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 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* and s* fill a Py_buffer structure. This locks the underlying buffer 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).
The es, es#, et and et# formats allocate the result buffer. You have to call PyMem_Free() after you have finished processing the data (or in any early abort case).
Other formats take a str or a read-only bytes-like object, such as bytes, and provide a const 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 PyBufferProcs.bf_releasebuffer field must be NULL. This disallows common mutable objects such as bytearray, but also some read-only objects such as memoryview of bytes.

Besides this bf_releasebuffer requirement, 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 PY_SSIZE_T_CLEAN must be defined before including Python.h. On Python 3.9 and older, the type of the length argument is Py_ssize_t if the PY_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 ValueError 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 preferable to use the O& format with PyUnicode_FSConverter() as converter.

Changed in version 3.5: Previously, TypeError was raised when embedded null code points were encountered in the Python string.

s* (str or bytes-like object) [Py_buffer]

This format accepts Unicode objects as well as bytes-like objects. 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, read-only bytes-like object) [const char *, Py_ssize_t]

Like s*, except that it provides a 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 (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-like 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, read-only bytes-like object or None) [const char *, Py_ssize_t]

Like s#, but the Python object may also be None, in which case the C pointer is set to NULL.

y (read-only bytes-like object) [const char *]

This format converts a bytes-like object to a C pointer to a borrowed character string; it does not accept Unicode objects. The bytes buffer must not contain embedded null bytes; if it does, a ValueError exception is raised.

Changed in version 3.5: Previously, TypeError was 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-only bytes-like object) [const char *, Py_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 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) [const 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). The Python string must not contain embedded null code points; if it does, a ValueError exception is raised.

Changed in version 3.5: Previously, TypeError was raised when embedded null code points were encountered in the Python string.

Deprecated since version 3.3, will be removed in version 3.12: Part of the old-style Py_UNICODE API; please migrate to using PyUnicode_AsWideCharString().

u# (str) [const Py_UNICODE *, Py_ssize_t]

This variant on u stores into two C variables, the first one a pointer to a Unicode data buffer, the second one its length. This variant allows null code points.

Deprecated since version 3.3, will be removed in version 3.12: Part of the old-style Py_UNICODE API; please migrate to using PyUnicode_AsWideCharString().

Z (str or None) [const Py_UNICODE *]

Like u, but the Python object may also be None, in which case the Py_UNICODE pointer is set to NULL.

Deprecated since version 3.3, will be removed in version 3.12: Part of the old-style Py_UNICODE API; please migrate to using PyUnicode_AsWideCharString().

Z# (str or None) [const Py_UNICODE *, Py_ssize_t]

Like u#, but the Python object may also be None, in which case the Py_UNICODE pointer is set to NULL.

Deprecated since version 3.3, will be removed in version 3.12: Part of the old-style Py_UNICODE API; please migrate to using PyUnicode_AsWideCharString().

U (str) [PyObject *]

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*.

w* (read-write bytes-like object) [Py_buffer]

This format accepts any object which implements the read-write buffer interface. It fills a Py_buffer structure provided by the caller. The buffer may contain embedded null bytes. The caller have to call PyBuffer_Release() when it is done with the buffer.

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, Py_ssize_t *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, Py_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.

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) [long long]: Convert a Python integer to a C long long.
K (int) [unsigned long long]: Convert a Python integer to a C unsigned long long without overflow checking.
n (int) [Py_ssize_t]: Convert a Python integer to a C Py_ssize_t.
c (bytes or bytearray of length 1) [char]: Convert a Python byte, represented as a bytes or bytearray object of length 1, to a C char.

Changed in version 3.3: Allow bytearray objects.
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.

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 1 if the expression was true and 0 if it was false. This accepts any valid Python value. See Truth Value Testing for more information about how Python tests values for truth.

New in version 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 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).
$: PyArg_ParseTupleAndKeywords() only: 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.

New in version 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 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, ...)¶: Part of the Stable ABI.
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)¶: Part of the Stable ABI.
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[], ...)¶: Part of the Stable ABI.
Parse the parameters of a function that takes both positional and keyword parameters into local variables. The keywords argument is a NULL-terminated array of keyword parameter names. Empty names denote positional-only parameters. Returns true on success; on failure, it returns false and raises the appropriate exception.

Changed in version 3.6: Added support for positional-only parameters.

int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)¶: Part of the Stable ABI.
Identical to PyArg_ParseTupleAndKeywords(), except that it accepts a va_list rather than a variable number of arguments.

int PyArg_ValidateKeywordArguments(PyObject*)¶: Part of the Stable ABI.
Ensure that the keys in the keywords argument dictionary are strings. This is only needed if PyArg_ParseTupleAndKeywords() is not used, since the latter already does this check.

New in version 3.2.

int PyArg_Parse(PyObject *args, const char *format, ...)¶: Part of the Stable ABI.
Function used to deconstruct the argument lists of “old-style” functions — these are functions which use the METH_OLDARGS parameter parsing method, which has been removed in Python 3. 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, ...)¶

Part of the Stable ABI.

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. Part of the Stable ABI.

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) [const 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) [const char *, Py_ssize_t]: 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) [const char *]: This converts a C string to a Python bytes object. If the C string pointer is NULL, None is returned.
y# (bytes) [const char *, Py_ssize_t]: 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) [const char *]: Same as s.
z# (str or None) [const char *, Py_ssize_t]: Same as s#.
u (str) [const wchar_t *]: Convert a null-terminated wchar_t buffer of Unicode (UTF-16 or UCS-4) data to a Python Unicode object. If the Unicode buffer pointer is NULL, None is returned.
u# (str) [const wchar_t *, Py_ssize_t]: Convert a Unicode (UTF-16 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) [const char *]: Same as s.
U# (str or None) [const char *, Py_ssize_t]: Same as s#.
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) [long long]: Convert a C long long to a Python integer object.
K (int) [unsigned long long]: Convert a C unsigned long long to a Python integer object.
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)¶: Return value: New reference. Part of the Stable ABI.
Identical to Py_BuildValue(), except that it accepts a va_list rather than a variable number of arguments.

Parsing arguments and building values¶