
``struct`` --- Interpret bytes as packed binary data
****************************************************

This module performs conversions between Python values and C structs
represented as Python ``bytes`` objects.  It uses *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 struct.error

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

struct.pack(fmt, v1, v2, ...)

   Return a bytes containing the values ``v1, v2, ...`` packed
   according to the given format.  The arguments must match the values
   required by the format exactly.

struct.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.

struct.unpack(fmt, bytes)

   Unpack the bytes (presumably packed by ``pack(fmt, ...)``)
   according to the given format.  The result is a tuple even if it
   contains exactly one item.  The bytes must contain exactly the
   amount of data required by the format (``len(bytes)`` must equal
   ``calcsize(fmt)``).

struct.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)``).

struct.calcsize(fmt)

   Return the size of the struct (and hence of the bytes)
   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``    | ``char``                  | bytes of length 1    |         |
+----------+---------------------------+----------------------+---------+
| ``b``    | ``signed char``           | integer              | (1)     |
+----------+---------------------------+----------------------+---------+
| ``B``    | ``unsigned char``         | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``?``    | ``_Bool``                 | bool                 | (2)     |
+----------+---------------------------+----------------------+---------+
| ``h``    | ``short``                 | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``H``    | ``unsigned short``        | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``i``    | ``int``                   | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``I``    | ``unsigned int``          | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``l``    | ``long``                  | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``L``    | ``unsigned long``         | integer              |         |
+----------+---------------------------+----------------------+---------+
| ``q``    | ``long long``             | integer              | (3)     |
+----------+---------------------------+----------------------+---------+
| ``Q``    | ``unsigned long long``    | integer              | (3)     |
+----------+---------------------------+----------------------+---------+
| ``f``    | ``float``                 | float                |         |
+----------+---------------------------+----------------------+---------+
| ``d``    | ``double``                | float                |         |
+----------+---------------------------+----------------------+---------+
| ``s``    | ``char[]``                | bytes                | (1)     |
+----------+---------------------------+----------------------+---------+
| ``p``    | ``char[]``                | bytes                | (1)     |
+----------+---------------------------+----------------------+---------+
| ``P``    | ``void *``                | integer              |         |
+----------+---------------------------+----------------------+---------+

Notes:

1. The ``c``, ``s`` and ``p`` conversion codes operate on ``bytes``
   objects, but packing with such codes also supports ``str`` objects,
   which are encoded using UTF-8.

2. The ``'?'`` conversion code corresponds to the ``_Bool`` type
   defined by C99. If this type is not available, it is simulated
   using a ``char``. In standard mode, it is always represented by one
   byte.

3. The ``'q'`` and ``'Q'`` conversion codes are available in native
   mode only if the platform C compiler supports C ``long long``, or,
   on Windows, ``__int64``.  They are always available in standard
   modes.

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
length of the bytes, 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 bytes object 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 ``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 ``unpack()``, the ``'p'`` format character consumes
count bytes, but that the string returned can never contain more than
255 bytes.

For the ``'?'`` format character, the return value is either ``True``
or ``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); ``short`` is 2 bytes;
``int`` and ``long`` are 4 bytes; ``long long`` (``__int64`` on
Windows) is 8 bytes; ``float`` and ``double`` are 32-bit and 64-bit
IEEE floating point numbers, respectively. ``_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)
   b'\x00\x01\x00\x02\x00\x00\x00\x03'
   >>> unpack('hhl', b'\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 = b'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', record))
   Student(name=b'raymond   ', serialnum=4658, school=264, gradelevel=8)

See also:

   Module ``array``
      Packed binary storage of homogeneous data.

   Module ``xdrlib``
      Packing and unpacking of XDR data.


Struct Objects
==============

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

class struct.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
   ``struct`` functions with the same format since the format string
   only needs to be compiled once.

   Compiled Struct objects support the following methods and
   attributes:

   pack(v1, v2, ...)

      Identical to the ``pack()`` function, using the compiled format.
      (``len(result)`` will equal ``self.size``.)

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

      Identical to the ``pack_into()`` function, using the compiled
      format.

   unpack(bytes)

      Identical to the ``unpack()`` function, using the compiled
      format. (``len(bytes)`` must equal ``self.size``).

   unpack_from(buffer[, offset=0])

      Identical to the ``unpack_from()`` function, using the compiled
      format. (``len(buffer[offset:])`` must be at least
      ``self.size``).

   format

      The format string used to construct this Struct object.

   size

      The calculated size of the struct (and hence of the bytes)
      corresponding to ``format``.
