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To: python-list@python.org
From: Stefan Behnel <stefan_ml@behnel.de>
Subject: Re: PyArg_ParseTuple() when the type could be anything?
Date: Sat, 03 Aug 2013 16:31:37 +0200
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David M. Cotter, 03.08.2013 02:55:
> I'd like to be able to use PyArg_ParseTuple() in a generic way.
> 
> for example, i'd like to have all commands start with 1 integer parameter, and this "commandID" will inform me of what parameters come next (via LUT).
> 
> knowing that i can then call ParseTuple again with the proper parameters.
> 
> like this:
> 
> if (PyArg_ParseTuple(args, "i|", &commandID)) {
> 
> 	switch (commandID) {
> 	
> 		case cmd_with_str: {
> 			const char *strZ = NULL;
> 			
> 			if (PyArg_ParseTuple(args, "is", &commandID, &strZ)) {
> 				//	do something with string
> 			}
> 			break;
> 		}
> 	
> 		case cmd_with_float: {
> 			float	valF = -1;
> 			
> 			if (PyArg_ParseTuple(args, "if", &commandID, &valF)) {
> 				//	do something with float
> 			}
> 			break;
> 		}
> 	}
> }
> 
> is there a way to achieve this?  the "i|" at the start is not working

If you're willing to switch to Cython, here's an (untested) example:

    cdef enum:
        cmd_with_str = 1
        cmd_with_float = 2

    cdef int command_id = args[0]
    if command_id == cmd_with_str:
        str_z = args[1]          # it's an object, so use it as such
        print(str_z)
    elif command_id == cmd_with_float:
        val_f = <float>args[1]   # converting to C float here
        ...
    else:
        raise ValueError("unknown command")

Two comments:

1) you can obviously do the same in C, by writing a bit more code. It would
likely be a lot slower, though, and you'd have to take care of error
handling etc.

2) you might want to rethink your design as this is a rather unpythonic
API. Although it depends on who (or what) you are expecting to use it.

Stefan