CTYPES structure passing

[Bryan]

Correction/addendum: I wrote:
> try to to
> be to upset when you find out how your three days were wasted...

Of course that's, "try not to be too upset..."

[...]
> You're probably familiar with the Python
> convention of special member names with double-underscores at each
> end? Well, ctypes doesn't follow it.

The double-ended-double-underscore convention is for names defined by
the Python language. As a library module, ctypes is technically right
not to use it. It's still a diabolical gotcha.

[...]
> If you spell _fields_ as ctypes
> requires, it will complain about your assignments, in that you are
> trying to assign an in to a union.

Obviously a typo, for "trying to assign an int to a union." Come to
think of it, I bet initially Peter West had that bit right. Because of
the _fields_ issue, the correct assignment failed:

  FrameFormat.XUnion.subSample = 0

Without _fields_, ctypes did not create a FrameFormat.XUnion member,
so the assignment fails with "AttributeError: 'LUCAM_FRAME_FORMAT'
object has no attribute 'XUnion'".


--
--Bryan Olson