
Parsing arguments and building values
*************************************

These functions are useful when creating your own extensions functions
and methods.  Additional information and examples are available in
*Extending and Embedding the Python Interpreter*.

The first three of these functions described, ``PyArg_ParseTuple()``,
``PyArg_ParseTupleAndKeywords()``, and ``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 ``Py_buffer`` structure gets filled, the underlying
buffer is locked so that the caller can subsequently use the buffer
even inside a ``Py_BEGIN_ALLOW_THREADS`` block without the risk of
mutable data being resized or destroyed.  As a result, **you have to
call** ``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 ``Py_ssize_t``) is controlled by
  defining the macro ``PY_SSIZE_T_CLEAN`` before including
  ``Python.h``.  If the macro was defined, length is a ``Py_ssize_t``
  rather than an ``int``. This behavior will change in a future Python
  version to only support ``Py_ssize_t`` and drop ``int`` support. It
  is best to always define ``PY_SSIZE_T_CLEAN``.

``s`` (``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 ``TypeError`` exception is raised. Unicode objects are converted
   to C strings using ``'utf-8'`` encoding. If this conversion fails,
   a ``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
     ``PyUnicode_FSConverter()`` as *converter*.

``s*`` (``str``, ``bytes``, ``bytearray`` or buffer compatible object)
[Py_buffer]
   This format accepts Unicode objects as well as objects supporting
   the buffer protocol. It fills a ``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#`` (``str``, ``bytes`` or read-only buffer compatible object)
[const char *, int or ``Py_ssize_t``]
   Like ``s*``, except that it doesn't accept mutable buffer-like
   objects such as ``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`` (``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*`` (``str``, ``bytes``, ``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 ``Py_buffer`` structure is set to
   *NULL*.

``z#`` (``str``, ``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`` (``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
   ``TypeError`` exception is raised.

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

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

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

``u`` (``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
   ``Py_UNICODE`` pointer variable, which will be filled with the
   pointer to an existing Unicode buffer.  Please note that the width
   of a ``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 ``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#`` (``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`` (``str`` or ``None``) [Py_UNICODE *]
   Like ``u``, but the Python object may also be ``None``, in which
   case the ``Py_UNICODE`` pointer is set to *NULL*.

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

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

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

``es`` (``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 ``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
   ``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.

   ``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 ``PyMem_Free()`` to free the allocated buffer after
   use.

``et`` (``str``, ``bytes`` or ``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#`` (``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 ``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 ``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 ``PyMem_Free()`` to free the
   allocated buffer after usage.

   If **buffer* points to a non-*NULL* pointer (an already allocated
   buffer), ``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
   ``ValueError`` will be set.

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

``et#`` (``str``, ``bytes`` or ``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`` (``int``) [unsigned char]
   Convert a nonnegative Python integer to an unsigned tiny int,
   stored in a C ``unsigned char``.

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

``h`` (``int``) [short int]
   Convert a Python integer to a C ``short int``.

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

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

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

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

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

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

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

``n`` (``int``) [Py_ssize_t]
   Convert a Python integer to a C ``Py_ssize_t``.

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

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

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

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

``D`` (``complex``) [Py_complex]
   Convert a Python complex number to a C ``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 ``PyObject*``) into which the object pointer is stored.  If
   the Python object does not have the required type, ``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 ``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 ``void*`` argument that was passed to the ``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.

   Changed in version 3.1: ``Py_CLEANUP_SUPPORTED`` was added.

``(items)`` (``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 ``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, ``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 ``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 ``PyArg_Parse*()``
functions return true, otherwise they return false and raise an
appropriate exception. When the ``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
-------------

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.

int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)

   Identical to ``PyArg_ParseTuple()``, except that it accepts a
   va_list rather than a variable number of arguments.

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.

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

   Identical to ``PyArg_ParseTupleAndKeywords()``, except that it
   accepts a va_list rather than a variable number of arguments.

int PyArg_ValidateKeywordArguments(PyObject *)

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

   New in version 3.2.

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 ``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.

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
   ``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 ``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 ``_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 ``PyArg_UnpackTuple()`` in this example is entirely
   equivalent to this call to ``PyArg_ParseTuple()``:

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


Building values
===============

PyObject* Py_BuildValue(const char *format, ...)
    Return value: New reference.

   Create a new value based on a format string similar to those
   accepted by the ``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.

   ``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 ``Py_BuildValue()``.  In other
   words, if your code invokes ``malloc()`` and passes the allocated
   memory to ``Py_BuildValue()``, your code is responsible for calling
   ``free()`` for that memory once ``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`` (``str`` or ``None``) [char *]
      Convert a null-terminated C string to a Python ``str`` object
      using ``'utf-8'`` encoding. If the C string pointer is *NULL*,
      ``None`` is used.

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

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

   ``y#`` (``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`` (``str`` or ``None``) [char *]
      Same as ``s``.

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

   ``u`` (``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#`` (``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`` (``str`` or ``None``) [char *]
      Same as ``s``.

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

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

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

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

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

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

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

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

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

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

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

   ``n`` (``int``) [Py_ssize_t]
      Convert a C ``Py_ssize_t`` to a Python integer.

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

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

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

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

   ``D`` (``complex``) [Py_complex *]
      Convert a C ``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, ``Py_BuildValue()`` will return *NULL* but won't
      raise an exception.  If no exception has been raised yet,
      ``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 ``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}`` (``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 ``SystemError``
   exception is set and *NULL* returned.

PyObject* Py_VaBuildValue(const char *format, va_list vargs)

   Identical to ``Py_BuildValue()``, except that it accepts a va_list
   rather than a variable number of arguments.
