How to use Cython typed memoryviews to accept strings from Python?

Question

How can I write a Cython function that takes a byte string object (a normal string, a bytearray, or another object that follows the buffer protocol) as a typed memoryview?

Answer 1

This issue was fixed in Cython 0.28, released 2018-03-13 (PR #1869). The changelog says:

The const modifier can be applied to memoryview declarations to allow read-only buffers as input.

There is also a new section in the documentation.

The example you gave will work in Cython 0.28 if you write your function like this:

cpdef object printbuf(const unsigned char[:] buf):
    chars = [chr(x) for x in buf]
    print repr(''.join(chars))

Answer 2

Despite the documentation suggesting otherwise, Cython (at least up to version 0.22) does not support coercing read-only buffer objects into typed memoryview objects. Cython always passes the PyBUF_WRITABLE flag to PyObject_GetBuffer(), even when it doesn't need write access. This causes read-only buffer objects to raise an exception.

I raised this issue on the Cython developer mailing list, and even included a (very rough) patch. I never got a reply, so I assume the Cython developers are not interested in fixing this bug.