c
These functions are useful when creating your own extensions functions and methods. Additional information and examples are available in extending-index.
The first three of these functions described, :cPyArg_ParseTuple, :cPyArg_ParseTupleAndKeywords, and :cPyArg_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 accessing an object as a contiguous chunk of memory. You don't have to provide raw storage for the returned unicode or bytes area.
Unless otherwise stated, buffers are not NUL-terminated.
There are three ways strings and buffers can be converted to C:
- Formats such as
y*ands*fill a :cPy_bufferstructure. This locks the underlying buffer so that the caller can subsequently use the buffer even inside a :cPy_BEGIN_ALLOW_THREADSblock without the risk of mutable data being resized or destroyed. As a result, you have to call :cPyBuffer_Releaseafter you have finished processing the data (or in any early abort case). - The
es,es#,etandet#formats allocate the result buffer. You have to call :cPyMem_Freeafter you have finished processing the data (or in any early abort case). Other formats take a
stror a read-onlybytes-like object, such asbytes, and provide aconst char *pointer to its buffer. In this case the buffer is "borrowed": it is managed by the corresponding Python object, and shares the lifetime of this object. You won't have to release any memory yourself.To ensure that the underlying buffer may be safely borrowed, the object's :c
PyBufferProcs.bf_releasebufferfield must beNULL. This disallows common mutable objects such asbytearray, but also some read-only objects such asmemoryviewofbytes.Besides this
bf_releasebufferrequirement, there is no check to verify whether the input object is immutable (e.g. whether it would honor a request for a writable buffer, or whether another thread can mutate the data).
Note
For all # variants of formats (s#, y#, etc.), the macro :cPY_SSIZE_T_CLEAN must be defined before including Python.h. On Python 3.9 and older, the type of the length argument is :cPy_ssize_t if the :cPY_SSIZE_T_CLEAN macro is defined, or int otherwise.
s(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 null code points; if it does, a
ValueErrorexception is raised. Unicode objects are converted to C strings using'utf-8'encoding. If this conversion fails, aUnicodeErroris raised.Note
This format does not accept
bytes-like objects <bytes-like object>. If you want to accept filesystem paths and convert them to C character strings, it is preferable to use theO&format with :cPyUnicode_FSConverteras converter.3.5 Previously,
TypeErrorwas raised when embedded null code points were encountered in the Python string.s*(strorbytes-like object) [Py_buffer]This format accepts Unicode objects as well as bytes-like objects. It fills a :c
Py_bufferstructure provided by the caller. In this case the resulting C string may contain embedded NUL bytes. Unicode objects are converted to C strings using'utf-8'encoding.s#(str, read-onlybytes-like object) [const char *, :cPy_ssize_t]Like
s*, except that it provides aborrowed buffer <c-arg-borrowed-buffer>. 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(strorNone) [const char *]Like
s, but the Python object may also beNone, in which case the C pointer is set toNULL.z*(str,bytes-like objectorNone) [Py_buffer]Like
s*, but the Python object may also beNone, in which case thebufmember of the :cPy_bufferstructure is set toNULL.z#(str, read-onlybytes-like objectorNone) [const char *, :cPy_ssize_t]Like
s#, but the Python object may also beNone, in which case the C pointer is set toNULL.y(read-onlybytes-like object) [const char *]This format converts a bytes-like object to a C pointer to a
borrowed <c-arg-borrowed-buffer>character string; it does not accept Unicode objects. The bytes buffer must not contain embedded null bytes; if it does, aValueErrorexception is raised.3.5 Previously,
TypeErrorwas raised when embedded null bytes were encountered in the bytes buffer.y*(bytes-like object) [Py_buffer]This variant on
s*doesn't accept Unicode objects, only bytes-like objects. This is the recommended way to accept binary data.y#(read-onlybytes-like object) [const char *, :cPy_ssize_t]This variant on
s#doesn't accept Unicode objects, only bytes-like objects.S(bytes) [PyBytesObject *]Requires that the Python object is a
bytesobject, without attempting any conversion. RaisesTypeErrorif the object is not a bytes object. The C variable may also be declared as :cPyObject*.Y(bytearray) [PyByteArrayObject *]Requires that the Python object is a
bytearrayobject, without attempting any conversion. RaisesTypeErrorif the object is not abytearrayobject. The C variable may also be declared as :cPyObject*.U(str) [PyObject *]Requires that the Python object is a Unicode object, without attempting any conversion. Raises
TypeErrorif the object is not a Unicode object. The C variable may also be declared as :cPyObject*.w*(read-writebytes-like object) [Py_buffer]This format accepts any object which implements the read-write buffer interface. It fills a :c
Py_bufferstructure provided by the caller. The buffer may contain embedded null bytes. The caller have to call :cPyBuffer_Releasewhen it is done with the buffer.es(str) [const char *encoding, char **buffer]This variant on
sis 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
const char*which points to the name of an encoding as a NUL-terminated string, orNULL, 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 :cchar**; 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
PyArg_ParseTuplewill 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 :cPyMem_Freeto free the allocated buffer after use.et(str,bytesorbytearray) [const char *encoding, char **buffer]Same as
esexcept 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#(str) [const char *encoding, char **buffer, :cPy_ssize_t*buffer_length]This variant on
s#is used for encoding Unicode into a character buffer. Unlike theesformat, 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
const char*which points to the name of an encoding as a NUL-terminated string, orNULL, 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 :cchar**; 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
NULLpointer, 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 :cPyMem_Freeto free the allocated buffer after usage.If *buffer points to a non-
NULLpointer (an already allocated buffer), :cPyArg_ParseTuplewill 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, aValueErrorwill be set.In both cases, *buffer_length is set to the length of the encoded data without the trailing NUL byte.
et#(str,bytesorbytearray) [const char *encoding, char **buffer, :cPy_ssize_t*buffer_length]Same as
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.
3.12 u, u#, Z, and Z# are removed because they used a legacy Py_UNICODE* representation.
b(int) [unsigned char]Convert a nonnegative Python integer to an unsigned tiny int, stored in a C :c
unsigned char.B(int) [unsigned char]Convert a Python integer to a tiny int without overflow checking, stored in a C :c
unsigned char.h(int) [short int]Convert a Python integer to a C :c
short int.H(int) [unsigned short int]Convert a Python integer to a C :c
unsigned short int, without overflow checking.i(int) [int]Convert a Python integer to a plain C :c
int.I(int) [unsigned int]Convert a Python integer to a C :c
unsigned int, without overflow checking.l(int) [long int]Convert a Python integer to a C :c
long int.k(int) [unsigned long]Convert a Python integer to a C :c
unsigned longwithout overflow checking.L(int) [long long]Convert a Python integer to a C :c
long long.K(int) [unsigned long long]Convert a Python integer to a C :c
unsigned long longwithout overflow checking.n(int) [:cPy_ssize_t]Convert a Python integer to a C :c
Py_ssize_t.c(bytesorbytearrayof length 1) [char]Convert a Python byte, represented as a
bytesorbytearrayobject of length 1, to a C :cchar.3.3 Allow
bytearrayobjects.C(strof length 1) [int]Convert a Python character, represented as a
strobject of length 1, to a C :cint.f(float) [float]Convert a Python floating point number to a C :c
float.d(float) [double]Convert a Python floating point number to a C :c
double.D(complex) [Py_complex]Convert a Python complex number to a C :c
Py_complexstructure.
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 :cPyObject*) into which the object pointer is stored. If the Python object does not have the required type,TypeErroris 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
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
void*argument that was passed to thePyArg_Parse*function. The returned status should be1for a successful conversion and0if 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 beNULL; address will have the same value as in the original call.3.1
Py_CLEANUP_SUPPORTEDwas added.p(bool) [int]Tests the value passed in for truth (a boolean predicate) and converts the result to its equivalent C true/false integer value. Sets the int to
1if the expression was true and0if it was false. This accepts any valid Python value. Seetruthfor more information about how Python tests values for truth.3.3
(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 :cLONG_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, :c
PyArg_ParseTupledoes not touch the contents of the corresponding C variable(s).$:c
PyArg_ParseTupleAndKeywordsonly: Indicates that the remaining arguments in the Python argument list are keyword-only. Currently, all keyword-only arguments must also be optional arguments, so|must always be specified before$in the format string.3.3
: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 :c
PyArg_ParseTupleraises).;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 PyArg_Parse* functions return true, otherwise they return false and raise an appropriate exception. When the 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.