5 Parsing arguments and building values
6 =====================================
8 These functions are useful when creating your own extensions functions and
9 methods. Additional information and examples are available in
10 :ref:`extending-index`.
12 The first three of these functions described, :cfunc:`PyArg_ParseTuple`,
13 :cfunc:`PyArg_ParseTupleAndKeywords`, and :cfunc:`PyArg_Parse`, all use *format
14 strings* which are used to tell the function about the expected arguments. The
15 format strings use the same syntax for each of these functions.
17 A format string consists of zero or more "format units." A format unit
18 describes one Python object; it is usually a single character or a parenthesized
19 sequence of format units. With a few exceptions, a format unit that is not a
20 parenthesized sequence normally corresponds to a single address argument to
21 these functions. In the following description, the quoted form is the format
22 unit; the entry in (round) parentheses is the Python object type that matches
23 the format unit; and the entry in [square] brackets is the type of the C
24 variable(s) whose address should be passed.
26 ``s`` (string or Unicode object) [const char \*]
27 Convert a Python string or Unicode object to a C pointer to a character string.
28 You must not provide storage for the string itself; a pointer to an existing
29 string is stored into the character pointer variable whose address you pass.
30 The C string is NUL-terminated. The Python string must not contain embedded NUL
31 bytes; if it does, a :exc:`TypeError` exception is raised. Unicode objects are
32 converted to C strings using the default encoding. If this conversion fails, a
33 :exc:`UnicodeError` is raised.
35 ``s#`` (string, Unicode or any read buffer compatible object) [const char \*, int]
36 This variant on ``s`` stores into two C variables, the first one a pointer to a
37 character string, the second one its length. In this case the Python string may
38 contain embedded null bytes. Unicode objects pass back a pointer to the default
39 encoded string version of the object if such a conversion is possible. All
40 other read-buffer compatible objects pass back a reference to the raw internal
43 ``z`` (string or ``None``) [const char \*]
44 Like ``s``, but the Python object may also be ``None``, in which case the C
45 pointer is set to *NULL*.
47 ``z#`` (string or ``None`` or any read buffer compatible object) [const char \*, int]
48 This is to ``s#`` as ``z`` is to ``s``.
50 ``u`` (Unicode object) [Py_UNICODE \*]
51 Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of
52 16-bit Unicode (UTF-16) data. As with ``s``, there is no need to provide
53 storage for the Unicode data buffer; a pointer to the existing Unicode data is
54 stored into the :ctype:`Py_UNICODE` pointer variable whose address you pass.
56 ``u#`` (Unicode object) [Py_UNICODE \*, int]
57 This variant on ``u`` stores into two C variables, the first one a pointer to a
58 Unicode data buffer, the second one its length. Non-Unicode objects are handled
59 by interpreting their read-buffer pointer as pointer to a :ctype:`Py_UNICODE`
62 ``es`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
63 This variant on ``s`` is used for encoding Unicode and objects convertible to
64 Unicode into a character buffer. It only works for encoded data without embedded
67 This format requires two arguments. The first is only used as input, and
68 must be a :ctype:`const char\*` which points to the name of an encoding as a
69 NUL-terminated string, or *NULL*, in which case the default encoding is used.
70 An exception is raised if the named encoding is not known to Python. The
71 second argument must be a :ctype:`char\*\*`; the value of the pointer it
72 references will be set to a buffer with the contents of the argument text.
73 The text will be encoded in the encoding specified by the first argument.
75 :cfunc:`PyArg_ParseTuple` will allocate a buffer of the needed size, copy the
76 encoded data into this buffer and adjust *\*buffer* to reference the newly
77 allocated storage. The caller is responsible for calling :cfunc:`PyMem_Free` to
78 free the allocated buffer after use.
80 ``et`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
81 Same as ``es`` except that 8-bit string objects are passed through without
82 recoding them. Instead, the implementation assumes that the string object uses
83 the encoding passed in as parameter.
85 ``es#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer, int \*buffer_length]
86 This variant on ``s#`` is used for encoding Unicode and objects convertible to
87 Unicode into a character buffer. Unlike the ``es`` format, this variant allows
88 input data which contains NUL characters.
90 It requires three arguments. The first is only used as input, and must be a
91 :ctype:`const char\*` which points to the name of an encoding as a
92 NUL-terminated string, or *NULL*, in which case the default encoding is used.
93 An exception is raised if the named encoding is not known to Python. The
94 second argument must be a :ctype:`char\*\*`; the value of the pointer it
95 references will be set to a buffer with the contents of the argument text.
96 The text will be encoded in the encoding specified by the first argument.
97 The third argument must be a pointer to an integer; the referenced integer
98 will be set to the number of bytes in the output buffer.
100 There are two modes of operation:
102 If *\*buffer* points a *NULL* pointer, the function will allocate a buffer of
103 the needed size, copy the encoded data into this buffer and set *\*buffer* to
104 reference the newly allocated storage. The caller is responsible for calling
105 :cfunc:`PyMem_Free` to free the allocated buffer after usage.
107 If *\*buffer* points to a non-*NULL* pointer (an already allocated buffer),
108 :cfunc:`PyArg_ParseTuple` will use this location as the buffer and interpret the
109 initial value of *\*buffer_length* as the buffer size. It will then copy the
110 encoded data into the buffer and NUL-terminate it. If the buffer is not large
111 enough, a :exc:`ValueError` will be set.
113 In both cases, *\*buffer_length* is set to the length of the encoded data
114 without the trailing NUL byte.
116 ``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
117 Same as ``es#`` except that string objects are passed through without recoding
118 them. Instead, the implementation assumes that the string object uses the
119 encoding passed in as parameter.
121 ``b`` (integer) [char]
122 Convert a Python integer to a tiny int, stored in a C :ctype:`char`.
124 ``B`` (integer) [unsigned char]
125 Convert a Python integer to a tiny int without overflow checking, stored in a C
126 :ctype:`unsigned char`.
128 .. versionadded:: 2.3
130 ``h`` (integer) [short int]
131 Convert a Python integer to a C :ctype:`short int`.
133 ``H`` (integer) [unsigned short int]
134 Convert a Python integer to a C :ctype:`unsigned short int`, without overflow
137 .. versionadded:: 2.3
139 ``i`` (integer) [int]
140 Convert a Python integer to a plain C :ctype:`int`.
142 ``I`` (integer) [unsigned int]
143 Convert a Python integer to a C :ctype:`unsigned int`, without overflow
146 .. versionadded:: 2.3
148 ``l`` (integer) [long int]
149 Convert a Python integer to a C :ctype:`long int`.
151 ``k`` (integer) [unsigned long]
152 Convert a Python integer or long integer to a C :ctype:`unsigned long` without
155 .. versionadded:: 2.3
157 ``L`` (integer) [PY_LONG_LONG]
158 Convert a Python integer to a C :ctype:`long long`. This format is only
159 available on platforms that support :ctype:`long long` (or :ctype:`_int64` on
162 ``K`` (integer) [unsigned PY_LONG_LONG]
163 Convert a Python integer or long integer to a C :ctype:`unsigned long long`
164 without overflow checking. This format is only available on platforms that
165 support :ctype:`unsigned long long` (or :ctype:`unsigned _int64` on Windows).
167 .. versionadded:: 2.3
169 ``n`` (integer) [Py_ssize_t]
170 Convert a Python integer or long integer to a C :ctype:`Py_ssize_t`.
172 .. versionadded:: 2.5
174 ``c`` (string of length 1) [char]
175 Convert a Python character, represented as a string of length 1, to a C
178 ``f`` (float) [float]
179 Convert a Python floating point number to a C :ctype:`float`.
181 ``d`` (float) [double]
182 Convert a Python floating point number to a C :ctype:`double`.
184 ``D`` (complex) [Py_complex]
185 Convert a Python complex number to a C :ctype:`Py_complex` structure.
187 ``O`` (object) [PyObject \*]
188 Store a Python object (without any conversion) in a C object pointer. The C
189 program thus receives the actual object that was passed. The object's reference
190 count is not increased. The pointer stored is not *NULL*.
192 ``O!`` (object) [*typeobject*, PyObject \*]
193 Store a Python object in a C object pointer. This is similar to ``O``, but
194 takes two C arguments: the first is the address of a Python type object, the
195 second is the address of the C variable (of type :ctype:`PyObject\*`) into which
196 the object pointer is stored. If the Python object does not have the required
197 type, :exc:`TypeError` is raised.
199 ``O&`` (object) [*converter*, *anything*]
200 Convert a Python object to a C variable through a *converter* function. This
201 takes two arguments: the first is a function, the second is the address of a C
202 variable (of arbitrary type), converted to :ctype:`void \*`. The *converter*
203 function in turn is called as follows::
205 status = converter(object, address);
207 where *object* is the Python object to be converted and *address* is the
208 :ctype:`void\*` argument that was passed to the :cfunc:`PyArg_Parse\*` function.
209 The returned *status* should be ``1`` for a successful conversion and ``0`` if
210 the conversion has failed. When the conversion fails, the *converter* function
211 should raise an exception and leave the content of *address* unmodified.
213 ``S`` (string) [PyStringObject \*]
214 Like ``O`` but requires that the Python object is a string object. Raises
215 :exc:`TypeError` if the object is not a string object. The C variable may also
216 be declared as :ctype:`PyObject\*`.
218 ``U`` (Unicode string) [PyUnicodeObject \*]
219 Like ``O`` but requires that the Python object is a Unicode object. Raises
220 :exc:`TypeError` if the object is not a Unicode object. The C variable may also
221 be declared as :ctype:`PyObject\*`.
223 ``t#`` (read-only character buffer) [char \*, int]
224 Like ``s#``, but accepts any object which implements the read-only buffer
225 interface. The :ctype:`char\*` variable is set to point to the first byte of
226 the buffer, and the :ctype:`int` is set to the length of the buffer. Only
227 single-segment buffer objects are accepted; :exc:`TypeError` is raised for all
230 ``w`` (read-write character buffer) [char \*]
231 Similar to ``s``, but accepts any object which implements the read-write buffer
232 interface. The caller must determine the length of the buffer by other means,
233 or use ``w#`` instead. Only single-segment buffer objects are accepted;
234 :exc:`TypeError` is raised for all others.
236 ``w#`` (read-write character buffer) [char \*, int]
237 Like ``s#``, but accepts any object which implements the read-write buffer
238 interface. The :ctype:`char \*` variable is set to point to the first byte of
239 the buffer, and the :ctype:`int` is set to the length of the buffer. Only
240 single-segment buffer objects are accepted; :exc:`TypeError` is raised for all
243 ``(items)`` (tuple) [*matching-items*]
244 The object must be a Python sequence whose length is the number of format units
245 in *items*. The C arguments must correspond to the individual format units in
246 *items*. Format units for sequences may be nested.
250 Prior to Python version 1.5.2, this format specifier only accepted a tuple
251 containing the individual parameters, not an arbitrary sequence. Code which
252 previously caused :exc:`TypeError` to be raised here may now proceed without an
253 exception. This is not expected to be a problem for existing code.
255 It is possible to pass Python long integers where integers are requested;
256 however no proper range checking is done --- the most significant bits are
257 silently truncated when the receiving field is too small to receive the value
258 (actually, the semantics are inherited from downcasts in C --- your mileage may
261 A few other characters have a meaning in a format string. These may not occur
262 inside nested parentheses. They are:
265 Indicates that the remaining arguments in the Python argument list are optional.
266 The C variables corresponding to optional arguments should be initialized to
267 their default value --- when an optional argument is not specified,
268 :cfunc:`PyArg_ParseTuple` does not touch the contents of the corresponding C
272 The list of format units ends here; the string after the colon is used as the
273 function name in error messages (the "associated value" of the exception that
274 :cfunc:`PyArg_ParseTuple` raises).
277 The list of format units ends here; the string after the semicolon is used as
278 the error message *instead* of the default error message. Clearly, ``:`` and
279 ``;`` mutually exclude each other.
281 Note that any Python object references which are provided to the caller are
282 *borrowed* references; do not decrement their reference count!
284 Additional arguments passed to these functions must be addresses of variables
285 whose type is determined by the format string; these are used to store values
286 from the input tuple. There are a few cases, as described in the list of format
287 units above, where these parameters are used as input values; they should match
288 what is specified for the corresponding format unit in that case.
290 For the conversion to succeed, the *arg* object must match the format
291 and the format must be exhausted. On success, the
292 :cfunc:`PyArg_Parse\*` functions return true, otherwise they return
293 false and raise an appropriate exception. When the
294 :cfunc:`PyArg_Parse\*` functions fail due to conversion failure in one
295 of the format units, the variables at the addresses corresponding to that
296 and the following format units are left untouched.
299 .. cfunction:: int PyArg_ParseTuple(PyObject *args, const char *format, ...)
301 Parse the parameters of a function that takes only positional parameters into
302 local variables. Returns true on success; on failure, it returns false and
303 raises the appropriate exception.
306 .. cfunction:: int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)
308 Identical to :cfunc:`PyArg_ParseTuple`, except that it accepts a va_list rather
309 than a variable number of arguments.
312 .. cfunction:: int PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], ...)
314 Parse the parameters of a function that takes both positional and keyword
315 parameters into local variables. Returns true on success; on failure, it
316 returns false and raises the appropriate exception.
319 .. cfunction:: int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)
321 Identical to :cfunc:`PyArg_ParseTupleAndKeywords`, except that it accepts a
322 va_list rather than a variable number of arguments.
325 .. cfunction:: int PyArg_Parse(PyObject *args, const char *format, ...)
327 Function used to deconstruct the argument lists of "old-style" functions ---
328 these are functions which use the :const:`METH_OLDARGS` parameter parsing
329 method. This is not recommended for use in parameter parsing in new code, and
330 most code in the standard interpreter has been modified to no longer use this
331 for that purpose. It does remain a convenient way to decompose other tuples,
332 however, and may continue to be used for that purpose.
335 .. cfunction:: int PyArg_UnpackTuple(PyObject *args, const char *name, Py_ssize_t min, Py_ssize_t max, ...)
337 A simpler form of parameter retrieval which does not use a format string to
338 specify the types of the arguments. Functions which use this method to retrieve
339 their parameters should be declared as :const:`METH_VARARGS` in function or
340 method tables. The tuple containing the actual parameters should be passed as
341 *args*; it must actually be a tuple. The length of the tuple must be at least
342 *min* and no more than *max*; *min* and *max* may be equal. Additional
343 arguments must be passed to the function, each of which should be a pointer to a
344 :ctype:`PyObject\*` variable; these will be filled in with the values from
345 *args*; they will contain borrowed references. The variables which correspond
346 to optional parameters not given by *args* will not be filled in; these should
347 be initialized by the caller. This function returns true on success and false if
348 *args* is not a tuple or contains the wrong number of elements; an exception
349 will be set if there was a failure.
351 This is an example of the use of this function, taken from the sources for the
352 :mod:`_weakref` helper module for weak references::
355 weakref_ref(PyObject *self, PyObject *args)
358 PyObject *callback = NULL;
359 PyObject *result = NULL;
361 if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) {
362 result = PyWeakref_NewRef(object, callback);
367 The call to :cfunc:`PyArg_UnpackTuple` in this example is entirely equivalent to
368 this call to :cfunc:`PyArg_ParseTuple`::
370 PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
372 .. versionadded:: 2.2
375 .. cfunction:: PyObject* Py_BuildValue(const char *format, ...)
377 Create a new value based on a format string similar to those accepted by the
378 :cfunc:`PyArg_Parse\*` family of functions and a sequence of values. Returns
379 the value or *NULL* in the case of an error; an exception will be raised if
382 :cfunc:`Py_BuildValue` does not always build a tuple. It builds a tuple only if
383 its format string contains two or more format units. If the format string is
384 empty, it returns ``None``; if it contains exactly one format unit, it returns
385 whatever object is described by that format unit. To force it to return a tuple
386 of size 0 or one, parenthesize the format string.
388 When memory buffers are passed as parameters to supply data to build objects, as
389 for the ``s`` and ``s#`` formats, the required data is copied. Buffers provided
390 by the caller are never referenced by the objects created by
391 :cfunc:`Py_BuildValue`. In other words, if your code invokes :cfunc:`malloc`
392 and passes the allocated memory to :cfunc:`Py_BuildValue`, your code is
393 responsible for calling :cfunc:`free` for that memory once
394 :cfunc:`Py_BuildValue` returns.
396 In the following description, the quoted form is the format unit; the entry in
397 (round) parentheses is the Python object type that the format unit will return;
398 and the entry in [square] brackets is the type of the C value(s) to be passed.
400 The characters space, tab, colon and comma are ignored in format strings (but
401 not within format units such as ``s#``). This can be used to make long format
402 strings a tad more readable.
404 ``s`` (string) [char \*]
405 Convert a null-terminated C string to a Python object. If the C string pointer
406 is *NULL*, ``None`` is used.
408 ``s#`` (string) [char \*, int]
409 Convert a C string and its length to a Python object. If the C string pointer
410 is *NULL*, the length is ignored and ``None`` is returned.
412 ``z`` (string or ``None``) [char \*]
415 ``z#`` (string or ``None``) [char \*, int]
418 ``u`` (Unicode string) [Py_UNICODE \*]
419 Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python
420 Unicode object. If the Unicode buffer pointer is *NULL*, ``None`` is returned.
422 ``u#`` (Unicode string) [Py_UNICODE \*, int]
423 Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python
424 Unicode object. If the Unicode buffer pointer is *NULL*, the length is ignored
425 and ``None`` is returned.
427 ``i`` (integer) [int]
428 Convert a plain C :ctype:`int` to a Python integer object.
430 ``b`` (integer) [char]
431 Convert a plain C :ctype:`char` to a Python integer object.
433 ``h`` (integer) [short int]
434 Convert a plain C :ctype:`short int` to a Python integer object.
436 ``l`` (integer) [long int]
437 Convert a C :ctype:`long int` to a Python integer object.
439 ``B`` (integer) [unsigned char]
440 Convert a C :ctype:`unsigned char` to a Python integer object.
442 ``H`` (integer) [unsigned short int]
443 Convert a C :ctype:`unsigned short int` to a Python integer object.
445 ``I`` (integer/long) [unsigned int]
446 Convert a C :ctype:`unsigned int` to a Python integer object or a Python long
447 integer object, if it is larger than ``sys.maxint``.
449 ``k`` (integer/long) [unsigned long]
450 Convert a C :ctype:`unsigned long` to a Python integer object or a Python long
451 integer object, if it is larger than ``sys.maxint``.
453 ``L`` (long) [PY_LONG_LONG]
454 Convert a C :ctype:`long long` to a Python long integer object. Only available
455 on platforms that support :ctype:`long long`.
457 ``K`` (long) [unsigned PY_LONG_LONG]
458 Convert a C :ctype:`unsigned long long` to a Python long integer object. Only
459 available on platforms that support :ctype:`unsigned long long`.
461 ``n`` (int) [Py_ssize_t]
462 Convert a C :ctype:`Py_ssize_t` to a Python integer or long integer.
464 .. versionadded:: 2.5
466 ``c`` (string of length 1) [char]
467 Convert a C :ctype:`int` representing a character to a Python string of length
470 ``d`` (float) [double]
471 Convert a C :ctype:`double` to a Python floating point number.
473 ``f`` (float) [float]
476 ``D`` (complex) [Py_complex \*]
477 Convert a C :ctype:`Py_complex` structure to a Python complex number.
479 ``O`` (object) [PyObject \*]
480 Pass a Python object untouched (except for its reference count, which is
481 incremented by one). If the object passed in is a *NULL* pointer, it is assumed
482 that this was caused because the call producing the argument found an error and
483 set an exception. Therefore, :cfunc:`Py_BuildValue` will return *NULL* but won't
484 raise an exception. If no exception has been raised yet, :exc:`SystemError` is
487 ``S`` (object) [PyObject \*]
490 ``N`` (object) [PyObject \*]
491 Same as ``O``, except it doesn't increment the reference count on the object.
492 Useful when the object is created by a call to an object constructor in the
495 ``O&`` (object) [*converter*, *anything*]
496 Convert *anything* to a Python object through a *converter* function. The
497 function is called with *anything* (which should be compatible with :ctype:`void
498 \*`) as its argument and should return a "new" Python object, or *NULL* if an
501 ``(items)`` (tuple) [*matching-items*]
502 Convert a sequence of C values to a Python tuple with the same number of items.
504 ``[items]`` (list) [*matching-items*]
505 Convert a sequence of C values to a Python list with the same number of items.
507 ``{items}`` (dictionary) [*matching-items*]
508 Convert a sequence of C values to a Python dictionary. Each pair of consecutive
509 C values adds one item to the dictionary, serving as key and value,
512 If there is an error in the format string, the :exc:`SystemError` exception is
513 set and *NULL* returned.