BUG: np.dtype(ctypes.Structure) does not respect _pack_ field

class Foo(ctypes.Structure):
    _fields_ = [('one', ctypes.c_uint8), ('two', ctypes.c_uint32)]
    _pack_ = 2

>>> ctypes.sizeof(Foo())  # packed to alignment 2
6
>>> np.dtype(Foo).itemsize  # default alignment incorrectly used
8

Fix should be fairly straightforward by changing:

https://github.com/numpy/numpy/blob/44810f07ec6a281d96d6a69c105ce6ae59aec718/numpy/core/_dtype_ctypes.py#L35-L46

[mattip]

I would not like to encourage deeper use of ctypes. The code behind them has many open issues. The package defines yet-another-dsl for describing c-like interfaces. It has so many layers it will always be slow and buggy. Why not directly inherit from a new dtypes.baseclass that returns NotImplemented where needed instead?

[eric-wieser]

@tylerjereddy:

Incidentally, I’ve been thinking a lot about using ctypes to define custom dtypes lately,

A neat but rather pointless trick

@np.dtype
class Foo(ctypes.Structure):
    _fields_ = [
        ('x', ctypes.c_uint32),
        ('y', ctypes.c_uint32)
    ]