7.3. struct — Interpret strings as packed binary data
This module performs conversions between Python values and C structs represented
as Python strings. This can be used in handling binary data stored in files or
from network connections, among other sources. It uses
Format Strings as compact descriptions of the layout of the C
structs and the intended conversion to/from Python values.
Note
By default, the result of packing a given C struct includes pad bytes in
order to maintain proper alignment for the C types involved; similarly,
alignment is taken into account when unpacking. This behavior is chosen so
that the bytes of a packed struct correspond exactly to the layout in memory
of the corresponding C struct. To handle platform-independent data formats
or omit implicit pad bytes, use standard size and alignment instead of
native size and alignment: see Byte Order, Size, and Alignment for details.
7.3.1. Functions and Exceptions
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 string 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.
New in version 2.5:
New in version 2.5.
-
struct.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)).
-
struct.unpack_from(fmt, buffer[, offset=0])
Unpack the buffer according to the 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)).
New in version 2.5:
New in version 2.5.
-
struct.calcsize(fmt)
Return the size of the struct (and hence of the string) corresponding to the
given format.
For the 'f' and 'd' conversion codes, the packed representation uses
the IEEE 754 binary32 (for 'f') or binary64 (for 'd') format,
regardless of the floating-point format used by the platform.
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.
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 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 characters.
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 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.
7.3.2.3. Examples
Note
All examples assume a native byte order, size, and alignment with a
big-endian machine.
A basic example of packing/unpacking three integers:
>>> 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
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)
The ordering of format characters may have an impact on size since the padding
needed to satisfy alignment requirements is different:
>>> pack('ci', '*', 0x12131415)
'*\x00\x00\x00\x12\x13\x14\x15'
>>> pack('ic', 0x12131415, '*')
'\x12\x13\x14\x15*'
>>> calcsize('ci')
8
>>> calcsize('ic')
5
The following format 'llh0l' specifies two pad bytes at the end, assuming
longs are aligned on 4-byte boundaries:
>>> pack('llh0l', 1, 2, 3)
'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'
This only works when native size and alignment are in effect; standard size and
alignment does not enforce any alignment.
See also
- Module
array
- Packed binary storage of homogeneous data.
- Module
xdrlib
- Packing and unpacking of XDR data.