Implement Py_BuildValue #59

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1. sbinet commented on Jan 19, 2018

   

   sbinet

   on Jan 19, 2018

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   @elbaro, unfortunately, cgo doesn't handle C functions that are variadic (IIRC).

   so, essentially, one would need to implement Py_BuildValue in terms of e.g. PyTuple_New + PyTuple_SET_ITEM.
   and, for this to work, one would need to also implement a parser for the Py_BuildValue(format) string to infer the tuple size and the various types for the elements of that tuple:

   • https://docs.python.org/2/c-api/arg.html

   that said that could be done. PR accepted :P
2. sbinet commented on Jan 19, 2018

   

   sbinet

   on Jan 19, 2018

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   (note that, according to Python2 C-API Py_BuildValue doesn't always return a slice, so this should also be taken care of...)
3. icholy commented on Jun 14, 2019

   

   icholy

   on Jun 14, 2019

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   What about a hacky solution where there are a bunch of C wrapping functions with a fixed number of parameters. This code is obviously wrong, but it gets the idea across:

   PyObject* Py_BuildValue_1(const char *format, PyObject* a1) {
     return Py_BuildValue(format, a1);
   }
   
   PyObject* Py_BuildValue_2(const char *format, PyObject* a1, PyObject* a2) {
     return Py_BuildValue(format, a1, a2);
   }
   
   PyObject* Py_BuildValue_3(const char *format, PyObject* a1, PyObject* a2, PyObject* a3) {
     return Py_BuildValue(format, a1, a2, a3);
   }

   func Py_BuildValue(format string, args ...*PyObject) *PyObject {
     switch len(args) {
     case 1:
       return C.Py_BuildValue_1(format, args[0])
     case 2:
       return C.Py_BuildValue_2(format, args[0], args[1])
     case 3:
       return C.Py_BuildValue_3(format, args[0], args[1], args[3])
     default:
       return Py_None
     }
   }
4. sbinet commented on Jun 14, 2019

   

   sbinet

   on Jun 14, 2019

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   SGTM.
   that's actually what I did there:
   https://github.com/sbinet/go-python/blob/master/go-python.c#L16 for PyObject_CallFunction