Parsing arguments and building values
These functions are useful when creating your own extensions functions and
methods. Additional information and examples are available in
Extending and Embedding the Python Interpreter.
The first three of these functions described, PyArg_ParseTuple(),
PyArg_ParseTupleAndKeywords(), and 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.
Parsing arguments
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.
Strings and buffers
These formats do not expect you to provide raw storage for the returned string
or bytes. Also, you won’t have to release any memory yourself, except with
the es, es#, et and et# formats.
However, when a Py_buffer structure gets filled, the underlying
buffer is locked so that the caller can subsequently use the buffer even
inside a Py_BEGIN_ALLOW_THREADS block without the risk of mutable data
being resized or destroyed. As a result, you have to call
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 Py_ssize_t) is controlled by
defining the macro PY_SSIZE_T_CLEAN before including
Python.h. If the macro was defined, length is a
Py_ssize_t rather than an int. This behavior will change
in a future Python version to only support Py_ssize_t and
drop int support. It is best to always define PY_SSIZE_T_CLEAN.
- 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 NUL bytes; if it does,
a TypeError exception is raised. Unicode objects are converted
to C strings using 'utf-8' encoding. If this conversion fails, a
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
preferrable to use the O& format with PyUnicode_FSConverter()
as converter.
- s* (str, bytes, bytearray or buffer compatible object) [Py_buffer]
- This format accepts Unicode objects as well as objects supporting the
buffer protocol.
It fills a Py_buffer structure 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, bytes or read-only buffer compatible object) [const char *, int or Py_ssize_t]
- Like s*, except that it doesn’t accept mutable buffer-like objects
such as 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 (str 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* (str, bytes, bytearray, buffer compatible object or None) [Py_buffer]
- Like s*, but the Python object may also be None, in which case the
buf member of the Py_buffer structure is set to NULL.
- z# (str, bytes, read-only buffer compatible object or None) [const char *, int]
- Like s#, but the Python object may also be None, in which case the C
pointer is set to NULL.
- y (bytes) [const char *]
- This format converts a bytes-like object to a C pointer to a character
string; it does not accept Unicode objects. The bytes buffer must not
contain embedded NUL bytes; if it does, a TypeError
exception is raised.
- y* (bytes, bytearray or buffer compatible object) [Py_buffer]
- This variant on s* doesn’t accept Unicode objects, only objects
supporting the buffer protocol. This is the recommended way to accept
binary data.
- y# (bytes) [const char *, int]
- This variant on s# doesn’t accept Unicode objects, only bytes-like
objects.
- S (bytes) [PyBytesObject *]
- Requires that the Python object is a bytes object, without
attempting any conversion. Raises TypeError if the object is not
a bytes object. The C variable may also be declared as PyObject*.
- Y (bytearray) [PyByteArrayObject *]
- Requires that the Python object is a bytearray object, without
attempting any conversion. Raises TypeError if the object is not
a bytearray object. The C variable may also be declared as PyObject*.
- u (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 Py_UNICODE
pointer variable, which will be filled with the pointer to an existing
Unicode buffer. Please note that the width of a Py_UNICODE
character depends on compilation options (it is either 16 or 32 bits).
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# (str) [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.
- Z (str or None) [Py_UNICODE *]
- Like u, but the Python object may also be None, in which case the
Py_UNICODE pointer is set to NULL.
- Z# (str or None) [Py_UNICODE *, int]
- Like u#, but the Python object may also be None, in which case the
Py_UNICODE pointer is set to NULL.
- U (str) [PyUnicodeObject *]
- Requires that the Python object is a Unicode object, without attempting
any conversion. Raises TypeError if the object is not a Unicode
object. The C variable may also be declared as PyObject*.
- t# (bytes, bytearray or read-only character buffer) [char *, int]
- Like s#, but accepts any object which implements the read-only buffer
interface. The char* variable is set to point to the first byte of
the buffer, and the int is set to the length of the buffer. Only
single-segment buffer objects are accepted; TypeError is raised for all
others.
- w (bytearray or read-write character buffer) [char *]
- Similar to y, 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;
TypeError is raised for all others.
- w* (bytearray or read-write byte-oriented buffer) [Py_buffer]
- This is to w what y* is to y.
- w# (bytearray or read-write character buffer) [char *, int]
- Like y#, but accepts any object which implements the read-write buffer
interface. The char * variable is set to point to the first byte
of the buffer, and the int is set to the length of the buffer.
Only single-segment buffer objects are accepted; TypeError is raised
for all others.
- es (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 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 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.
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 PyMem_Free() to
free the allocated buffer after use.
- et (str, bytes or bytearray) [const char *encoding, char **buffer]
- 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.
- es# (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
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 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
PyMem_Free() to free the allocated buffer after usage.
If *buffer points to a non-NULL pointer (an already allocated buffer),
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 ValueError will be set.
In both cases, *buffer_length is set to the length of the encoded data
without the trailing NUL byte.
- et# (str, bytes or bytearray) [const char *encoding, char **buffer, int *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.
Numbers
- b (int) [unsigned char]
- Convert a nonnegative Python integer to an unsigned tiny int, stored in a C
unsigned char.
- B (int) [unsigned char]
- Convert a Python integer to a tiny int without overflow checking, stored in a C
unsigned char.
- h (int) [short int]
- Convert a Python integer to a C short int.
- H (int) [unsigned short int]
- Convert a Python integer to a C unsigned short int, without overflow
checking.
- i (int) [int]
- Convert a Python integer to a plain C int.
- I (int) [unsigned int]
- Convert a Python integer to a C unsigned int, without overflow
checking.
- l (int) [long int]
- Convert a Python integer to a C long int.
- k (int) [unsigned long]
- Convert a Python integer to a C unsigned long without
overflow checking.
- L (int) [PY_LONG_LONG]
- Convert a Python integer to a C long long. This format is only
available on platforms that support long long (or _int64 on
Windows).
- K (int) [unsigned PY_LONG_LONG]
- Convert a Python integer to a C unsigned long long
without overflow checking. This format is only available on platforms that
support unsigned long long (or unsigned _int64 on Windows).
- n (int) [Py_ssize_t]
- Convert a Python integer to a C Py_ssize_t.
- c (bytes of length 1) [char]
- Convert a Python byte, represented as a bytes object of length 1,
to a C char.
- C (str of length 1) [int]
- Convert a Python character, represented as a str object of
length 1, to a C int.
- f (float) [float]
- Convert a Python floating point number to a C float.
- d (float) [double]
- Convert a Python floating point number to a C double.
- D (complex) [Py_complex]
- Convert a Python complex number to a C Py_complex structure.
Other objects
- 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 PyObject*) into which
the object pointer is stored. If the Python object does not have the required
type, 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 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
void* argument that was passed to the 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.
Changed in version 3.1: Py_CLEANUP_SUPPORTED was added.
- (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 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,
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
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
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.
API Functions
-
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.
-
int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)
- Identical to PyArg_ParseTuple(), except that it accepts a va_list rather
than a variable number of arguments.
-
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.
-
int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)
- Identical to PyArg_ParseTupleAndKeywords(), except that it accepts a
va_list rather than a variable number of arguments.
-
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 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.
-
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 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
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
_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 PyArg_UnpackTuple() in this example is entirely equivalent to
this call to PyArg_ParseTuple():
PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
Building values
-
PyObject* Py_BuildValue(const char *format, ...)
- Return value: New reference.
Create a new value based on a format string similar to those accepted by the
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.
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
Py_BuildValue(). In other words, if your code invokes malloc()
and passes the allocated memory to Py_BuildValue(), your code is
responsible for calling free() for that memory once
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 (str or None) [char *]
- Convert a null-terminated C string to a Python str object using 'utf-8'
encoding. If the C string pointer is NULL, None is used.
- s# (str or None) [char *, int]
- Convert a C string and its length to a Python str object using 'utf-8'
encoding. If the C string pointer is NULL, the length is ignored and
None is returned.
- y (bytes) [char *]
- This converts a C string to a Python bytes() object. If the C
string pointer is NULL, None is returned.
- y# (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 (str or None) [char *]
- Same as s.
- z# (str or None) [char *, int]
- Same as s#.
- u (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# (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 (str or None) [char *]
- Convert a null-terminated C string to a Python unicode object. If the C string
pointer is NULL, None is used.
- U# (str or None) [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 (int) [int]
- Convert a plain C int to a Python integer object.
- b (int) [char]
- Convert a plain C char to a Python integer object.
- h (int) [short int]
- Convert a plain C short int to a Python integer object.
- l (int) [long int]
- Convert a C long int to a Python integer object.
- B (int) [unsigned char]
- Convert a C unsigned char to a Python integer object.
- H (int) [unsigned short int]
- Convert a C unsigned short int to a Python integer object.
- I (int) [unsigned int]
- Convert a C unsigned int to a Python integer object.
- k (int) [unsigned long]
- Convert a C unsigned long to a Python integer object.
- L (int) [PY_LONG_LONG]
- Convert a C long long to a Python integer object. Only available
on platforms that support long long (or _int64 on
Windows).
- K (int) [unsigned PY_LONG_LONG]
- Convert a C unsigned long long to a Python integer object. Only
available on platforms that support unsigned long long (or
unsigned _int64 on Windows).
- n (int) [Py_ssize_t]
- Convert a C Py_ssize_t to a Python integer.
- c (bytes of length 1) [char]
- Convert a C int representing a byte to a Python bytes object of
length 1.
- C (str of length 1) [int]
- Convert a C int representing a character to Python str
object of length 1.
- d (float) [double]
- Convert a C double to a Python floating point number.
- f (float) [float]
- Convert a C float to a Python floating point number.
- D (complex) [Py_complex *]
- Convert a C 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, Py_BuildValue() will return NULL but won’t
raise an exception. If no exception has been raised yet, 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 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} (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 SystemError exception is
set and NULL returned.
-
PyObject* Py_VaBuildValue(const char *format, va_list vargs)
- Identical to Py_BuildValue(), except that it accepts a va_list
rather than a variable number of arguments.