/Doc/c-api/arg.rst
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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 (or :ctype:`Py_ssize_t`, see below)]
- 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.
- Starting with Python 2.5 the type of the length argument can be controlled
- by defining the macro :cmacro:`PY_SSIZE_T_CLEAN` before including
- :file:`Python.h`. If the macro is defined, length is a :ctype:`Py_ssize_t`
- rather than an int.
- ``s*`` (string, Unicode, or any buffer compatible object) [Py_buffer \*]
- Similar to ``s#``, this code fills a Py_buffer structure provided by the
- caller. The buffer gets 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.
- .. versionadded:: 2.6
- ``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``.
- ``z*`` (string or ``None`` or any buffer compatible object) [Py_buffer*]
- This is to ``s*`` as ``z`` is to ``s``.
- .. versionadded:: 2.6
- ``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) [unsigned char]
- Convert a nonnegative Python integer to an unsigned tiny int, stored in a C
- :ctype:`unsigned 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 \*, Py_ssize_t]
- 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:`Py_ssize_t` 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 byte-oriented buffer) [Py_buffer \*]
- This is to ``w`` what ``s*`` is to ``s``.
- .. versionadded:: 2.6
- ``(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. ``:`` 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
- .. versionchanged:: 2.5
- This function used an :ctype:`int` type for *min* and *max*. This might
- require changes in your code for properly supporting 64-bit systems.
- .. 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.
- .. cfunction:: PyObject* Py_VaBuildValue(const char *format, va_list vargs)
- Identical to :cfunc:`Py_BuildValue`, except that it accepts a va_list
- rather than a variable number of arguments.