This module performs conversions between Python values and C structs represented
bytes objects. 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.
struct functions take a buffer
argument. This refers to objects of class bytearray.
Functions and Exceptions¶
The module defines the following exception and functions:
Exception raised on various occasions;
pack(fmt, v1, v2, ...)¶
Return a bytes object containing the values v1, v2, ... packed according to the format string fmt. 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 format string fmt and write the packed bytes into the writable buffer buffer starting at position offset. Note that offset is a required argument.
Unpack from the buffer buffer (presumably packed by
pack(fmt, ...)) according to the format string fmt. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes must match the size required by the format, as reflected by
unpack_from(fmt, buffer, offset=0)¶
Unpack from buffer starting at position offset, according to the format string fmt. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes, minus offset, must be at least the size required by the format, as reflected by
Return the size of the struct (and hence of the bytes object produced by
pack(fmt, ...)) corresponding to the format string fmt.
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.
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.
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.
There is no way to indicate non-native byte order (force byte-swapping); use the
appropriate choice of
- Padding is only automatically added between successive structure members. No padding is added at the beginning or the end of the encoded struct.
- No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’, ‘=’, and ‘!’.
- 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. See Examples.
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|
||bytes of length 1||1|
'?'conversion code corresponds to the
_Booltype 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.
'Q'conversion codes are not supported!
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.
Changed in version 3.2: Use of the
__index__()method for non-integers is new in 3.2.
'N'conversion codes are only available for the native size (selected as the default or with the
'@'byte order character). For the standard size, you can use whichever of the other integer formats fits your application.
'e'conversion codes, the packed representation uses the IEEE 754 binary32, binary64 or binary16 format (for
'e'respectively), regardless of the floating-point format used by the platform. The
'e'format is not supported!
'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.
The IEEE 754 binary16 “half precision” type was introduced in the 2008 revision of the IEEE 754 standard. It has a sign bit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored), and can represent numbers between approximately
6.5e+04at full precision. This type is not widely supported by C compilers: on a typical machine, an unsigned short can be used for storage, but not for math operations. See the Wikipedia page on the half-precision floating-point format for more information.
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 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.
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 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).
When packing a value
x using one of the integer formats (
x is outside the valid range for that format
struct.error is raised.
'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
'?' 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) b'