| Python 3.6.5 Documentation > Parsing arguments and building values
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.
In general, when a format sets a pointer to a buffer, the buffer is
managed by the corresponding Python object, and the buffer shares the
lifetime of this object. You won’t have to release any memory
yourself. The only exceptions are "es", "es#", "et" and "et#".
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.
Some formats require a read-only *bytes-like object*, and set a
pointer instead of a buffer structure. They work by checking that the
object’s "PyBufferProcs.bf_releasebuffer" field is *NULL*, which
disallows mutable objects such as "bytearray".
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 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 *, int or
"Py_ssize_t"]
Like "s*", except that it doesn’t accept mutable objects. 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 *,
int]
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
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 *, 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). 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.
"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. This
variant allows null code points.
"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") [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, 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") [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 **p**redicate) 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, ...)
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. 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)
Identical to "PyArg_ParseTupleAndKeywords()", except that it
accepts a va_list rather than a variable number of arguments.
int PyArg_ValidateKeywordArguments(PyObject *)
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, ...)
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, ...)
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 *]
Same as "s".
"U#" ("str" or "None") [char *, int]
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)
Identical to "Py_BuildValue()", except that it accepts a va_list
rather than a variable number of arguments.
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