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, :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.

``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.

``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`.

   .. versionadded:: 2.3

``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.

   .. versionadded:: 2.3

``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.

   .. versionadded:: 2.3

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

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

   .. versionadded:: 2.3

``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 or long 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).

   .. versionadded:: 2.3

``n`` (integer) [Py_ssize_t]
   Convert a Python integer or long integer to a C :ctype:`Py_ssize_t`.

   .. versionadded:: 2.5

``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 and leave the content of *address* unmodified.

``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.

   .. note::

      Prior to Python version 1.5.2, this format specifier only accepted a tuple
      containing the individual parameters, not an arbitrary sequence.  Code which
      previously caused :exc:`TypeError` to be raised here may now proceed without an
      exception.  This is not expected to be a problem for existing code.

It is possible to pass Python long integers where integers are requested;
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. When the
:cfunc:`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.


.. 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.


.. 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)

   .. versionadded:: 2.2


.. 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.

   ``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 integer object or a Python long
      integer object, if it is larger than ``sys.maxint``.

   ``k`` (integer/long) [unsigned long]
      Convert a C :ctype:`unsigned long` to a Python integer object or a Python long
      integer object, if it is larger than ``sys.maxint``.

   ``L`` (long) [PY_LONG_LONG]
      Convert a C :ctype:`long long` to a Python long 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 long 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 or long integer.

      .. versionadded:: 2.5

   ``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.