Numpy's PyArray_New for dummies

README.md

When it comes to usign the Numpy C API to create arrays, there are plenty of resources to use the simpler functions, such as the PyArray_SimpleNewXXX() series. To use the most generic ones, you are supposed to be fairly well versed in Numpy. Unfortunately, there are functions of intermediate complexity, and resources for these are scarce.

Today, I had to create a Numpy array as a view (without copying data) of some fields in a vector of structs. Clearly, not a job for the PyArray_SimpleNewXXX() functions, but for PyArray_New(). Besides basic info such as the buffer, the type, and the strides, this function requires some arcane arguments, such as the PyTypeObject* subtype.

For the life of me, I could not figure out what to pass to the function as that argument. The Numpy sources were of no help. since they passed a pointer to a function (&PyArray_Type) as that argument (?!?!?!). After much experimentation and frustration, I found the answer to my predicament in this thread:

https://groups.google.com/forum/#!topic/cython-users/fFBiCCwblD0

Argh!!!! Just np.ndarray!!!!!! So simple, yet so cryptic! I am sure that the answer to this question was somehow in the Numpy docs, but WHY NOT PUT AN EXAMPLE CALL FOR EACH FUNCTION?

Anyway, let's see a snippet of code with the solution...

arrayAsView.pyx

#cython code

cimport numpy as cnp
import numpy as np

cnp.import_array()

cdef extern from "numpy/ndarraytypes.h" nogil:
  int NPY_ARRAY_CARRAY

cimport cpython.ref as ref

#"parent" is the owner of "data"
cdef cnp.ndarray get2DArrayAsView(object parent, void *data, cdef cnp.npy_intp *strides, cdef cnp.npy_intp *dims):
  cdef cnp.ndarray  result = cnp.PyArray_New(np.ndarray, 2, dims, cnp.NPY_INT64, strides,
                                             data, -1, NPY_ARRAY_CARRAY, <object>NULL)
  #result.base is of type PyObject*, so no reference counting with this assignment
  result.base              = <ref.PyObject*>parent
  ref.Py_INCREF(parent) #so, make sure that "result" owns a reference to "parent"
  return result