510 lines
23 KiB
ReStructuredText
510 lines
23 KiB
ReStructuredText
.. highlightlang:: c
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.. _arg-parsing:
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Parsing arguments and building values
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=====================================
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These functions are useful when creating your own extensions functions and
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methods. Additional information and examples are available in
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:ref:`extending-index`.
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The first three of these functions described, :cfunc:`PyArg_ParseTuple`,
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:cfunc:`PyArg_ParseTupleAndKeywords`, and :cfunc:`PyArg_Parse`, all use *format
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strings* which are used to tell the function about the expected arguments. The
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format strings use the same syntax for each of these functions.
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A format string consists of zero or more "format units." A format unit
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describes one Python object; it is usually a single character or a parenthesized
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sequence of format units. With a few exceptions, a format unit that is not a
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parenthesized sequence normally corresponds to a single address argument to
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these functions. In the following description, the quoted form is the format
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unit; the entry in (round) parentheses is the Python object type that matches
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the format unit; and the entry in [square] brackets is the type of the C
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variable(s) whose address should be passed.
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``s`` (string or Unicode object) [const char \*]
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Convert a Python string or Unicode object to a C pointer to a character string.
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You must not provide storage for the string itself; a pointer to an existing
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string is stored into the character pointer variable whose address you pass.
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The C string is NUL-terminated. The Python string must not contain embedded NUL
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bytes; if it does, a :exc:`TypeError` exception is raised. Unicode objects are
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converted to C strings using the default encoding. If this conversion fails, a
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:exc:`UnicodeError` is raised.
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``s#`` (string, Unicode or any read buffer compatible object) [const char \*, int]
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This variant on ``s`` stores into two C variables, the first one a pointer to a
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character string, the second one its length. In this case the Python string may
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contain embedded null bytes. Unicode objects pass back a pointer to the default
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encoded string version of the object if such a conversion is possible. All
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other read-buffer compatible objects pass back a reference to the raw internal
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data representation.
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``z`` (string or ``None``) [const char \*]
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Like ``s``, but the Python object may also be ``None``, in which case the C
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pointer is set to *NULL*.
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``z#`` (string or ``None`` or any read buffer compatible object) [const char \*, int]
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This is to ``s#`` as ``z`` is to ``s``.
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``u`` (Unicode object) [Py_UNICODE \*]
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Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of
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16-bit Unicode (UTF-16) data. As with ``s``, there is no need to provide
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storage for the Unicode data buffer; a pointer to the existing Unicode data is
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stored into the :ctype:`Py_UNICODE` pointer variable whose address you pass.
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``u#`` (Unicode object) [Py_UNICODE \*, int]
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This variant on ``u`` stores into two C variables, the first one a pointer to a
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Unicode data buffer, the second one its length. Non-Unicode objects are handled
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by interpreting their read-buffer pointer as pointer to a :ctype:`Py_UNICODE`
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array.
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``es`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
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This variant on ``s`` is used for encoding Unicode and objects convertible to
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Unicode into a character buffer. It only works for encoded data without embedded
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NUL bytes.
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This format requires two arguments. The first is only used as input, and
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must be a :ctype:`const char\*` which points to the name of an encoding as a
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NUL-terminated string, or *NULL*, in which case the default encoding is used.
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An exception is raised if the named encoding is not known to Python. The
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second argument must be a :ctype:`char\*\*`; the value of the pointer it
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references will be set to a buffer with the contents of the argument text.
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The text will be encoded in the encoding specified by the first argument.
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:cfunc:`PyArg_ParseTuple` will allocate a buffer of the needed size, copy the
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encoded data into this buffer and adjust *\*buffer* to reference the newly
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allocated storage. The caller is responsible for calling :cfunc:`PyMem_Free` to
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free the allocated buffer after use.
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``et`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
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Same as ``es`` except that 8-bit string objects are passed through without
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recoding them. Instead, the implementation assumes that the string object uses
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the encoding passed in as parameter.
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``es#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer, int \*buffer_length]
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This variant on ``s#`` is used for encoding Unicode and objects convertible to
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Unicode into a character buffer. Unlike the ``es`` format, this variant allows
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input data which contains NUL characters.
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It requires three arguments. The first is only used as input, and must be a
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:ctype:`const char\*` which points to the name of an encoding as a
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NUL-terminated string, or *NULL*, in which case the default encoding is used.
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An exception is raised if the named encoding is not known to Python. The
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second argument must be a :ctype:`char\*\*`; the value of the pointer it
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references will be set to a buffer with the contents of the argument text.
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The text will be encoded in the encoding specified by the first argument.
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The third argument must be a pointer to an integer; the referenced integer
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will be set to the number of bytes in the output buffer.
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There are two modes of operation:
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If *\*buffer* points a *NULL* pointer, the function will allocate a buffer of
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the needed size, copy the encoded data into this buffer and set *\*buffer* to
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reference the newly allocated storage. The caller is responsible for calling
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:cfunc:`PyMem_Free` to free the allocated buffer after usage.
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If *\*buffer* points to a non-*NULL* pointer (an already allocated buffer),
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:cfunc:`PyArg_ParseTuple` will use this location as the buffer and interpret the
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initial value of *\*buffer_length* as the buffer size. It will then copy the
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encoded data into the buffer and NUL-terminate it. If the buffer is not large
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enough, a :exc:`ValueError` will be set.
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In both cases, *\*buffer_length* is set to the length of the encoded data
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without the trailing NUL byte.
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``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
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Same as ``es#`` except that string objects are passed through without recoding
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them. Instead, the implementation assumes that the string object uses the
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encoding passed in as parameter.
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``b`` (integer) [char]
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Convert a Python integer to a tiny int, stored in a C :ctype:`char`.
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``B`` (integer) [unsigned char]
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Convert a Python integer to a tiny int without overflow checking, stored in a C
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:ctype:`unsigned char`.
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.. versionadded:: 2.3
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``h`` (integer) [short int]
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Convert a Python integer to a C :ctype:`short int`.
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``H`` (integer) [unsigned short int]
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Convert a Python integer to a C :ctype:`unsigned short int`, without overflow
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checking.
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.. versionadded:: 2.3
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``i`` (integer) [int]
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Convert a Python integer to a plain C :ctype:`int`.
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``I`` (integer) [unsigned int]
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Convert a Python integer to a C :ctype:`unsigned int`, without overflow
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checking.
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.. versionadded:: 2.3
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``l`` (integer) [long int]
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Convert a Python integer to a C :ctype:`long int`.
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``k`` (integer) [unsigned long]
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Convert a Python integer or long integer to a C :ctype:`unsigned long` without
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overflow checking.
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.. versionadded:: 2.3
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``L`` (integer) [PY_LONG_LONG]
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Convert a Python integer to a C :ctype:`long long`. This format is only
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available on platforms that support :ctype:`long long` (or :ctype:`_int64` on
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Windows).
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``K`` (integer) [unsigned PY_LONG_LONG]
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Convert a Python integer or long integer to a C :ctype:`unsigned long long`
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without overflow checking. This format is only available on platforms that
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support :ctype:`unsigned long long` (or :ctype:`unsigned _int64` on Windows).
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.. versionadded:: 2.3
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``n`` (integer) [Py_ssize_t]
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Convert a Python integer or long integer to a C :ctype:`Py_ssize_t`.
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.. versionadded:: 2.5
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``c`` (string of length 1) [char]
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Convert a Python character, represented as a string of length 1, to a C
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:ctype:`char`.
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``f`` (float) [float]
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Convert a Python floating point number to a C :ctype:`float`.
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``d`` (float) [double]
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Convert a Python floating point number to a C :ctype:`double`.
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``D`` (complex) [Py_complex]
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Convert a Python complex number to a C :ctype:`Py_complex` structure.
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``O`` (object) [PyObject \*]
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Store a Python object (without any conversion) in a C object pointer. The C
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program thus receives the actual object that was passed. The object's reference
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count is not increased. The pointer stored is not *NULL*.
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``O!`` (object) [*typeobject*, PyObject \*]
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Store a Python object in a C object pointer. This is similar to ``O``, but
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takes two C arguments: the first is the address of a Python type object, the
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second is the address of the C variable (of type :ctype:`PyObject\*`) into which
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the object pointer is stored. If the Python object does not have the required
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type, :exc:`TypeError` is raised.
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``O&`` (object) [*converter*, *anything*]
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Convert a Python object to a C variable through a *converter* function. This
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takes two arguments: the first is a function, the second is the address of a C
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variable (of arbitrary type), converted to :ctype:`void \*`. The *converter*
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function in turn is called as follows::
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status = converter(object, address);
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where *object* is the Python object to be converted and *address* is the
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:ctype:`void\*` argument that was passed to the :cfunc:`PyArg_Parse\*` function.
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The returned *status* should be ``1`` for a successful conversion and ``0`` if
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the conversion has failed. When the conversion fails, the *converter* function
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should raise an exception.
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``S`` (string) [PyStringObject \*]
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Like ``O`` but requires that the Python object is a string object. Raises
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:exc:`TypeError` if the object is not a string object. The C variable may also
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be declared as :ctype:`PyObject\*`.
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``U`` (Unicode string) [PyUnicodeObject \*]
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Like ``O`` but requires that the Python object is a Unicode object. Raises
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:exc:`TypeError` if the object is not a Unicode object. The C variable may also
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be declared as :ctype:`PyObject\*`.
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``t#`` (read-only character buffer) [char \*, int]
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Like ``s#``, but accepts any object which implements the read-only buffer
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interface. The :ctype:`char\*` variable is set to point to the first byte of
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the buffer, and the :ctype:`int` is set to the length of the buffer. Only
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single-segment buffer objects are accepted; :exc:`TypeError` is raised for all
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others.
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``w`` (read-write character buffer) [char \*]
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Similar to ``s``, but accepts any object which implements the read-write buffer
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interface. The caller must determine the length of the buffer by other means,
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or use ``w#`` instead. Only single-segment buffer objects are accepted;
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:exc:`TypeError` is raised for all others.
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``w#`` (read-write character buffer) [char \*, int]
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Like ``s#``, but accepts any object which implements the read-write buffer
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interface. The :ctype:`char \*` variable is set to point to the first byte of
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the buffer, and the :ctype:`int` is set to the length of the buffer. Only
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single-segment buffer objects are accepted; :exc:`TypeError` is raised for all
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others.
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``(items)`` (tuple) [*matching-items*]
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The object must be a Python sequence whose length is the number of format units
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in *items*. The C arguments must correspond to the individual format units in
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*items*. Format units for sequences may be nested.
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.. note::
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Prior to Python version 1.5.2, this format specifier only accepted a tuple
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containing the individual parameters, not an arbitrary sequence. Code which
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previously caused :exc:`TypeError` to be raised here may now proceed without an
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exception. This is not expected to be a problem for existing code.
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It is possible to pass Python long integers where integers are requested;
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however no proper range checking is done --- the most significant bits are
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silently truncated when the receiving field is too small to receive the value
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(actually, the semantics are inherited from downcasts in C --- your mileage may
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vary).
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A few other characters have a meaning in a format string. These may not occur
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inside nested parentheses. They are:
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``|``
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Indicates that the remaining arguments in the Python argument list are optional.
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The C variables corresponding to optional arguments should be initialized to
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their default value --- when an optional argument is not specified,
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:cfunc:`PyArg_ParseTuple` does not touch the contents of the corresponding C
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variable(s).
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``:``
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The list of format units ends here; the string after the colon is used as the
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function name in error messages (the "associated value" of the exception that
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:cfunc:`PyArg_ParseTuple` raises).
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``;``
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The list of format units ends here; the string after the semicolon is used as
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the error message *instead* of the default error message. Clearly, ``:`` and
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``;`` mutually exclude each other.
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Note that any Python object references which are provided to the caller are
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*borrowed* references; do not decrement their reference count!
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Additional arguments passed to these functions must be addresses of variables
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whose type is determined by the format string; these are used to store values
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from the input tuple. There are a few cases, as described in the list of format
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units above, where these parameters are used as input values; they should match
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what is specified for the corresponding format unit in that case.
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For the conversion to succeed, the *arg* object must match the format and the
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format must be exhausted. On success, the :cfunc:`PyArg_Parse\*` functions
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return true, otherwise they return false and raise an appropriate exception.
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.. cfunction:: int PyArg_ParseTuple(PyObject *args, const char *format, ...)
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Parse the parameters of a function that takes only positional parameters into
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local variables. Returns true on success; on failure, it returns false and
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raises the appropriate exception.
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.. cfunction:: int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)
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Identical to :cfunc:`PyArg_ParseTuple`, except that it accepts a va_list rather
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than a variable number of arguments.
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.. cfunction:: int PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], ...)
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Parse the parameters of a function that takes both positional and keyword
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parameters into local variables. Returns true on success; on failure, it
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returns false and raises the appropriate exception.
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.. cfunction:: int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)
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Identical to :cfunc:`PyArg_ParseTupleAndKeywords`, except that it accepts a
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va_list rather than a variable number of arguments.
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.. cfunction:: int PyArg_Parse(PyObject *args, const char *format, ...)
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Function used to deconstruct the argument lists of "old-style" functions ---
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these are functions which use the :const:`METH_OLDARGS` parameter parsing
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method. This is not recommended for use in parameter parsing in new code, and
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most code in the standard interpreter has been modified to no longer use this
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for that purpose. It does remain a convenient way to decompose other tuples,
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however, and may continue to be used for that purpose.
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.. cfunction:: int PyArg_UnpackTuple(PyObject *args, const char *name, Py_ssize_t min, Py_ssize_t max, ...)
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A simpler form of parameter retrieval which does not use a format string to
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specify the types of the arguments. Functions which use this method to retrieve
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their parameters should be declared as :const:`METH_VARARGS` in function or
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method tables. The tuple containing the actual parameters should be passed as
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*args*; it must actually be a tuple. The length of the tuple must be at least
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*min* and no more than *max*; *min* and *max* may be equal. Additional
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arguments must be passed to the function, each of which should be a pointer to a
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:ctype:`PyObject\*` variable; these will be filled in with the values from
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*args*; they will contain borrowed references. The variables which correspond
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to optional parameters not given by *args* will not be filled in; these should
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be initialized by the caller. This function returns true on success and false if
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*args* is not a tuple or contains the wrong number of elements; an exception
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will be set if there was a failure.
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This is an example of the use of this function, taken from the sources for the
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:mod:`_weakref` helper module for weak references::
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static PyObject *
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weakref_ref(PyObject *self, PyObject *args)
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{
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PyObject *object;
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PyObject *callback = NULL;
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PyObject *result = NULL;
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if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) {
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result = PyWeakref_NewRef(object, callback);
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}
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return result;
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}
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The call to :cfunc:`PyArg_UnpackTuple` in this example is entirely equivalent to
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this call to :cfunc:`PyArg_ParseTuple`::
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PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
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.. versionadded:: 2.2
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.. cfunction:: PyObject* Py_BuildValue(const char *format, ...)
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Create a new value based on a format string similar to those accepted by the
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:cfunc:`PyArg_Parse\*` family of functions and a sequence of values. Returns
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the value or *NULL* in the case of an error; an exception will be raised if
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*NULL* is returned.
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:cfunc:`Py_BuildValue` does not always build a tuple. It builds a tuple only if
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its format string contains two or more format units. If the format string is
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empty, it returns ``None``; if it contains exactly one format unit, it returns
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whatever object is described by that format unit. To force it to return a tuple
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of size 0 or one, parenthesize the format string.
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When memory buffers are passed as parameters to supply data to build objects, as
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for the ``s`` and ``s#`` formats, the required data is copied. Buffers provided
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by the caller are never referenced by the objects created by
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:cfunc:`Py_BuildValue`. In other words, if your code invokes :cfunc:`malloc`
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and passes the allocated memory to :cfunc:`Py_BuildValue`, your code is
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responsible for calling :cfunc:`free` for that memory once
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:cfunc:`Py_BuildValue` returns.
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In the following description, the quoted form is the format unit; the entry in
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(round) parentheses is the Python object type that the format unit will return;
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and the entry in [square] brackets is the type of the C value(s) to be passed.
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The characters space, tab, colon and comma are ignored in format strings (but
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not within format units such as ``s#``). This can be used to make long format
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strings a tad more readable.
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``s`` (string) [char \*]
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Convert a null-terminated C string to a Python object. If the C string pointer
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is *NULL*, ``None`` is used.
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``s#`` (string) [char \*, int]
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Convert a C string and its length to a Python object. If the C string pointer
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is *NULL*, the length is ignored and ``None`` is returned.
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``z`` (string or ``None``) [char \*]
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Same as ``s``.
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``z#`` (string or ``None``) [char \*, int]
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Same as ``s#``.
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``u`` (Unicode string) [Py_UNICODE \*]
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Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python
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Unicode object. If the Unicode buffer pointer is *NULL*, ``None`` is returned.
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``u#`` (Unicode string) [Py_UNICODE \*, int]
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Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python
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Unicode object. If the Unicode buffer pointer is *NULL*, the length is ignored
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and ``None`` is returned.
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``i`` (integer) [int]
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Convert a plain C :ctype:`int` to a Python integer object.
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``b`` (integer) [char]
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Convert a plain C :ctype:`char` to a Python integer object.
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``h`` (integer) [short int]
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Convert a plain C :ctype:`short int` to a Python integer object.
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``l`` (integer) [long int]
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Convert a C :ctype:`long int` to a Python integer object.
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``B`` (integer) [unsigned char]
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Convert a C :ctype:`unsigned char` to a Python integer object.
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``H`` (integer) [unsigned short int]
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Convert a C :ctype:`unsigned short int` to a Python integer object.
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``I`` (integer/long) [unsigned int]
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Convert a C :ctype:`unsigned int` to a Python integer object or a Python long
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integer object, if it is larger than ``sys.maxint``.
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``k`` (integer/long) [unsigned long]
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Convert a C :ctype:`unsigned long` to a Python integer object or a Python long
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integer object, if it is larger than ``sys.maxint``.
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``L`` (long) [PY_LONG_LONG]
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Convert a C :ctype:`long long` to a Python long integer object. Only available
|
|
on platforms that support :ctype:`long long`.
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``K`` (long) [unsigned PY_LONG_LONG]
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Convert a C :ctype:`unsigned long long` to a Python long integer object. Only
|
|
available on platforms that support :ctype:`unsigned long long`.
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``n`` (int) [Py_ssize_t]
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Convert a C :ctype:`Py_ssize_t` to a Python integer or long integer.
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.. versionadded:: 2.5
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``c`` (string of length 1) [char]
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|
Convert a C :ctype:`int` representing a character to a Python string of length
|
|
1.
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``d`` (float) [double]
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Convert a C :ctype:`double` to a Python floating point number.
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``f`` (float) [float]
|
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Same as ``d``.
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``D`` (complex) [Py_complex \*]
|
|
Convert a C :ctype:`Py_complex` structure to a Python complex number.
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``O`` (object) [PyObject \*]
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|
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, :cfunc:`Py_BuildValue` will return *NULL* but won't
|
|
raise an exception. If no exception has been raised yet, :exc:`SystemError` is
|
|
set.
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|
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``S`` (object) [PyObject \*]
|
|
Same as ``O``.
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|
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|
``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.
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|
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|
``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 :ctype:`void
|
|
\*`) as its argument and should return a "new" Python object, or *NULL* if an
|
|
error occurred.
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|
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|
``(items)`` (tuple) [*matching-items*]
|
|
Convert a sequence of C values to a Python tuple with the same number of items.
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``[items]`` (list) [*matching-items*]
|
|
Convert a sequence of C values to a Python list with the same number of items.
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|
|
|
``{items}`` (dictionary) [*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.
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|
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|
If there is an error in the format string, the :exc:`SystemError` exception is
|
|
set and *NULL* returned.
|