:mod:`struct` --- Interpret strings as packed binary data

=========================================================

.. module:: struct

   :synopsis: Interpret strings as packed binary data.

.. index::

   pair: C; structures

   triple: packing; binary; data

This module performs conversions between Python values and C structs represented

as Python strings.  It uses :dfn:`format strings` (explained below) as compact

descriptions of the lay-out of the C structs and the intended conversion to/from

Python values.  This can be used in handling binary data stored in files or from

network connections, among other sources.

The module defines the following exception and functions:

.. exception:: error

   Exception raised on various occasions; argument is a string describing what is

   wrong.

.. function:: pack(fmt, v1, v2, ...)

   Return a string containing the values ``v1, v2, ...`` packed according to the

   given format.  The arguments must match the values required by the format

   exactly.

.. function:: pack_into(fmt, buffer, offset, v1, v2, ...)

   Pack the values ``v1, v2, ...`` according to the given format, write the packed

   bytes into the writable *buffer* starting at *offset*. Note that the offset is

   a required argument.

   .. versionadded:: 2.5

.. function:: unpack(fmt, string)

   Unpack the string (presumably packed by ``pack(fmt, ...)``) according to the

   given format.  The result is a tuple even if it contains exactly one item.  The

   string must contain exactly the amount of data required by the format

   (``len(string)`` must equal ``calcsize(fmt)``).

.. function:: unpack_from(fmt, buffer[,offset=0])

   Unpack the *buffer* according to tthe given format. The result is a tuple even

   if it contains exactly one item. The *buffer* must contain at least the amount

   of data required by the format (``len(buffer[offset:])`` must be at least

   ``calcsize(fmt)``).

   .. versionadded:: 2.5

.. function:: calcsize(fmt)

   Return the size of the struct (and hence of the string) corresponding to the

   given format.

Format characters have the following meaning; the conversion between C and

Python values should be obvious given their types:

+--------+-------------------------+--------------------+-------+

| Format | C Type                  | Python             | Notes |

+========+=========================+====================+=======+

| ``x``  | pad byte                | no value           |       |

+--------+-------------------------+--------------------+-------+

| ``c``  | :ctype:`char`           | string of length 1 |       |

+--------+-------------------------+--------------------+-------+

| ``b``  | :ctype:`signed char`    | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``B``  | :ctype:`unsigned char`  | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``?``  | :ctype:`_Bool`          | bool               | \(1)  |

+--------+-------------------------+--------------------+-------+

| ``h``  | :ctype:`short`          | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``H``  | :ctype:`unsigned short` | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``i``  | :ctype:`int`            | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``I``  | :ctype:`unsigned int`   | integer or long    |       |

+--------+-------------------------+--------------------+-------+

| ``l``  | :ctype:`long`           | integer            |       |

+--------+-------------------------+--------------------+-------+

| ``L``  | :ctype:`unsigned long`  | long               |       |

+--------+-------------------------+--------------------+-------+

| ``q``  | :ctype:`long long`      | long               | \(2)  |

+--------+-------------------------+--------------------+-------+

| ``Q``  | :ctype:`unsigned long   | long               | \(2)  |

|        | long`                   |                    |       |

+--------+-------------------------+--------------------+-------+

| ``f``  | :ctype:`float`          | float              |       |

+--------+-------------------------+--------------------+-------+

| ``d``  | :ctype:`double`         | float              |       |

+--------+-------------------------+--------------------+-------+

| ``s``  | :ctype:`char[]`         | string             |       |

+--------+-------------------------+--------------------+-------+

| ``p``  | :ctype:`char[]`         | string             |       |

+--------+-------------------------+--------------------+-------+

| ``P``  | :ctype:`void \*`        | long               |       |

+--------+-------------------------+--------------------+-------+

Notes:

(1)

   The ``'?'`` conversion code corresponds to the :ctype:`_Bool` type defined by

   C99. If this type is not available, it is simulated using a :ctype:`char`. In

   standard mode, it is always represented by one byte.

   .. versionadded:: 2.6

(2)

   The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if

   the platform C compiler supports C :ctype:`long long`, or, on Windows,

   :ctype:`__int64`.  They are always available in standard modes.

   .. versionadded:: 2.2

A format character may be preceded by an integral repeat count.  For example,

the format string ``'4h'`` means exactly the same as ``'hhhh'``.

Whitespace characters between formats are ignored; a count and its format must

not contain whitespace though.

For the ``'s'`` format character, the count is interpreted as the size of the

string, not a repeat count like for the other format characters; for example,

``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.

For packing, the string is truncated or padded with null bytes as appropriate to

make it fit. For unpacking, the resulting string always has exactly the

specified number of bytes.  As a special case, ``'0s'`` means a single, empty

string (while ``'0c'`` means 0 characters).

The ``'p'`` format character encodes a "Pascal string", meaning a short

variable-length string stored in a fixed number of bytes. The count is the total

number of bytes stored.  The first byte stored is the length of the string, or

255, whichever is smaller.  The bytes of the string follow.  If the string

passed in to :func:`pack` is too long (longer than the count minus 1), only the

leading count-1 bytes of the string are stored.  If the string is shorter than

count-1, it is padded with null bytes so that exactly count bytes in all are

used.  Note that for :func:`unpack`, the ``'p'`` format character consumes count

bytes, but that the string returned can never contain more than 255 characters.

For the ``'I'``, ``'L'``, ``'q'`` and ``'Q'`` format characters, the return

value is a Python long integer.

For the ``'P'`` format character, the return value is a Python integer or long

integer, depending on the size needed to hold a pointer when it has been cast to

an integer type.  A *NULL* pointer will always be returned as the Python integer

``0``. When packing pointer-sized values, Python integer or long integer objects

may be used.  For example, the Alpha and Merced processors use 64-bit pointer

values, meaning a Python long integer will be used to hold the pointer; other

platforms use 32-bit pointers and will use a Python integer.

For the ``'?'`` format character, the return value is either :const:`True` or

:const:`False`. When packing, the truth value of the argument object is used.

Either 0 or 1 in the native or standard bool representation will be packed, and

any non-zero value will be True when unpacking.

By default, C numbers are represented in the machine's native format and byte

order, and properly aligned by skipping pad bytes if necessary (according to the

rules used by the C compiler).

Alternatively, the first character of the format string can be used to indicate

the byte order, size and alignment of the packed data, according to the

following table:

+-----------+------------------------+--------------------+

| Character | Byte order             | Size and alignment |

+===========+========================+====================+

| ``@``     | native                 | native             |

+-----------+------------------------+--------------------+

| ``=``     | native                 | standard           |

+-----------+------------------------+--------------------+

| ``<``     | little-endian          | standard           |

+-----------+------------------------+--------------------+

| ``>``     | big-endian             | standard           |

+-----------+------------------------+--------------------+

| ``!``     | network (= big-endian) | standard           |

+-----------+------------------------+--------------------+

If the first character is not one of these, ``'@'`` is assumed.

Native byte order is big-endian or little-endian, depending on the host system.

For example, Motorola and Sun processors are big-endian; Intel and DEC

processors are little-endian.

Native size and alignment are determined using the C compiler's

``sizeof`` expression.  This is always combined with native byte order.

Standard size and alignment are as follows: no alignment is required for any

type (so you have to use pad bytes); :ctype:`short` is 2 bytes; :ctype:`int` and

:ctype:`long` are 4 bytes; :ctype:`long long` (:ctype:`__int64` on Windows) is 8

bytes; :ctype:`float` and :ctype:`double` are 32-bit and 64-bit IEEE floating

point numbers, respectively. :ctype:`_Bool` is 1 byte.

Note the difference between ``'@'`` and ``'='``: both use native byte order, but

the size and alignment of the latter is standardized.

The form ``'!'`` is available for those poor souls who claim they can't remember

whether network byte order is big-endian or little-endian.

There is no way to indicate non-native byte order (force byte-swapping); use the

appropriate choice of ``'<'`` or ``'>'``.

The ``'P'`` format character is only available for the native byte ordering

(selected as the default or with the ``'@'`` byte order character). The byte

order character ``'='`` chooses to use little- or big-endian ordering based on

the host system. The struct module does not interpret this as native ordering,

so the ``'P'`` format is not available.

Examples (all using native byte order, size and alignment, on a big-endian

machine)::

   >>> from struct import *

   >>> pack('hhl', 1, 2, 3)

   '\x00\x01\x00\x02\x00\x00\x00\x03'

   >>> unpack('hhl', '\x00\x01\x00\x02\x00\x00\x00\x03')

   (1, 2, 3)

   >>> calcsize('hhl')

   8

Hint: to align the end of a structure to the alignment requirement of a

particular type, end the format with the code for that type with a repeat count

of zero.  For example, the format ``'llh0l'`` specifies two pad bytes at the

end, assuming longs are aligned on 4-byte boundaries.  This only works when

native size and alignment are in effect; standard size and alignment does not

enforce any alignment.

Unpacked fields can be named by assigning them to variables or by wrapping

the result in a named tuple::

    >>> record = 'raymond   \x32\x12\x08\x01\x08'

    >>> name, serialnum, school, gradelevel = unpack('<10sHHb', record)

    >>> from collections import namedtuple

    >>> Student = namedtuple('Student', 'name serialnum school gradelevel')

    >>> Student._make(unpack('<10sHHb', s))

    Student(name='raymond   ', serialnum=4658, school=264, gradelevel=8)

.. seealso::

   Module :mod:`array`

      Packed binary storage of homogeneous data.

   Module :mod:`xdrlib`

      Packing and unpacking of XDR data.

.. _struct-objects:

Struct Objects

--------------

The :mod:`struct` module also defines the following type:

.. class:: Struct(format)

   Return a new Struct object which writes and reads binary data according to the

   format string *format*.  Creating a Struct object once and calling its methods

   is more efficient than calling the :mod:`struct` functions with the same format

   since the format string only needs to be compiled once.

   .. versionadded:: 2.5

   Compiled Struct objects support the following methods and attributes:

   .. method:: pack(v1, v2, ...)

      Identical to the :func:`pack` function, using the compiled format.

      (``len(result)`` will equal :attr:`self.size`.)

   .. method:: pack_into(buffer, offset, v1, v2, ...)

      Identical to the :func:`pack_into` function, using the compiled format.

   .. method:: unpack(string)

      Identical to the :func:`unpack` function, using the compiled format.

      (``len(string)`` must equal :attr:`self.size`).

   .. method:: unpack_from(buffer[, offset=0])

      Identical to the :func:`unpack_from` function, using the compiled format.

      (``len(buffer[offset:])`` must be at least :attr:`self.size`).

   .. attribute:: format

      The format string used to construct this Struct object.

   .. attribute:: size

      The calculated size of the struct (and hence of the string) corresponding

      to :attr:`format`.