[C++-sig] checking passed arg is python type

[Neal Becker]

Jim Bosch wrote:

> On 03/14/2013 06:12 PM, Neal Becker wrote:
>> I'm expecting a numpy.random.RandomState object to be passed to me.  I want
>> to
>> check it is.  The best I could think of is below.  Anything more
>> boost::python- ish to do this task?
>>
>> static object rs_obj;
>>
>> template<typename out_type=uint32_t>
>> struct engine {
>>    typedef out_type result_type;
>>    bp::object rs;		// RandomState object
>>
>>    void verify (object const& o) {
>>      if (!PyObject_IsInstance(o.ptr(), rs_obj.ptr()))
>>        throw std::runtime_error ("engine: expected RandomState");
>>    }
>>
>>    engine (bp::object const& _rs) : rs (_rs) { verify (_rs); }
>> ...
>>
>> BOOST_PYTHON_MODULE...
>>
>>    object mod = object (handle<> ((PyImport_ImportModule("numpy.random"))));
>>    rs_obj = mod.attr("RandomState");
>>
> 
> I believe you can replace the penultimate line with:
> 
> bp::object mod = bp::import("numpy.random");
> 
> But I can't think of any ways to Boost.Python-ize the rest; there really
> should be an "isinstance" wrapper in Boost.Python, but there isn't.  You may
> also want to be a little careful with the return value of PyObject_IsInstance
> - I believe that it's psosible for it to raise an exception instead of simply
> returning 0, and in that case you'd want to use bp::throw_error_already_set()
> to propagate the exception to Python.
> 
> 
> Jim

Thanks, that seems to work, and looks ever so much prettier.