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+.. 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, :cfunc:`PyArg_ParseTuple`,
+:cfunc:`PyArg_ParseTupleAndKeywords`, and :cfunc:`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 :exc:`TypeError` exception is raised. Unicode objects are
+   converted to C strings using the default encoding.  If this conversion fails, a
+   :exc:`UnicodeError` is raised.
+
+``s#`` (string, Unicode or any read buffer compatible 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.  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.
+
+``y`` (bytes object) [const char \*]
+   This variant on ``s`` convert a Python bytes object to a C pointer to a
+   character string. The bytes object must not contain embedded NUL bytes; if it
+   does, a :exc:`TypeError` exception is raised.
+
+``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.
+
+``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 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 :ctype:`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 :ctype:`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 :ctype:`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 :ctype:`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.
+
+   :cfunc:`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 :cfunc:`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
+   :ctype:`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 :ctype:`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
+   :cfunc:`PyMem_Free` to free the allocated buffer after usage.
+
+   If *\*buffer* points to a non-*NULL* pointer (an already allocated buffer),
+   :cfunc:`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#`` (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) [char]
+   Convert a Python integer to a tiny int, stored in a C :ctype:`char`.
+
+``B`` (integer) [unsigned char]
+   Convert a Python integer to a tiny int without overflow checking, stored in a C
+   :ctype:`unsigned char`.
+
+``h`` (integer) [short int]
+   Convert a Python integer to a C :ctype:`short int`.
+
+``H`` (integer) [unsigned short int]
+   Convert a Python integer to a C :ctype:`unsigned short int`, without overflow
+   checking.
+
+``i`` (integer) [int]
+   Convert a Python integer to a plain C :ctype:`int`.
+
+``I`` (integer) [unsigned int]
+   Convert a Python integer to a C :ctype:`unsigned int`, without overflow
+   checking.
+
+``l`` (integer) [long int]
+   Convert a Python integer to a C :ctype:`long int`.
+
+``k`` (integer) [unsigned long]
+   Convert a Python integer to a C :ctype:`unsigned long` without
+   overflow checking.
+
+``L`` (integer) [PY_LONG_LONG]
+   Convert a Python integer to a C :ctype:`long long`.  This format is only
+   available on platforms that support :ctype:`long long` (or :ctype:`_int64` on
+   Windows).
+
+``K`` (integer) [unsigned PY_LONG_LONG]
+   Convert a Python integer to a C :ctype:`unsigned long long`
+   without overflow checking.  This format is only available on platforms that
+   support :ctype:`unsigned long long` (or :ctype:`unsigned _int64` on Windows).
+
+``n`` (integer) [Py_ssize_t]
+   Convert a Python integer to a C :ctype:`Py_ssize_t`.
+
+``c`` (string of length 1) [char]
+   Convert a Python character, represented as a string of length 1, to a C
+   :ctype:`char`.
+
+``f`` (float) [float]
+   Convert a Python floating point number to a C :ctype:`float`.
+
+``d`` (float) [double]
+   Convert a Python floating point number to a C :ctype:`double`.
+
+``D`` (complex) [Py_complex]
+   Convert a Python complex number to a C :ctype:`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 :ctype:`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 :ctype:`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
+   :ctype:`void\*` argument that was passed to the :cfunc:`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.
+
+``S`` (string) [PyStringObject \*]
+   Like ``O`` but requires that the Python object is a string object.  Raises
+   :exc:`TypeError` if the object is not a string object.  The C variable may also
+   be declared as :ctype:`PyObject\*`.
+
+``U`` (Unicode string) [PyUnicodeObject \*]
+   Like ``O`` but requires that the Python object is a Unicode object.  Raises
+   :exc:`TypeError` if the object is not a Unicode object.  The C variable may also
+   be declared as :ctype:`PyObject\*`.
+
+``t#`` (read-only character buffer) [char \*, int]
+   Like ``s#``, but accepts any object which implements the read-only buffer
+   interface.  The :ctype:`char\*` variable is set to point to the first byte of
+   the buffer, and the :ctype:`int` is set to the length of the buffer.  Only
+   single-segment buffer objects are accepted; :exc:`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;
+   :exc:`TypeError` is raised for all others.
+
+``w#`` (read-write character buffer) [char \*, int]
+   Like ``s#``, but accepts any object which implements the read-write buffer
+   interface.  The :ctype:`char \*` variable is set to point to the first byte of
+   the buffer, and the :ctype:`int` is set to the length of the buffer.  Only
+   single-segment buffer objects are accepted; :exc:`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 :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,
+   :cfunc:`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
+   :cfunc:`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.  Clearly, ``:`` 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 :cfunc:`PyArg_Parse\*` functions
+return true, otherwise they return false and raise an appropriate exception.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)
+
+   Identical to :cfunc:`PyArg_ParseTuple`, except that it accepts a va_list rather
+   than a variable number of arguments.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)
+
+   Identical to :cfunc:`PyArg_ParseTupleAndKeywords`, except that it accepts a
+   va_list rather than a variable number of arguments.
+
+
+.. XXX deprecated, will be removed
+.. cfunction:: 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.
+
+
+.. cfunction:: 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
+   :ctype:`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 :cfunc:`PyArg_UnpackTuple` in this example is entirely equivalent to
+   this call to :cfunc:`PyArg_ParseTuple`::
+
+      PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
+
+
+.. cfunction:: PyObject* Py_BuildValue(const char *format, ...)
+
+   Create a new value based on a format string similar to those accepted by the
+   :cfunc:`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.
+
+   :cfunc:`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
+   :cfunc:`Py_BuildValue`.  In other words, if your code invokes :cfunc:`malloc`
+   and passes the allocated memory to :cfunc:`Py_BuildValue`, your code is
+   responsible for calling :cfunc:`free` for that memory once
+   :cfunc:`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.
+
+   ``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 :ctype:`int` to a Python integer object.
+
+   ``b`` (integer) [char]
+      Convert a plain C :ctype:`char` to a Python integer object.
+
+   ``h`` (integer) [short int]
+      Convert a plain C :ctype:`short int` to a Python integer object.
+
+   ``l`` (integer) [long int]
+      Convert a C :ctype:`long int` to a Python integer object.
+
+   ``B`` (integer) [unsigned char]
+      Convert a C :ctype:`unsigned char` to a Python integer object.
+
+   ``H`` (integer) [unsigned short int]
+      Convert a C :ctype:`unsigned short int` to a Python integer object.
+
+   ``I`` (integer/long) [unsigned int]
+      Convert a C :ctype:`unsigned int` to a Python long integer object.
+
+   ``k`` (integer/long) [unsigned long]
+      Convert a C :ctype:`unsigned long` to a Python long integer object.
+
+   ``L`` (long) [PY_LONG_LONG]
+      Convert a C :ctype:`long long` to a Python integer object. Only available
+      on platforms that support :ctype:`long long`.
+
+   ``K`` (long) [unsigned PY_LONG_LONG]
+      Convert a C :ctype:`unsigned long long` to a Python integer object. Only
+      available on platforms that support :ctype:`unsigned long long`.
+
+   ``n`` (int) [Py_ssize_t]
+      Convert a C :ctype:`Py_ssize_t` to a Python integer.
+
+   ``c`` (string of length 1) [char]
+      Convert a C :ctype:`int` representing a character to a Python string of length
+      1.
+
+   ``d`` (float) [double]
+      Convert a C :ctype:`double` to a Python floating point number.
+
+   ``f`` (float) [float]
+      Same as ``d``.
+
+   ``D`` (complex) [Py_complex \*]
+      Convert a C :ctype:`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, :cfunc:`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 :ctype:`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 :exc:`SystemError` exception is
+   set and *NULL* returned.