Magic memoryview() style casting for Cython
A helper / hack to allow us to cast a mmap.mmap or other buffer to a Cython pointer of the correct type.
Cython is capable of casting a lot of things to a C pointer of the correct type, especially with the aid of memoryview. However, in Python 2, memoryview lacks the memoryview.cast method (so Cython won’t let us change the dimensions of the array). Further, both Python 2 and 3 require the memory map to be writable (making the pointer type const does not seem to help here either).
This class takes a (possibly read-only) memmap object, and produces a Python object with a __getbuffer__ method that returns The Right Thing. It pretends that the underlying buffer is writable to make Cython happy. If you give it a read-only buffer, and try to write to the result, then you will have a bad time.
When a Python object is cast by Cython to a pointer, it holds a reference to the underlying Python object in order to prevent the memory to which it refers being garbage collected. The MagicMemoryView in turn keeps a reference to the underlying data, so everything should behave as expected.
from magicmemoryview import MagicMemoryView cdef double data[:, :, :] data = MagicMemoryView(source_buffer, (24, 12, 25), "d")