So why does this barf?
> 
>     >>> [].__getitem__

I was describing how to tell if instances are sequence-like.  Before
we get to make that judgement, first we have to look at the C method
table.  So:

    x is sequence-like if it has tp_as_sequence;
        all instances have tp_as_sequence;
            an instance is sequence-like if it has __getitem__() but not keys()

    x is mapping-like if it has tp_as_mapping;
        all instances have tp_as_mapping;
            an instance is mapping-like if it has both __getitem__() and keys()

The "in" operator is implemented this way.

    x customizes "in" if it has sq_contains;
        all instances have sq_contains;
            an instance customizes "in" if it has __contains__()

If sq_contains is missing, or if an instance has no __contains__ method,
we supply the default behaviour by comparing the operand to each member
of x in turn.  This default behaviour is implemented twice: once in
PyObject_Contains, and once in instance_contains.

So i proposed this same structure for sq_iter and __iter__.

    x customizes "for ... in x" if it has sq_iter;
        all instances have sq_iter;
            an instance customizes "in" if it has __iter__()

If sq_iter is missing, or if an instance has no __iter__ method,
we supply the default behaviour by calling PyObject_GetItem on x
and incrementing the index until IndexError.


-- ?!ng

"The only `intuitive' interface is the nipple.  After that, it's all learned."
    -- Bruce Ediger, on user interfaces