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, :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 TypeError exception is raised. Unicode objects are converted to C strings using the default encoding. If this conversion fails, a UnicodeError is raised.

Starting with Python 2.5 the type of the length argument can be controlled by defining the macro :cmacro:`PY_SSIZE_T_CLEAN` before including Python.h. If the macro is defined, length is a :ctype:`Py_ssize_t` rather than an int.

s* (string, Unicode, or any buffer compatible object) [Py_buffer *]

This is similar to s, but the code fills a :ctype:`Py_buffer` structure provided by the caller. 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. The underlying buffer is 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.

s# (string, Unicode or any read buffer compatible object) [const char *, int or :ctype:`Py_ssize_t`]

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. Since this format doesn’t allow writable buffer compatible objects like byte arrays, s* is to be preferred.

The type of the length argument (int or :ctype:`Py_ssize_t`) is controlled by defining the macro :cmacro:`PY_SSIZE_T_CLEAN` before including Python.h. If the macro was defined, length is a :ctype:`Py_ssize_t` rather than an int. This behavior will change in a future Python version to only support :ctype:`Py_ssize_t` and drop int support. It is best to always define :cmacro:`PY_SSIZE_T_CLEAN`.

y (bytes object) [const char *]

This variant on s converts a Python bytes or bytearray object to a C pointer to a character string. The bytes object must not contain embedded NUL bytes; if it does, a TypeError exception is raised.

y* (bytes object) [Py_buffer *]

This is to s* as y is to s.

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, no byte arrays.

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 buffer compatible object) [Py_buffer*]

This is to s* as z is to s.

z# (string or None or any read buffer compatible object) [const char *, int]

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 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) [unsigned char]

Convert a nonnegative Python integer to an unsigned tiny int, stored in a C :ctype:`unsigned 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, 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 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 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; 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; TypeError is raised for all others.

w* (read-write byte-oriented buffer) [Py_buffer *]

This is to w what s* is to s.

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; TypeError is raised for all others.

(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, :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. : 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.