Simple illustration of Cython error

setup.py

from distutils.core import setup
from distutils.extension import Extension
from Cython.Build import cythonize

import numpy as np

include_dir = np.get_include()

extensions = [
        Extension('testlib', ['testlib.pyx', ], include_dirs=[include_dir, ])
        ]

setup(
    ext_modules=cythonize(extensions)
)

test.py

import numpy as np

import testlib

print('####################')
print('Internally created struct: {} (expected: 1.0)'.format(testlib.foo_test_struct()))
print('####################')
print('Internally created np array: {} (expected: 1.0)'.format(testlib.foo_test_nparray()))
print('####################')
print('####################')
print('Testing np record from python -> cython')
N = 5
dtype = [('x', np.float64), ('y', 'S4'), ('z', np.int64)]
x = np.zeros(N, dtype=dtype)

x['x'] = np.arange(N).astype(np.float64) + 1
x['y'] = b'ab'
x['z'] = np.arange(N, 0, -1)

# Set first record to have a string that takes up the entire 4 elements
x['y'][0] = 'abcd'

# This works
print('Externally created np array w/full string: {} (expected: 1.0)'.format(testlib.get_foo_x(x[0])))

# This crashes
print('Externally created np array w/full string: {} (expected: 2.0)'.format(testlib.get_foo_x(x[1])))

testlib.pyx

import numpy as np
cimport numpy as np


cdef packed struct foo_type:
    np.float64_t x
    char[4] y
    np.int64_t z


cpdef double get_foo_x(foo_type f):
    return f.x


cpdef double foo_test_struct():
    cdef:
        foo_type f

    f.x = 1.0
    for i, c in enumerate(b'ab'):
        f.y[i] = c
    f.z = 2

    return get_foo_x(f)


cpdef double foo_test_nparray():
    cdef:
        foo_type[:] x

    N = 5
    dtype = [('x', np.float64), ('y', 'S4'), ('z', np.int64)]
    xarr = np.zeros(N, dtype=dtype)

    xarr['x'] = np.arange(N) + 1.0
    xarr['y'] = b'ab'
    xarr['z'] = np.arange(N, 0, -1)

    x = xarr

    return get_foo_x(x[0])