[C++-sig] Re: accessing the python type system

[David Abrahams]

Stefan Seefeld <seefeld at sympatico.ca> writes:

> David Abrahams wrote:
>> Stefan Seefeld <stefan.seefeld at orthosoft.ca> writes:
>> 
>>>hi there,
>>>
>>>are there any functions/operators in boost.python to
>>>let me access the python type system from within C++ ?
>>>
>>>For example holding two python objects I'd like to test
>>>whether one's type is derived from the other's type,
>>>whether they are derived from a given type, i.e. things
>>>like 'type', 'isinstance', etc.
>> These are more functions in the same category as len().  See my reply
>> to Philippe Fremy today on this list.
>
> What about a 'type object' similar to 'type_info', i.e. with
> a 'name()' accessor and comparison operators ?

Well, if you have isinstance you don't need that.

In principle, there's no reason we shouldn't have something like you
propose, only it should be a wrapper around Python's 'type' just the
same way that boost::python::dict is a wrapper around Python's
'dict'...

...but once you start go go down that road you get in trouble.  Python
makes type(x) return a reference to x.__class__ (unless x is a
"classic" instance, in which case it returns "instance").  Because
type(type(x)) also has to work (get x's metatype), it can't have a
regular copy constructor.  When copies of an object are not equivalent
to the original in C++ it plays havoc.

I guess 'type' would have to be an instance of a class derived from
boost::python::object, whose unary function call operator returns its
argument's type.  I wouldn't want a 'name()' function because Python
type objects don't have that:

    >>> class X(object): pass
    ...
    >>> X.name()
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    AttributeError: type object 'X' has no attribute 'name'
    >>> X.__name__
    'X'

The equivalent in C++ would be X.attr("__name__")

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com