path: root/Doc/c-api/arg.rst

.. highlightlang:: c

.. _arg-parsing:

Parsing arguments and building values
=====================================

These functions are useful when creating your own extensions functions and
methods.  Additional information and examples are available in
:ref:`extending-index`.

The first three of these functions described, :c:func:`PyArg_ParseTuple`,
:c:func:`PyArg_ParseTupleAndKeywords`, and :c:func:`PyArg_Parse`, all use *format
strings* which are used to tell the function about the expected arguments.  The
format strings use the same syntax for each of these functions.

-----------------
Parsing arguments
-----------------

A format string consists of zero or more "format units."  A format unit
describes one Python object; it is usually a single character or a parenthesized
sequence of format units.  With a few exceptions, a format unit that is not a
parenthesized sequence normally corresponds to a single address argument to
these functions.  In the following description, the quoted form is the format
unit; the entry in (round) parentheses is the Python object type that matches
the format unit; and the entry in [square] brackets is the type of the C
variable(s) whose address should be passed.

Strings and buffers
-------------------

These formats allow to access an object as a contiguous chunk of memory.
You don't have to provide raw storage for the returned unicode or bytes
area.  Also, you won't have to release any memory yourself, except with the
``es``, ``es#``, ``et`` and ``et#`` formats.

However, when a :c:type:`Py_buffer` structure gets filled, the underlying
buffer is locked so that the caller can subsequently use the buffer even
inside a :c:type:`Py_BEGIN_ALLOW_THREADS` block without the risk of mutable data
being resized or destroyed.  As a result, **you have to call**
:c:func:`PyBuffer_Release` after you have finished processing the data (or
in any early abort case).

Unless otherwise stated, buffers are not NUL-terminated.

.. note::
   For all ``#`` variants of formats (``s#``, ``y#``, etc.), the type of
   the length argument (int or :c:type:`Py_ssize_t`) is controlled by
   defining the macro :c:macro:`PY_SSIZE_T_CLEAN` before including
   :file:`Python.h`.  If the macro was defined, length is a
   :c:type:`Py_ssize_t` rather than an :c:type:`int`. This behavior will change
   in a future Python version to only support :c:type:`Py_ssize_t` and
   drop :c:type:`int` support. It is best to always define :c:macro:`PY_SSIZE_T_CLEAN`.


``s`` (:class:`str`) [const char \*]
   Convert a Unicode object to a C pointer to a character string.
   A pointer to an existing string is stored in the character pointer
   variable whose address you pass.  The C string is NUL-terminated.
   The Python string must not contain embedded NUL bytes; if it does,
   a :exc:`TypeError` exception is raised. Unicode objects are converted
   to C strings using ``'utf-8'`` encoding. If this conversion fails, a
   :exc:`UnicodeError` is raised.

   .. note::
      This format does not accept bytes-like objects.  If you want to accept
      filesystem paths and convert them to C character strings, it is
      preferable to use the ``O&`` format with :c:func:`PyUnicode_FSConverter`
      as *converter*.

``s*`` (:class:`str`, :class:`bytes`, :class:`bytearray` or buffer compatible object) [Py_buffer]
   This format accepts Unicode objects as well as objects supporting the
   buffer protocol.
   It fills a :c:type:`Py_buffer` structure provided by the caller.
   In this case the resulting C string may contain embedded NUL bytes.
   Unicode objects are converted to C strings using ``'utf-8'`` encoding.

``s#`` (:class:`str`, :class:`bytes` or read-only buffer compatible object) [const char \*, int or :c:type:`Py_ssize_t`]
   Like ``s*``, except that it doesn't accept mutable buffer-like objects
   such as :class:`bytearray`.  The result is stored into two C variables,
   the first one a pointer to a C string, the second one its length.
   The string may contain embedded null bytes. Unicode objects are converted
   to C strings using ``'utf-8'`` encoding.

``z`` (:class:`str` or ``None``) [const char \*]
   Like ``s``, but the Python object may also be ``None``, in which case the C
   pointer is set to *NULL*.

``z*`` (:class:`str`, :class:`bytes`, :class:`bytearray`, buffer compatible object or ``None``) [Py_buffer]
   Like ``s*``, but the Python object may also be ``None``, in which case the
   ``buf`` member of the :c:type:`Py_buffer` structure is set to *NULL*.

``z#`` (:class:`str`, :class:`bytes`, read-only buffer compatible object or ``None``) [const char \*, int]
   Like ``s#``, but the Python object may also be ``None``, in which case the C
   pointer is set to *NULL*.

``y`` (:class:`bytes`) [const char \*]
   This format converts a bytes-like object to a C pointer to a character
   string; it does not accept Unicode objects.  The bytes buffer must not
   contain embedded NUL bytes; if it does, a :exc:`TypeError`
   exception is raised.

``y*`` (:class:`bytes`, :class:`bytearray` or buffer compatible object) [Py_buffer]
   This variant on ``s*`` doesn't accept Unicode objects, only objects
   supporting the buffer protocol.  **This is the recommended way to accept
   binary data.**

``y#`` (:class:`bytes`) [const char \*, int]
   This variant on ``s#`` doesn't accept Unicode objects, only bytes-like
   objects.

``S`` (:class:`bytes`) [PyBytesObject \*]
   Requires that the Python object is a :class:`bytes` object, without
   attempting any conversion.  Raises :exc:`TypeError` if the object is not
   a bytes object.  The C variable may also be declared as :c:type:`PyObject\*`.

``Y`` (:class:`bytearray`) [PyByteArrayObject \*]
   Requires that the Python object is a :class:`bytearray` object, without
   attempting any conversion.  Raises :exc:`TypeError` if the object is not
   a :class:`bytearray` object. The C variable may also be declared as :c:type:`PyObject\*`.

``u`` (:class:`str`) [Py_UNICODE \*]
   Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of
   Unicode characters.  You must pass the address of a :c:type:`Py_UNICODE`
   pointer variable, which will be filled with the pointer to an existing
   Unicode buffer.  Please note that the width of a :c:type:`Py_UNICODE`
   character depends on compilation options (it is either 16 or 32 bits).
   The Python string must not contain embedded NUL characters; if it does,
   a :exc:`TypeError` exception is raised.

   .. note::
      Since ``u`` doesn't give you back the length of the string, and it
      may contain embedded NUL characters, it is recommended to use ``u#``
      or ``U`` instead.

``u#`` (:class:`str`) [Py_UNICODE \*, int]
   This variant on ``u`` stores into two C variables, the first one a pointer to a
   Unicode data buffer, the second one its length.

``Z`` (:class:`str` or ``None``) [Py_UNICODE \*]
   Like ``u``, but the Python object may also be ``None``, in which case the
   :c:type:`Py_UNICODE` pointer is set to *NULL*.

``Z#`` (:class:`str` or ``None``) [Py_UNICODE \*, int]
   Like ``u#``, but the Python object may also be ``None``, in which case the
   :c:type:`Py_UNICODE` pointer is set to *NULL*.

``U`` (:class:`str`) [PyUnicodeObject \*]
   Requires that the Python object is a Unicode object, without attempting
   any conversion.  Raises :exc:`TypeError` if the object is not a Unicode
   object.  The C variable may also be declared as :c:type:`PyObject\*`.

``w*`` (:class:`bytearray` or read-write byte-oriented buffer) [Py_buffer]
   This format accepts any object which implements the read-write buffer
   interface. It fills a :c:type:`Py_buffer` structure provided by the caller.
   The buffer may contain embedded null bytes. The caller have to call
   :c:func:`PyBuffer_Release` when it is done with the buffer.

``es`` (:class:`str`) [const char \*encoding, char \*\*buffer]
   This variant on ``s`` is used for encoding Unicode into a character buffer.
   It only works for encoded data without embedded NUL bytes.

   This format requires two arguments.  The first is only used as input, and
   must be a :c:type:`const char\*` which points to the name of an encoding as a
   NUL-terminated string, or *NULL*, in which case ``'utf-8'`` encoding is used.
   An exception is raised if the named encoding is not known to Python.  The
   second argument must be a :c:type:`char\*\*`; the value of the pointer it
   references will be set to a buffer with the contents of the argument text.
   The text will be encoded in the encoding specified by the first argument.

   :c:func:`PyArg_ParseTuple` will allocate a buffer of the needed size, copy the
   encoded data into this buffer and adjust *\*buffer* to reference the newly
   allocated storage.  The caller is responsible for calling :c:func:`PyMem_Free` to
   free the allocated buffer after use.

``et`` (:class:`str`, :class:`bytes` or :class:`bytearray`) [const char \*encoding, char \*\*buffer]
   Same as ``es`` except that byte string objects are passed through without
   recoding them.  Instead, the implementation assumes that the byte string object uses
   the encoding passed in as parameter.

``es#`` (:class:`str`) [const char \*encoding, char \*\*buffer, int \*buffer_length]
   This variant on ``s#`` is used for encoding Unicode into a character buffer.
   Unlike the ``es`` format, this variant allows input data which contains NUL
   characters.

   It requires three arguments.  The first is only used as input, and must be a
   :c:type:`const char\*` which points to the name of an encoding as a
   NUL-terminated string, or *NULL*, in which case ``'utf-8'`` encoding is used.
   An exception is raised if the named encoding is not known to Python.  The
   second argument must be a :c:type:`char\*\*`; the value of the pointer it
   references will be set to a buffer with the contents of the argument text.
   The text will be encoded in the encoding specified by the first argument.
   The third argument must be a pointer to an integer; the referenced integer
   will be set to the number of bytes in the output buffer.

   There are two modes of operation:

   If *\*buffer* points a *NULL* pointer, the function will allocate a buffer of
   the needed size, copy the encoded data into this buffer and set *\*buffer* to
   reference the newly allocated storage.  The caller is responsible for calling
   :c:func:`PyMem_Free` to free the allocated buffer after usage.

   If *\*buffer* points to a non-*NULL* pointer (an already allocated buffer),
   :c:func:`PyArg_ParseTuple` will use this location as the buffer and interpret the
   initial value of *\*buffer_length* as the buffer size.  It will then copy the
   encoded data into the buffer and NUL-terminate it.  If the buffer is not large
   enough, a :exc:`ValueError` will be set.

   In both cases, *\*buffer_length* is set to the length of the encoded data
   without the trailing NUL byte.

``et#`` (:class:`str`, :class:`bytes` or :class:`bytearray`) [const char \*encoding, char \*\*buffer, int \*buffer_length]
   Same as ``es#`` except that byte string objects are passed through without recoding
   them. Instead, the implementation assumes that the byte string object uses the
   encoding passed in as parameter.

Numbers
-------

``b`` (:class:`int`) [unsigned char]
   Convert a nonnegative Python integer to an unsigned tiny int, stored in a C
   :c:type:`unsigned char`.

``B`` (:class:`int`) [unsigned char]
   Convert a Python integer to a tiny int without overflow checking, stored in a C
   :c:type:`unsigned char`.

``h`` (:class:`int`) [short int]
   Convert a Python integer to a C :c:type:`short int`.

``H`` (:class:`int`) [unsigned short int]
   Convert a Python integer to a C :c:type:`unsigned short int`, without overflow
   checking.

``i`` (:class:`int`) [int]
   Convert a Python integer to a plain C :c:type:`int`.

``I`` (:class:`int`) [unsigned int]
   Convert a Python integer to a C :c:type:`unsigned int`, without overflow
   checking.

``l`` (:class:`int`) [long int]
   Convert a Python integer to a C :c:type:`long int`.

``k`` (:class:`int`) [unsigned long]
   Convert a Python integer to a C :c:type:`unsigned long` without
   overflow checking.

``L`` (:class:`int`) [PY_LONG_LONG]
   Convert a Python integer to a C :c:type:`long long`.  This format is only
   available on platforms that support :c:type:`long long` (or :c:type:`_int64` on
   Windows).

``K`` (:class:`int`) [unsigned PY_LONG_LONG]
   Convert a Python integer to a C :c:type:`unsigned long long`
   without overflow checking.  This format is only available on platforms that
   support :c:type:`unsigned long long` (or :c:type:`unsigned _int64` on Windows).

``n`` (:class:`int`) [Py_ssize_t]
   Convert a Python integer to a C :c:type:`Py_ssize_t`.

``c`` (:class:`bytes` of length 1) [char]
   Convert a Python byte, represented as a :class:`bytes` object of length 1,
   to a C :c:type:`char`.

``C`` (:class:`str` of length 1) [int]
   Convert a Python character, represented as a :class:`str` object of
   length 1, to a C :c:type:`int`.

``f`` (:class:`float`) [float]
   Convert a Python floating point number to a C :c:type:`float`.

``d`` (:class:`float`) [double]
   Convert a Python floating point number to a C :c:type:`double`.

``D`` (:class:`complex`) [Py_complex]
   Convert a Python complex number to a C :c:type:`Py_complex` structure.

Other objects
-------------

``O`` (object) [PyObject \*]
   Store a Python object (without any conversion) in a C object pointer.  The C
   program thus receives the actual object that was passed.  The object's reference
   count is not increased.  The pointer stored is not *NULL*.

``O!`` (object) [*typeobject*, PyObject \*]
   Store a Python object in a C object pointer.  This is similar to ``O``, but
   takes two C arguments: the first is the address of a Python type object, the
   second is the address of the C variable (of type :c:type:`PyObject\*`) into which
   the object pointer is stored.  If the Python object does not have the required
   type, :exc:`TypeError` is raised.

``O&`` (object) [*converter*, *anything*]
   Convert a Python object to a C variable through a *converter* function.  This
   takes two arguments: the first is a function, the second is the address of a C
   variable (of arbitrary type), converted to :c:type:`void \*`.  The *converter*
   function in turn is called as follows::

      status = converter(object, address);

   where *object* is the Python object to be converted and *address* is the
   :c:type:`void\*` argument that was passed to the :c:func:`PyArg_Parse\*` function.
   The returned *status* should be ``1`` for a successful conversion and ``0`` if
   the conversion has failed.  When the conversion fails, the *converter* function
   should raise an exception and leave the content of *address* unmodified.

   If the *converter* returns ``Py_CLEANUP_SUPPORTED``, it may get called a
   second time if the argument parsing eventually fails, giving the converter a
   chance to release any memory that it had already allocated. In this second
   call, the *object* parameter will be NULL; *address* will have the same value
   as in the original call.

   .. versionchanged:: 3.1
      ``Py_CLEANUP_SUPPORTED`` was added.

``(items)`` (:class:`tuple`) [*matching-items*]
   The object must be a Python sequence whose length is the number of format units
   in *items*.  The C arguments must correspond to the individual format units in
   *items*.  Format units for sequences may be nested.

It is possible to pass "long" integers (integers whose value exceeds the
platform's :const:`LONG_MAX`) however no proper range checking is done --- the
most significant bits are silently truncated when the receiving field is too
small to receive the value (actually, the semantics are inherited from downcasts
in C --- your mileage may vary).

A few other characters have a meaning in a format string.  These may not occur
inside nested parentheses.  They are:

``|``
   Indicates that the remaining arguments in the Python argument list are optional.
   The C variables corresponding to optional arguments should be initialized to
   their default value --- when an optional argument is not specified,
   :c:func:`PyArg_ParseTuple` does not touch the contents of the corresponding C
   variable(s).

``:``
   The list of format units ends here; the string after the colon is used as the
   function name in error messages (the "associated value" of the exception that
   :c:func:`PyArg_ParseTuple` raises).

``;``
   The list of format units ends here; the string after the semicolon is used as
   the error message *instead* of the default error message.  ``:`` and ``;``
   mutually exclude each other.

Note that any Python object references which are provided to the caller are
*borrowed* references; do not decrement their reference count!

Additional arguments passed to these functions must be addresses of variables
whose type is determined by the format string; these are used to store values
from the input tuple.  There are a few cases, as described in the list of format
units above, where these parameters are used as input values; they should match
what is specified for the corresponding format unit in that case.

For the conversion to succeed, the *arg* object must match the format
and the format must be exhausted.  On success, the
:c:func:`PyArg_Parse\*` functions return true, otherwise they return
false and raise an appropriate exception. When the
:c:func:`PyArg_Parse\*` functions fail due to conversion failure in one
of the format units, the variables at the addresses corresponding to that
and the following format units are left untouched.

API Functions
-------------

.. c:function:: int PyArg_ParseTuple(PyObject *args, const char *format, ...)

   Parse the parameters of a function that takes only positional parameters into
   local variables.  Returns true on success; on failure, it returns false and
   raises the appropriate exception.


.. c:function:: int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)

   Identical to :c:func:`PyArg_ParseTuple`, except that it accepts a va_list rather
   than a variable number of arguments.


.. c:function:: int PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], ...)

   Parse the parameters of a function that takes both positional and keyword
   parameters into local variables.  Returns true on success; on failure, it
   returns false and raises the appropriate exception.


.. c:function:: int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)

   Identical to :c:func:`PyArg_ParseTupleAndKeywords`, except that it accepts a
   va_list rather than a variable number of arguments.


.. c:function:: int PyArg_ValidateKeywordArguments(PyObject *)

   Ensure that the keys in the keywords argument dictionary are strings.  This
   is only needed if :c:func:`PyArg_ParseTupleAndKeywords` is not used, since the
   latter already does this check.

   .. versionadded:: 3.2


.. XXX deprecated, will be removed
.. c:function:: int PyArg_Parse(PyObject *args, const char *format, ...)

   Function used to deconstruct the argument lists of "old-style" functions ---
   these are functions which use the :const:`METH_OLDARGS` parameter parsing
   method.  This is not recommended for use in parameter parsing in new code, and
   most code in the standard interpreter has been modified to no longer use this
   for that purpose.  It does remain a convenient way to decompose other tuples,
   however, and may continue to be used for that purpose.


.. c:function:: int PyArg_UnpackTuple(PyObject *args, const char *name, Py_ssize_t min, Py_ssize_t max, ...)

   A simpler form of parameter retrieval which does not use a format string to
   specify the types of the arguments.  Functions which use this method to retrieve
   their parameters should be declared as :const:`METH_VARARGS` in function or
   method tables.  The tuple containing the actual parameters should be passed as
   *args*; it must actually be a tuple.  The length of the tuple must be at least
   *min* and no more than *max*; *min* and *max* may be equal.  Additional
   arguments must be passed to the function, each of which should be a pointer to a
   :c:type:`PyObject\*` variable; these will be filled in with the values from
   *args*; they will contain borrowed references.  The variables which correspond
   to optional parameters not given by *args* will not be filled in; these should
   be initialized by the caller. This function returns true on success and false if
   *args* is not a tuple or contains the wrong number of elements; an exception
   will be set if there was a failure.

   This is an example of the use of this function, taken from the sources for the
   :mod:`_weakref` helper module for weak references::

      static PyObject *
      weakref_ref(PyObject *self, PyObject *args)
      {
          PyObject *object;
          PyObject *callback = NULL;
          PyObject *result = NULL;

          if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) {
              result = PyWeakref_NewRef(object, callback);
          }
          return result;
      }

   The call to :c:func:`PyArg_UnpackTuple` in this example is entirely equivalent to
   this call to :c:func:`PyArg_ParseTuple`::

      PyArg_ParseTuple(args, "O|O:ref", &object, &callback)


---------------
Building values
---------------

.. c:function:: PyObject* Py_BuildValue(const char *format, ...)

   Create a new value based on a format string similar to those accepted by the
   :c:func:`PyArg_Parse\*` family of functions and a sequence of values.  Returns
   the value or *NULL* in the case of an error; an exception will be raised if
   *NULL* is returned.

   :c:func:`Py_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.

   When memory buffers are passed as parameters to supply data to build objects, as
   for the ``s`` and ``s#`` formats, the required data is copied.  Buffers provided
   by the caller are never referenced by the objects created by
   :c:func:`Py_BuildValue`.  In other words, if your code invokes :c:func:`malloc`
   and passes the allocated memory to :c:func:`Py_BuildValue`, your code is
   responsible for calling :c:func:`free` for that memory once
   :c:func:`Py_BuildValue` returns.

   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`` (:class:`str` or ``None``) [char \*]
      Convert a null-terminated C string to a Python :class:`str` object using ``'utf-8'``
      encoding. If the C string pointer is *NULL*, ``None`` is used.

   ``s#`` (:class:`str` or ``None``) [char \*, int]
      Convert a C string and its length to a Python :class:`str` object using ``'utf-8'``
      encoding. If the C string pointer is *NULL*, the length is ignored and
      ``None`` is returned.

   ``y`` (:class:`bytes`) [char \*]
      This converts a C string to a Python :func:`bytes` object.  If the C
      string pointer is *NULL*, ``None`` is returned.

   ``y#`` (:class:`bytes`) [char \*, int]
      This converts a C string and its lengths to a Python object.  If the C
      string pointer is *NULL*, ``None`` is returned.

   ``z`` (:class:`str` or ``None``) [char \*]
      Same as ``s``.

   ``z#`` (:class:`str` or ``None``) [char \*, int]
      Same as ``s#``.

   ``u`` (:class:`str`) [Py_UNICODE \*]
      Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python
      Unicode object.  If the Unicode buffer pointer is *NULL*, ``None`` is returned.

   ``u#`` (:class:`str`) [Py_UNICODE \*, int]
      Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python
      Unicode object.   If the Unicode buffer pointer is *NULL*, the length is ignored
      and ``None`` is returned.

   ``U`` (:class:`str` or ``None``) [char \*]
      Same as ``s``.

   ``U#`` (:class:`str` or ``None``) [char \*, int]
      Same as ``s#``.

   ``i`` (:class:`int`) [int]
      Convert a plain C :c:type:`int` to a Python integer object.

   ``b`` (:class:`int`) [char]
      Convert a plain C :c:type:`char` to a Python integer object.

   ``h`` (:class:`int`) [short int]
      Convert a plain C :c:type:`short int` to a Python integer object.

   ``l`` (:class:`int`) [long int]
      Convert a C :c:type:`long int` to a Python integer object.

   ``B`` (:class:`int`) [unsigned char]
      Convert a C :c:type:`unsigned char` to a Python integer object.

   ``H`` (:class:`int`) [unsigned short int]
      Convert a C :c:type:`unsigned short int` to a Python integer object.

   ``I`` (:class:`int`) [unsigned int]
      Convert a C :c:type:`unsigned int` to a Python integer object.

   ``k`` (:class:`int`) [unsigned long]
      Convert a C :c:type:`unsigned long` to a Python integer object.

   ``L`` (:class:`int`) [PY_LONG_LONG]
      Convert a C :c:type:`long long` to a Python integer object. Only available
      on platforms that support :c:type:`long long` (or :c:type:`_int64` on
      Windows).

   ``K`` (:class:`int`) [unsigned PY_LONG_LONG]
      Convert a C :c:type:`unsigned long long` to a Python integer object. Only
      available on platforms that support :c:type:`unsigned long long` (or
      :c:type:`unsigned _int64` on Windows).

   ``n`` (:class:`int`) [Py_ssize_t]
      Convert a C :c:type:`Py_ssize_t` to a Python integer.

   ``c`` (:class:`bytes` of length 1) [char]
      Convert a C :c:type:`int` representing a byte to a Python :class:`bytes` object of
      length 1.

   ``C`` (:class:`str` of length 1) [int]
      Convert a C :c:type:`int` representing a character to Python :class:`str`
      object of length 1.

   ``d`` (:class:`float`) [double]
      Convert a C :c:type:`double` to a Python floating point number.

   ``f`` (:class:`float`) [float]
      Convert a C :c:type:`float` to a Python floating point number.

   ``D`` (:class:`complex`) [Py_complex \*]
      Convert a C :c:type:`Py_complex` structure to a Python complex number.

   ``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, :c:func:`Py_BuildValue` will return *NULL* but won't
      raise an exception.  If no exception has been raised yet, :exc:`SystemError` is
      set.

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

   ``N`` (object) [PyObject \*]
      Same as ``O``, except it doesn't increment the reference count on the object.
      Useful when the object is created by a call to an object constructor in the
      argument list.

   ``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 :c:type:`void
      \*`) as its argument and should return a "new" Python object, or *NULL* if an
      error occurred.

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

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

   ``{items}`` (:class:`dict`) [*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 :exc:`SystemError` exception is
   set and *NULL* returned.

.. c:function:: PyObject* Py_VaBuildValue(const char *format, va_list vargs)

   Identical to :c:func:`Py_BuildValue`, except that it accepts a va_list
   rather than a variable number of arguments.