
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.

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.

``s`` (string or Unicode object) [const char *]
   Convert a Python string or Unicode object to a C pointer to a
   character string. You must not provide storage for the string
   itself; a pointer to an existing string is stored into 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 the default encoding.  If
   this conversion fails, a ``UnicodeError`` is raised.

   Starting with Python 2.5 the type of the length argument can be
   controlled by defining the macro ``PY_SSIZE_T_CLEAN`` before
   including ``Python.h``.  If the macro is defined, length is a
   ``Py_ssize_t`` rather than an int.

``s*`` (string, Unicode, or any buffer compatible object) [Py_buffer
*]
   This is similar to ``s``, but the code fills a ``Py_buffer``
   structure provided by the caller.  In this case the Python string
   may contain embedded null bytes.  Unicode objects pass back a
   pointer to the default encoded string version of the object if such
   a conversion is possible.  The underlying buffer is locked, so that
   the caller can subsequently use the buffer even inside a
   ``Py_BEGIN_ALLOW_THREADS`` block.  **The caller is responsible**
   for calling ``PyBuffer_Release`` with the structure after it has
   processed the data.

``s#`` (string, Unicode or any read buffer compatible object) [const
char *, int or ``Py_ssize_t``]
   This variant on ``s`` stores into two C variables, the first one a
   pointer to a character string, the second one its length.  In this
   case the Python string may contain embedded null bytes.  Unicode
   objects pass back a pointer to the default encoded string version
   of the object if such a conversion is possible.  All other read-
   buffer compatible objects pass back a reference to the raw internal
   data representation.  Since this format doesn't allow writable
   buffer compatible objects like byte arrays, ``s*`` is to be
   preferred.

   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``.

``y`` (bytes object) [const char *]
   This variant on ``s`` converts a Python bytes or bytearray object
   to a C pointer to a character string.  The bytes object must not
   contain embedded NUL bytes; if it does, a ``TypeError`` exception
   is raised.

``y*`` (bytes object) [Py_buffer *]
   This is to ``s*`` as ``y`` is to ``s``.

``y#`` (bytes object) [const char *, int]
   This variant on ``s#`` stores into two C variables, the first one a
   pointer to a character string, the second one its length.  This
   only accepts bytes objects, no byte arrays.

``z`` (string 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*`` (string or ``None`` or any buffer compatible object)
[Py_buffer*]
   This is to ``s*`` as ``z`` is to ``s``.

``z#`` (string or ``None`` or any read buffer compatible object)
[const char *, int]
   This is to ``s#`` as ``z`` is to ``s``.

``u`` (Unicode object) [Py_UNICODE *]
   Convert a Python Unicode object to a C pointer to a NUL-terminated
   buffer of 16-bit Unicode (UTF-16) data.  As with ``s``, there is no
   need to provide storage for the Unicode data buffer; a pointer to
   the existing Unicode data is stored into the ``Py_UNICODE`` pointer
   variable whose address you pass.

``u#`` (Unicode object) [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. Non-
   Unicode objects are handled by interpreting their read-buffer
   pointer as pointer to a ``Py_UNICODE`` array.

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

``Z#`` (Unicode or ``None``) [Py_UNICODE *, int]
   This is to ``u#`` as ``Z`` is to ``u``.

``es`` (string, Unicode object or character buffer compatible object)
[const char *encoding, char **buffer]
   This variant on ``s`` is used for encoding Unicode and objects
   convertible to 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 the
   default 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`` (string, Unicode object or character buffer compatible object)
[const char *encoding, char **buffer]
   Same as ``es`` except that 8-bit string objects are passed through
   without recoding them.  Instead, the implementation assumes that
   the string object uses the encoding passed in as parameter.

``es#`` (string, Unicode object or character buffer compatible object)
[const char *encoding, char **buffer, int *buffer_length]
   This variant on ``s#`` is used for encoding Unicode and objects
   convertible to 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 the default
   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#`` (string, Unicode object or character buffer compatible object)
[const char *encoding, char **buffer]
   Same as ``es#`` except that string objects are passed through
   without recoding them. Instead, the implementation assumes that the
   string object uses the encoding passed in as parameter.

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

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

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

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

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

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

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

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

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

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

``C`` (string of length 1) [int]
   Convert a Python character, represented as a unicode string 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.

``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.

``S`` (string) [PyStringObject *]
   Like ``O`` but requires that the Python object is a string object.
   Raises ``TypeError`` if the object is not a string object.  The C
   variable may also be declared as ``PyObject*``.

``U`` (Unicode string) [PyUnicodeObject *]
   Like ``O`` but requires that the Python object is a Unicode object.
   Raises ``TypeError`` if the object is not a Unicode object.  The C
   variable may also be declared as ``PyObject*``.

``t#`` (read-only character buffer) [char *, int]
   Like ``s#``, but accepts any object which implements the read-only
   buffer interface.  The ``char*`` variable is set to point to the
   first byte of the buffer, and the ``int`` is set to the length of
   the buffer.  Only single-segment buffer objects are accepted;
   ``TypeError`` is raised for all others.

``w`` (read-write character buffer) [char *]
   Similar to ``s``, but accepts any object which implements the read-
   write buffer interface.  The caller must determine the length of
   the buffer by other means, or use ``w#`` instead.  Only single-
   segment buffer objects are accepted; ``TypeError`` is raised for
   all others.

``w*`` (read-write byte-oriented buffer) [Py_buffer *]
   This is to ``w`` what ``s*`` is to ``s``.

``w#`` (read-write character buffer) [char *, int]
   Like ``s#``, but accepts any object which implements the read-write
   buffer interface.  The ``char *`` variable is set to point to the
   first byte of the buffer, and the ``int`` is set to the length of
   the buffer. Only single-segment buffer objects are accepted;
   ``TypeError`` is raised for all others.

``(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.

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_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)

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

   ``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.

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

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

   ``u`` (Unicode string) [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#`` (Unicode string) [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`` (string) [char *]
      Convert a null-terminated C string to a Python unicode object.
      If the C string pointer is *NULL*, ``None`` is used.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   ``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}`` (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 ``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.
