# The `Py_BuildValue()` Function

This function is the counterpart to `PyArg_ParseTuple()`. It is declared as follows:

        PyObject *Py_BuildValue(char *format, ...);

It recognizes a set of format units similar to the ones recognized by `PyArg_ParseTuple()`, but the arguments (which are input to the function, not output) must not be pointers, just values. It returns a new Python object, suitable for returning from a C function called from Python.

One difference with `PyArg_ParseTuple()`: while the latter requires its first argument to be a tuple (since Python argument lists are always represented as tuples internally), `BuildValue()` does not always build a tuple. It builds a tuple only if its format string contains two or more format units. If the format string is empty, it returns `None`; if it contains exactly one format unit, it returns whatever object is described by that format unit. To force it to return a tuple of size 0 or one, parenthesize the format string.

In the following description, the quoted form is the format unit; the entry in (round) parentheses is the Python object type that the format unit will return; and the entry in [square] brackets is the type of the C value(s) to be passed.

The characters space, tab, colon and comma are ignored in format strings (but not within format units such as ``s#`'). This can be used to make long format strings a tad more readable.

``s`' (string) [char *]
  Convert a null-terminated C string to a Python object. If the C string pointer is `NULL`, `None` is returned.

``s#`' (string) [char *, int]
  Convert a C string and its length to a Python object. If the C string pointer is `NULL`, the length is ignored and `None` is returned.

``z`' (string or `None`) [char *]
  Same as ``s`'.

``z#`' (string or `None`) [char *, int]
  Same as ``s#`'.

``i`' (integer) [int]
  Convert a plain C `int` to a Python integer object.

``b`' (integer) [char]
  Same as ``i`'.

``h`' (integer) [short int]
  Same as ``i`'.

``l`' (integer) [long int]
  Convert a C `long int` to a Python integer object.

``c`' (string of length 1) [char]
  Convert a C `int` representing a character to a Python string of length 1.

``d`' (float) [double]
  Convert a C `double` to a Python floating point number.

``f`' (float) [float]
  Same as ``d`'.

``O`' (object) [PyObject *]
  Pass a Python object untouched (except for its reference count, which is incremented by one). If the object passed in is a `NULL` pointer, it is assumed that this was caused because the call producing the argument found an error and set an exception. Therefore, `Py_BuildValue()` will return `NULL` but won't raise an exception. If no exception has been raised yet, `PyExc_SystemError` is set.

``S`' (object) [PyObject *]
  Same as ``O`'.

``O&`' (object) [converter, anything]
  Convert anything to a Python object through a converter function. The function is called with anything (which should be compatible with `void *`) as its argument and should return a ``new'' Python object, or `NULL` if an error occurred.

``(items)`' (tuple) [matching-items]
  Convert a sequence of C values to a Python tuple with the same number of items.

``[items]`' (list) [matching-items]
  Convert a sequence of C values to a Python list with the same number of items.

``{items}`' (dictionary) [matching-items]
  Convert a sequence of C values to a Python dictionary. Each pair of consecutive C values adds one item to the dictionary, serving as key and value, respectively.

If there is an error in the format string, the `PyExc_SystemError` exception is raised and `NULL` returned.

Examples (to the left the call, to the right the resulting Python value):

        Py_BuildValue("")                        None
        Py_BuildValue("i", 123)                  123
        Py_BuildValue("iii", 123, 456, 789)      (123, 456, 789)
        Py_BuildValue("s", "hello")              'hello'
        Py_BuildValue("ss", "hello", "world")    ('hello', 'world')
        Py_BuildValue("s#", "hello", 4)          'hell'
        Py_BuildValue("()")                      ()
        Py_BuildValue("(i)", 123)                (123,)
        Py_BuildValue("(ii)", 123, 456)          (123, 456)
        Py_BuildValue("(i,i)", 123, 456)         (123, 456)
        Py_BuildValue("[i,i]", 123, 456)         [123, 456]
        Py_BuildValue("{s:i,s:i}",
                      "abc", 123, "def", 456)    {'abc': 123, 'def': 456}
        Py_BuildValue("((ii)(ii)) (ii)",
                      1, 2, 3, 4, 5, 6)          (((1, 2), (3, 4)), (5, 6))