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.. highlightlang:: c
These functions are useful when creating your own extensions functions and methods. Additional information and examples are available in :ref:`extending-index`.
The first three of these functions described, :c:func:`PyArg_ParseTuple`, :c:func:`PyArg_ParseTupleAndKeywords`, and :c:func:`PyArg_Parse`, all use format strings which are used to tell the function about the expected arguments. The format strings use the same syntax for each of these functions.
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.
These formats allow to access an object as a contiguous chunk of memory.
You don't have to provide raw storage for the returned unicode or bytes
area. Also, you won't have to release any memory yourself, except with the
es
, es#
, et
and et#
formats.
However, when a :c:type:`Py_buffer` structure gets filled, the underlying buffer is locked so that the caller can subsequently use the buffer even inside a :c:type:`Py_BEGIN_ALLOW_THREADS` block without the risk of mutable data being resized or destroyed. As a result, you have to call :c:func:`PyBuffer_Release` after you have finished processing the data (or in any early abort case).
Unless otherwise stated, buffers are not NUL-terminated.
Note
For all #
variants of formats (s#
, y#
, etc.), the type of
the length argument (int or :c:type:`Py_ssize_t`) is controlled by
defining the macro :c:macro:`PY_SSIZE_T_CLEAN` before including
:file:`Python.h`. If the macro was defined, length is a
:c:type:`Py_ssize_t` rather than an :c:type:`int`. This behavior will change
in a future Python version to only support :c:type:`Py_ssize_t` and
drop :c:type:`int` support. It is best to always define :c:macro:`PY_SSIZE_T_CLEAN`.
s
(:class:`str`) [const char *]Convert a Unicode object to a C pointer to a character string.
A pointer to an existing string is stored in the character pointer
variable whose address you pass. The C string is NUL-terminated.
The Python string must not contain embedded NUL bytes; if it does,
a :exc:`TypeError` exception is raised. Unicode objects are converted
to C strings using 'utf-8'
encoding. If this conversion fails, a
:exc:`UnicodeError` is raised.
Note
This format does not accept bytes-like objects. If you want to accept
filesystem paths and convert them to C character strings, it is
preferable to use the O&
format with :c:func:`PyUnicode_FSConverter`
as converter.
s*
(:class:`str`, :class:`bytes`, :class:`bytearray` or buffer compatible object) [Py_buffer]'utf-8'
encoding.s#
(:class:`str`, :class:`bytes` or read-only buffer compatible object) [const char *, int or :c:type:`Py_ssize_t`]s*
, except that it doesn't accept mutable buffer-like objects
such as :class:`bytearray`. The result is stored into two C variables,
the first one a pointer to a C string, the second one its length.
The string may contain embedded null bytes. Unicode objects are converted
to C strings using 'utf-8'
encoding.z
(:class:`str` or None
) [const char *]s
, but the Python object may also be None
, in which case the C
pointer is set to NULL.z*
(:class:`str`, :class:`bytes`, :class:`bytearray`, buffer compatible object or None
) [Py_buffer]s*
, but the Python object may also be None
, in which case the
buf
member of the :c:type:`Py_buffer` structure is set to NULL.z#
(:class:`str`, :class:`bytes`, read-only buffer compatible object or None
) [const char *, int]s#
, but the Python object may also be None
, in which case the C
pointer is set to NULL.y
(:class:`bytes`) [const char *]y*
(:class:`bytes`, :class:`bytearray` or buffer compatible object) [Py_buffer]s*
doesn't accept Unicode objects, only objects
supporting the buffer protocol. This is the recommended way to accept
binary data.
y#
(:class:`bytes`) [const char *, int]s#
doesn't accept Unicode objects, only bytes-like
objects.S
(:class:`bytes`) [PyBytesObject *]Y
(:class:`bytearray`) [PyByteArrayObject *]u
(:class:`str`) [Py_UNICODE *]Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of Unicode characters. You must pass the address of a :c:type:`Py_UNICODE` pointer variable, which will be filled with the pointer to an existing Unicode buffer. Please note that the width of a :c:type:`Py_UNICODE` character depends on compilation options (it is either 16 or 32 bits). The Python string must not contain embedded NUL characters; if it does, a :exc:`TypeError` exception is raised.
Note
Since u
doesn't give you back the length of the string, and it
may contain embedded NUL characters, it is recommended to use u#
or U
instead.
u#
(:class:`str`) [Py_UNICODE *, int]u
stores into two C variables, the first one a pointer to a
Unicode data buffer, the second one its length.Z
(:class:`str` or None
) [Py_UNICODE *]u
, but the Python object may also be None
, in which case the
:c:type:`Py_UNICODE` pointer is set to NULL.Z#
(:class:`str` or None
) [Py_UNICODE *, int]u#
, but the Python object may also be None
, in which case the
:c:type:`Py_UNICODE` pointer is set to NULL.U
(:class:`str`) [PyUnicodeObject *]w*
(:class:`bytearray` or read-write byte-oriented buffer) [Py_buffer]es
(:class:`str`) [const char *encoding, char **buffer]This variant on s
is used for encoding Unicode into a character buffer.
It only works for encoded data without embedded NUL bytes.
This format requires two arguments. The first is only used as input, and
must be a :c:type:`const char\*` which points to the name of an encoding as a
NUL-terminated string, or NULL, in which case 'utf-8'
encoding is used.
An exception is raised if the named encoding is not known to Python. The
second argument must be a :c:type:`char\*\*`; the value of the pointer it
references will be set to a buffer with the contents of the argument text.
The text will be encoded in the encoding specified by the first argument.
:c:func:`PyArg_ParseTuple` will allocate a buffer of the needed size, copy the encoded data into this buffer and adjust *buffer to reference the newly allocated storage. The caller is responsible for calling :c:func:`PyMem_Free` to free the allocated buffer after use.
et
(:class:`str`, :class:`bytes` or :class:`bytearray`) [const char *encoding, char **buffer]es
except that byte string objects are passed through without
recoding them. Instead, the implementation assumes that the byte string object uses
the encoding passed in as parameter.es#
(:class:`str`) [const char *encoding, char **buffer, int *buffer_length]This variant on s#
is used for encoding Unicode into a character buffer.
Unlike the es
format, this variant allows input data which contains NUL
characters.
It requires three arguments. The first is only used as input, and must be a
:c:type:`const char\*` which points to the name of an encoding as a
NUL-terminated string, or NULL, in which case 'utf-8'
encoding is used.
An exception is raised if the named encoding is not known to Python. The
second argument must be a :c:type:`char\*\*`; the value of the pointer it
references will be set to a buffer with the contents of the argument text.
The text will be encoded in the encoding specified by the first argument.
The third argument must be a pointer to an integer; the referenced integer
will be set to the number of bytes in the output buffer.
There are two modes of operation:
If *buffer points a NULL pointer, the function will allocate a buffer of the needed size, copy the encoded data into this buffer and set *buffer to reference the newly allocated storage. The caller is responsible for calling :c:func:`PyMem_Free` to free the allocated buffer after usage.
If *buffer points to a non-NULL pointer (an already allocated buffer), :c:func:`PyArg_ParseTuple` will use this location as the buffer and interpret the initial value of *buffer_length as the buffer size. It will then copy the encoded data into the buffer and NUL-terminate it. If the buffer is not large enough, a :exc:`ValueError` will be set.
In both cases, *buffer_length is set to the length of the encoded data without the trailing NUL byte.
et#
(:class:`str`, :class:`bytes` or :class:`bytearray`) [const char *encoding, char **buffer, int *buffer_length]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.b
(:class:`int`) [unsigned char]B
(:class:`int`) [unsigned char]h
(:class:`int`) [short int]H
(:class:`int`) [unsigned short int]i
(:class:`int`) [int]I
(:class:`int`) [unsigned int]l
(:class:`int`) [long int]k
(:class:`int`) [unsigned long]L
(:class:`int`) [PY_LONG_LONG]K
(:class:`int`) [unsigned PY_LONG_LONG]n
(:class:`int`) [Py_ssize_t]c
(:class:`bytes` of length 1) [char]C
(:class:`str` of length 1) [int]f
(:class:`float`) [float]d
(:class:`float`) [double]D
(:class:`complex`) [Py_complex]O
(object) [PyObject *]O!
(object) [typeobject, PyObject *]O
, but
takes two C arguments: the first is the address of a Python type object, the
second is the address of the C variable (of type :c:type:`PyObject\*`) into which
the object pointer is stored. If the Python object does not have the required
type, :exc:`TypeError` is raised.O&
(object) [converter, anything]Convert a Python object to a C variable through a converter function. This takes two arguments: the first is a function, the second is the address of a C variable (of arbitrary type), converted to :c:type:`void \*`. The converter function in turn is called as follows:
status = converter(object, address);
where object is the Python object to be converted and address is the
:c:type:`void\*` argument that was passed to the :c:func:`PyArg_Parse\*` function.
The returned status should be 1
for a successful conversion and 0
if
the conversion has failed. When the conversion fails, the converter function
should raise an exception and leave the content of address unmodified.
If the converter returns Py_CLEANUP_SUPPORTED
, it may get called a
second time if the argument parsing eventually fails, giving the converter a
chance to release any memory that it had already allocated. In this second
call, the object parameter will be NULL; address will have the same value
as in the original call.
.. versionchanged:: 3.1 ``Py_CLEANUP_SUPPORTED`` was added.
(items)
(:class:`tuple`) [matching-items]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:
|
:
;
:
and ;
mutually exclude each other.Note that any Python object references which are provided to the caller are borrowed references; do not decrement their reference count!
Additional arguments passed to these functions must be addresses of variables whose type is determined by the format string; these are used to store values from the input tuple. There are a few cases, as described in the list of format units above, where these parameters are used as input values; they should match what is specified for the corresponding format unit in that case.
For the conversion to succeed, the arg object must match the format and the format must be exhausted. On success, the :c:func:`PyArg_Parse\*` functions return true, otherwise they return false and raise an appropriate exception. When the :c:func:`PyArg_Parse\*` functions fail due to conversion failure in one of the format units, the variables at the addresses corresponding to that and the following format units are left untouched.
.. c:function:: int PyArg_ParseTuple(PyObject *args, const char *format, ...) Parse the parameters of a function that takes only positional parameters into local variables. Returns true on success; on failure, it returns false and raises the appropriate exception.
.. c:function:: int PyArg_VaParse(PyObject *args, const char *format, va_list vargs) Identical to :c:func:`PyArg_ParseTuple`, except that it accepts a va_list rather than a variable number of arguments.
.. c:function:: int PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], ...) Parse the parameters of a function that takes both positional and keyword parameters into local variables. Returns true on success; on failure, it returns false and raises the appropriate exception.
.. c:function:: int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs) Identical to :c:func:`PyArg_ParseTupleAndKeywords`, except that it accepts a va_list rather than a variable number of arguments.
.. c:function:: int PyArg_ValidateKeywordArguments(PyObject *) Ensure that the keys in the keywords argument dictionary are strings. This is only needed if :c:func:`PyArg_ParseTupleAndKeywords` is not used, since the latter already does this check. .. versionadded:: 3.2
.. c:function:: int PyArg_Parse(PyObject *args, const char *format, ...) Function used to deconstruct the argument lists of "old-style" functions --- these are functions which use the :const:`METH_OLDARGS` parameter parsing method. This is not recommended for use in parameter parsing in new code, and most code in the standard interpreter has been modified to no longer use this for that purpose. It does remain a convenient way to decompose other tuples, however, and may continue to be used for that purpose.
.. c:function:: int PyArg_UnpackTuple(PyObject *args, const char *name, Py_ssize_t min, Py_ssize_t max, ...) A simpler form of parameter retrieval which does not use a format string to specify the types of the arguments. Functions which use this method to retrieve their parameters should be declared as :const:`METH_VARARGS` in function or method tables. The tuple containing the actual parameters should be passed as *args*; it must actually be a tuple. The length of the tuple must be at least *min* and no more than *max*; *min* and *max* may be equal. Additional arguments must be passed to the function, each of which should be a pointer to a :c:type:`PyObject\*` variable; these will be filled in with the values from *args*; they will contain borrowed references. The variables which correspond to optional parameters not given by *args* will not be filled in; these should be initialized by the caller. This function returns true on success and false if *args* is not a tuple or contains the wrong number of elements; an exception will be set if there was a failure. This is an example of the use of this function, taken from the sources for the :mod:`_weakref` helper module for weak references:: static PyObject * weakref_ref(PyObject *self, PyObject *args) { PyObject *object; PyObject *callback = NULL; PyObject *result = NULL; if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) { result = PyWeakref_NewRef(object, callback); } return result; } The call to :c:func:`PyArg_UnpackTuple` in this example is entirely equivalent to this call to :c:func:`PyArg_ParseTuple`:: PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
.. c:function:: PyObject* Py_BuildValue(const char *format, ...) Create a new value based on a format string similar to those accepted by the :c:func:`PyArg_Parse\*` family of functions and a sequence of values. Returns the value or *NULL* in the case of an error; an exception will be raised if *NULL* is returned. :c:func:`Py_BuildValue` does not always build a tuple. It builds a tuple only if its format string contains two or more format units. If the format string is empty, it returns ``None``; if it contains exactly one format unit, it returns whatever object is described by that format unit. To force it to return a tuple of size 0 or one, parenthesize the format string. When memory buffers are passed as parameters to supply data to build objects, as for the ``s`` and ``s#`` formats, the required data is copied. Buffers provided by the caller are never referenced by the objects created by :c:func:`Py_BuildValue`. In other words, if your code invokes :c:func:`malloc` and passes the allocated memory to :c:func:`Py_BuildValue`, your code is responsible for calling :c:func:`free` for that memory once :c:func:`Py_BuildValue` returns. In the following description, the quoted form is the format unit; the entry in (round) parentheses is the Python object type that the format unit will return; and the entry in [square] brackets is the type of the C value(s) to be passed. The characters space, tab, colon and comma are ignored in format strings (but not within format units such as ``s#``). This can be used to make long format strings a tad more readable. ``s`` (:class:`str` or ``None``) [char \*] Convert a null-terminated C string to a Python :class:`str` object using ``'utf-8'`` encoding. If the C string pointer is *NULL*, ``None`` is used. ``s#`` (:class:`str` or ``None``) [char \*, int] Convert a C string and its length to a Python :class:`str` object using ``'utf-8'`` encoding. If the C string pointer is *NULL*, the length is ignored and ``None`` is returned. ``y`` (:class:`bytes`) [char \*] This converts a C string to a Python :func:`bytes` object. If the C string pointer is *NULL*, ``None`` is returned. ``y#`` (:class:`bytes`) [char \*, int] This converts a C string and its lengths to a Python object. If the C string pointer is *NULL*, ``None`` is returned. ``z`` (:class:`str` or ``None``) [char \*] Same as ``s``. ``z#`` (:class:`str` or ``None``) [char \*, int] Same as ``s#``. ``u`` (:class:`str`) [Py_UNICODE \*] Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python Unicode object. If the Unicode buffer pointer is *NULL*, ``None`` is returned. ``u#`` (:class:`str`) [Py_UNICODE \*, int] Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is *NULL*, the length is ignored and ``None`` is returned. ``U`` (:class:`str` or ``None``) [char \*] Same as ``s``. ``U#`` (:class:`str` or ``None``) [char \*, int] Same as ``s#``. ``i`` (:class:`int`) [int] Convert a plain C :c:type:`int` to a Python integer object. ``b`` (:class:`int`) [char] Convert a plain C :c:type:`char` to a Python integer object. ``h`` (:class:`int`) [short int] Convert a plain C :c:type:`short int` to a Python integer object. ``l`` (:class:`int`) [long int] Convert a C :c:type:`long int` to a Python integer object. ``B`` (:class:`int`) [unsigned char] Convert a C :c:type:`unsigned char` to a Python integer object. ``H`` (:class:`int`) [unsigned short int] Convert a C :c:type:`unsigned short int` to a Python integer object. ``I`` (:class:`int`) [unsigned int] Convert a C :c:type:`unsigned int` to a Python integer object. ``k`` (:class:`int`) [unsigned long] Convert a C :c:type:`unsigned long` to a Python integer object. ``L`` (:class:`int`) [PY_LONG_LONG] Convert a C :c:type:`long long` to a Python integer object. Only available on platforms that support :c:type:`long long` (or :c:type:`_int64` on Windows). ``K`` (:class:`int`) [unsigned PY_LONG_LONG] Convert a C :c:type:`unsigned long long` to a Python integer object. Only available on platforms that support :c:type:`unsigned long long` (or :c:type:`unsigned _int64` on Windows). ``n`` (:class:`int`) [Py_ssize_t] Convert a C :c:type:`Py_ssize_t` to a Python integer. ``c`` (:class:`bytes` of length 1) [char] Convert a C :c:type:`int` representing a byte to a Python :class:`bytes` object of length 1. ``C`` (:class:`str` of length 1) [int] Convert a C :c:type:`int` representing a character to Python :class:`str` object of length 1. ``d`` (:class:`float`) [double] Convert a C :c:type:`double` to a Python floating point number. ``f`` (:class:`float`) [float] Convert a C :c:type:`float` to a Python floating point number. ``D`` (:class:`complex`) [Py_complex \*] Convert a C :c:type:`Py_complex` structure to a Python complex number. ``O`` (object) [PyObject \*] Pass a Python object untouched (except for its reference count, which is incremented by one). If the object passed in is a *NULL* pointer, it is assumed that this was caused because the call producing the argument found an error and set an exception. Therefore, :c:func:`Py_BuildValue` will return *NULL* but won't raise an exception. If no exception has been raised yet, :exc:`SystemError` is set. ``S`` (object) [PyObject \*] Same as ``O``. ``N`` (object) [PyObject \*] Same as ``O``, except it doesn't increment the reference count on the object. Useful when the object is created by a call to an object constructor in the argument list. ``O&`` (object) [*converter*, *anything*] Convert *anything* to a Python object through a *converter* function. The function is called with *anything* (which should be compatible with :c:type:`void \*`) as its argument and should return a "new" Python object, or *NULL* if an error occurred. ``(items)`` (:class:`tuple`) [*matching-items*] Convert a sequence of C values to a Python tuple with the same number of items. ``[items]`` (:class:`list`) [*matching-items*] Convert a sequence of C values to a Python list with the same number of items. ``{items}`` (:class:`dict`) [*matching-items*] Convert a sequence of C values to a Python dictionary. Each pair of consecutive C values adds one item to the dictionary, serving as key and value, respectively. If there is an error in the format string, the :exc:`SystemError` exception is set and *NULL* returned.
.. c:function:: PyObject* Py_VaBuildValue(const char *format, va_list vargs) Identical to :c:func:`Py_BuildValue`, except that it accepts a va_list rather than a variable number of arguments.
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