[ctypes-users] Set errcheck during the call?

[Mark H. <ma...@mi...>]

Ctype's errcheck attribute is a great way to maintain Python's error 
handling idiom when wrapping foreign libraries.  I have a case here for 
which I want to insure I'm using it most efficiently.  That is, I'm 
looking for a way to avoid the errcheck call on every clean ctypes 
function call.

I though this might be possible as it turns out the C library here has 
its own error trapping mechanism using callbacks which allows me to 
regain control under the Py interpreter for a moment.  Its not possible 
to raise an Exception at this point as the C lib demands a good return 
from the callback or it terminates everything with a C exit(), a 
behavior I believe  is fairly common.  Instead I try to set the 
foreign_func.errcheck = MyErrCheckhandler during the callback.   That 
is, for efficiency, I want to avoid an errcheck call upon every clean 
call; rather Id like it set only on errors for efficiency. 

This does not  work, so I'm assuming setting errcheck during the call is 
too late?


The code, generally:
-------------------------------------------------------------------------------------------
...
#foreign function.errcheck is not set, yet.
#Make this ctypes function call which throws an error inside the C library
foreign_lib.function()

# this callback comes from the library error handling mechanism
def foreign_lib_err_handler(..., foreign_func_name, ...)
    getattr( foreign_lib, foreign_func_name).errcheck = myerrcheck_handler
    #can not raise Exception here, since foreign lib must have this 
return code else it does BAD THINGS
    return library_success_code

def myerrcheck_handler( ... ):
    #this works fine if errcheck is set _before_ the call, not during :-(
    if foreign_lib.err_flag for == error_thrown:
       raise CustomLibException( some_err_message)
-------------------------------------------------------------------------------------------

And finally, if it is possible to make this 'after the call starts' 
approach work, it would be nice to reset the errcheck attribute so that 
it does not trip on future no-error calls.  Currently of course it can 
not be reset to None but only to call ables.

Regards,
Mark