Overloading embedded Python functions using PyArg_ParseTuple

Question

If I'm trying to overload an embedded Python function so that the second argument can be a long or an Object, is there a standard way to do it? Is this it?

What I'm trying now (names changed to protect the innocent):

  bool UseLongVar2 = true;
  if (!PyArg_ParseTuple(args, "ll:foo", &LongVar1, &LongVar2))
  {
      PyErr_Clear();
      if (!PyArg_ParseTuple(args, "lO&:foo", &LongVar1, convertObject, &Object))
      {
         UseLongVar2 = false;
         return NULL;
      }
  }

Mike · Accepted Answer · 2017-03-13 15:51:53Z

3

What I normally do is have two C functions that take the different arguments. The "python-facing" function's job is to parse out the arguments, call the appropriate C function, and build the return value if any.

This is pretty common when, for example, you want to allow both byte and Unicode strings.

Here is an example of what I mean.

// Silly example: get the length of a string, supporting Unicode and byte strings
static PyObject* getlen_py(PyObject *self, PyObject *args)
{
    // Unpack our argument (error handling omitted...)
    PyObject *arg = NULL;
    PyArg_UnpackTuple(args, "getlen", 1, 1, arg) ;

    if ( PyUnicode_Check(arg) )
    {
        // It's a Unicode string
        return PyInt_FromLong(getlen_w(PyUnicode_AS_UNICODE(arg))) ;
    }
    else
    {
        // It's a byte string
        return PyInt_FromLong(getlen_a(PyString_AS_STRING(arg))) ;
    }
}

edited Mar 13, 2017 at 15:51

Mike

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answered Feb 18, 2010 at 23:14

Ryan Ginstrom

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4 Comments

patros Over a year ago

How does that differ from what I'm doing here? After the example code I can say something along the lines of "result = UseLongVar2 ? fooLong(LongVar1, LongVar2) : fooObj(LongVar1, Object)"?

patros Over a year ago

Just to clarify: I'm interested in how to tell which types of arguments were passed from Python. Is there something better than PyArg_ParseTuple, or is there a cleaner shorthand for separating out different types of variable. The construct I'm using would be completely hideous for even a few arguments that could have 2 or 3 possible types.

Ryan Ginstrom Over a year ago

This is different because with your way, you're doing two tests. First the test to see if it passed, and then later you'll have to test the flags you set to see which arguments to parse. With the way I outlined, you only do one test. And no, I don't think that there's a cleaner way to check what args you got. Another good reason to keep the arguments in C extensions simple.

patros Over a year ago

You didn't outline what structure you use to parse out the arguments.

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