Can I work on this @eric-wieser ? Though I would require some guidance on how to work on this since I am begining with Open Source.

You'll want to change these lines:

Such that this test passes:

@euler16 - are you still working on this? If not I'd like to do it (I am also new to open source contributions).

yes I am, I will send a PR soon

@bmakos: If you want to work on something very similar, #10532 is probably only a little more complex

@bmakos: If you want to work on something very similar, #10532 is probably only a little more complex

Ok, I will take a look, see what I can do. Thank you!

@euler16: Are you close? It would be great if this could make it into 1.16 alongside @bmakos' work

Milestoned as 1.16 since the release notes added in #12392 claim this is already done!

So this indeed is simple, I wrote the code, but then decided to see how it behaves with a Union containing a packed structure.

>>>class Foo(ctypes.Structure):
    _fields_ = [('one', ctypes.c_uint8.__ctype_be__),('two', ctypes.c_uint32.__ctype_le__)]
    _pack_ = 1
    
>>>class Union(ctypes.Union):
    _fields_ = [
        ('a', ctypes.c_uint8),
        ('b', ctypes.c_uint16),
        ('c', ctypes.c_uint32),
        ('d', Foo),
    ] 
    
>>>ctypes.sizeof(Foo)
5
>>>ctypes.sizeof(Union)
8

In my code I constructed the dtype from Union with itemsize=ctypes.sizeof(Union))), which in this case gives size 8, while the correct size in my view would be 5.

As we are doing this to support ctypes the sensible choice is to still go with itemsize=ctypes.sizeof(Union))), but the ctypes Union constructor may need a fix (I will see how can I open a bug, never did this).

which in this case gives size 8, while the correct size in my view would be 5.

8 is correct - the struct has 3 bytes of padding inserted to meet the alignment requirements of uint32

I see now that if I also make the Union packed to 1 the size is 5. I assumed the rule to calculate the size was to take sizeof of the largest member as it is (in this case a packed Structure). Thank you for the clarification.