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.
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.
The module defines the following exception and functions:
pack(fmt, v1, v2, ...)
v1, v2, ...packed according to the given format. The arguments must match the values required by the format exactly.
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.
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 (
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
New in version 2.5.
Format strings are the mechanism used to specify the expected layout when packing and unpacking data. They are built up from Format Characters, which specify the type of data being packed/unpacked. In addition, there are special characters for controlling the Byte Order, Size, and Alignment.
By default, C types 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:
||network (= big-endian)||standard||none|
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, Intel x86 and AMD64 (x86-64) are little-endian;
Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature
switchable endianness (bi-endian). Use
sys.byteorder to check the
endianness of your system.
Native size and alignment are determined using the C compiler’s
sizeof expression. This is always combined with native byte order.
Standard size depends only on the format character; see the table in the Format Characters section.
Note the difference between
'=': both use native byte order, but
the size and alignment of the latter is standardized.
'!' 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
Format characters have the following meaning; the conversion between C and
Python values should be obvious given their types. The ‘Standard size’ column
refers to the size of the packed value in bytes when using standard size; that
is, when the format string starts with one of
'='. When using native size, the size of the packed value is
|Format||C Type||Python type||Standard size||Notes|
||pad byte||no value|
||string of length 1||1|
'?' conversion code corresponds to the
_Bool type defined by
C99. If this type is not available, it is simulated using a
standard mode, it is always represented by one byte.
New in version 2.6.
'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.
New in version 2.2.
When attempting to pack a non-integer using any of the integer conversion
codes, if the non-integer has a
__index__() method then that method is
called to convert the argument to an integer before packing. If no
__index__() method exists, or the call to
TypeError, then the
__int__() method is tried. However, the use
__int__() is deprecated, and will raise
Changed in version 2.7: Use of the
__index__() method for non-integers is new in 2.7.
'd' conversion codes, the packed representation uses
the IEEE 754 binary32 (for
'f') or binary64 (for
regardless of the floating-point format used by the platform.
'P' format character is only available for the native byte ordering
(selected as the default or with the
'@' byte order character). The byte
'=' 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
Whitespace characters between formats are ignored; a count and its format must not contain whitespace though.
'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.
If a count is not given, it defaults to 1. 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).
'p' format character encodes a “Pascal string”, meaning a short
variable-length string stored in a fixed number of bytes, given by the count.
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
'p' format character consumes count bytes, but that the
string returned can never contain more than 255 characters.
'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.
'?' format character, the return value is either
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.
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', record)) 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.
struct module also defines the following type:
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.
New in version 2.5.
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
pack_into(buffer, offset, v1, v2, ...)
Identical to the
pack_into() function, using the compiled format.
Identical to the
unpack() function, using the compiled format.
len(string) must equal
Identical to the
unpack_from() function, using the compiled format.
len(buffer[offset:]) must be at least
The format string used to construct this Struct object.
The calculated size of the struct (and hence of the string) corresponding