working with ctypes and complex data structures

[Michael Felt]

On 02-Oct-16 19:50, Michael Felt wrote:
> I am trying to understand the documentation re: ctypes and interfacing 
> with existing libraries.
>
> I am reading the documentation, and also other sites that have largely 
> just copied the documentation - as well as the "free chapter" at 
> O'Reilly (Python Cookbook).
>
Using O'Reilly and the standard documentation I now have these two 
excerpts. The second def works, the first does not.

Someone helping me understand what I am not reading properly in this bit 
of documentation - is much appreciated.

>>>from  ctypes  import  c_int,  WINFUNCTYPE,  windll
>>>from  ctypes.wintypes  import  HWND,  LPCSTR,  UINT
>>>prototype  =  WINFUNCTYPE(c_int,  HWND,  LPCSTR,  LPCSTR,  UINT)
>>>paramflags  =  (1,  "hwnd",  0),  (1,  "text",  "Hi"),  (1,  "caption",  None),  (1,  "flags",  0)
>>>MessageBox  =  prototype(("MessageBoxA",  windll.user32),  paramflags)
>>>

The MessageBox foreign function can now be called in these ways:

>>>

>>>MessageBox()
>>>MessageBox(text="Spam, spam, spam")
>>>MessageBox(flags=2,  text="foo bar")
>>>

Note that MessageBox() may give two arguments.

My code: note both def init() are meant to do the same thing, just different call syntax.

    +76  class cpu_total:
    +77      def __init__(self):
    +78          __perfstat__ = CDLL("libperfstat.a(shr_64.o)")
    +79          prototype = CFUNCTYPE(c_int, c_void_p, c_void_p, c_int, c_int)
    +80          args = (1, "name", None), (2, "buff", None), (1, "size", 0), (1, "count", 1)
    +81          cpu_total = prototype(("perfstat_cpu_total", __perfstat__), args)
    +82
    +83          buff = perfstat_cpu_total_t()
    +84          cpu_total(buff=buff, size=sizeof(buff))
    +85
    +86  class perfstat_cpu_total:
    +87      def __init__(self):
    +88          __perfstat__ = CDLL("libperfstat.a(shr_64.o)")
    +89          cpu_total = __perfstat__.perfstat_cpu_total
    +90
    +91          _perfstat_cpu_total = perfstat_cpu_total_t()
    +92          ret = cpu_total(None,
    +93                          byref(_perfstat_cpu_total),
    +94                          sizeof(perfstat_cpu_total_t), 1)
    +95          self.boot = _perfstat_cpu_total.lbolt.v / 100
    +96          self.ncpus = _perfstat_cpu_total.ncpus
    +97          self._v = _perfstat_cpu_total


When I call the second I can print values and even call functions, reload the _v, etc..

The first class fails with this message:

a = cpu_total()
Traceback (most recent call last):
   File "<stdin>", line 135, in <module>
   File "<stdin>", line 84, in __init__
TypeError: call takes exactly 1 arguments (2 given)

Thanks for your attention.