| .. 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. |
| |
| ``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. |
| |
| ``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. |
| |
| ``y*`` (bytes object) [Py_buffer \*] |
| This is to ``s*`` as ``y`` is to ``s``. |
| |
| ``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``. |
| |
| ``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 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. |
| |
| ``w*`` (read-write byte-oriented buffer) [Py_buffer \*] |
| This is to ``w`` what ``s*`` is to ``s``. |
| |
| ``(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. 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. |
| |
| |
| .. 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. |