[Numpy-discussion] PyInt and Numpy's int64 conversion

Travis Oliphant travis at continuum.io
Fri Jan 6 16:31:45 EST 2012

>>> No. All of the PyTypeObject objects for the NumPy array scalars are
>>> explicitly part of the NumPy C API so you have no choice but to depend
>>> on that (to get the best performance). If you want to ONLY check for
>>> int64 at the C API level, I did a bit of digging and the relevant type
>>> definitions are in
>>> https://github.com/numpy/numpy/blob/master/numpy/core/include/numpy/npy_common.h
>>> so you'll want to do:
>>> int is_int64(PyObject* obj){
>>> return PyObject_TypeCheck(obj, &PyInt64ArrType_Type);
>>> }
>>> and that will *only* detect np.int64
>>> - Wes
>> Ok many thanks !
>> One last thing, do you happen to know how to actually convert an np int64 to
>> a C int ?
>> - x.
> Not sure off-hand. You'll have to look at the NumPy scalar API in the C code

What is it you want to do?   Do you want to get the C int out of the np.int64 *Python* object?  

If so, you do: 

npy_int64 val
PyArray_ScalarAsCtype(obj, &val);

If you want to get the C int of a *different* type out of the scalar Python object, you do: 

npy_int32 val
PyArray_Descr * outcode = PyArray_DescrFromType(NPY_INT32);
PyArray_CastScalarToCtype(obj, &val, outcode);


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