Movatterモバイル変換


[0]ホーム

URL:


Navigation

struct — Interpret bytes as packed binary data

This module performs conversions between Python values and C structs representedas Pythonbytes objects. It usesformat strings (explainedbelow) as compact descriptions of the lay-out of the C structs and theintended conversion to/from Python values. This can be used in handlingbinary data stored in files or from network connections, among other sources.

The module defines the following exception and functions:

exceptionstruct.error
Exception raised on various occasions; argument is a string describing what iswrong.
struct.pack(fmt,v1,v2,...)
Return a bytes containing the valuesv1,v2,... packed according to thegiven format. The arguments must match the values required by the formatexactly.
struct.pack_into(fmt,buffer,offset,v1,v2,...)
Pack the valuesv1,v2,... according to the given format, write the packedbytes into the writablebuffer starting atoffset. Note that the offset isa required argument.
struct.unpack(fmt,bytes)
Unpack the bytes (presumably packed bypack(fmt,...)) according to thegiven format. The result is a tuple even if it contains exactly one item. Thebytes must contain exactly the amount of data required by the format(len(bytes) must equalcalcsize(fmt)).
struct.unpack_from(fmt,buffer[,offset=0])
Unpack thebuffer according to tthe given format. The result is a tuple evenif it contains exactly one item. Thebuffer must contain at least the amountof data required by the format (len(buffer[offset:]) must be at leastcalcsize(fmt)).
struct.calcsize(fmt)
Return the size of the struct (and hence of the bytes) corresponding to thegiven format.

Format characters have the following meaning; the conversion between C andPython values should be obvious given their types:

FormatC TypePythonNotes
xpad byteno value 
ccharbytes of length 1 
bsignedcharinteger(1)
Bunsignedcharinteger 
?_Boolbool(2)
hshortinteger 
Hunsignedshortinteger 
iintinteger 
Iunsignedintinteger 
llonginteger 
Lunsignedlonginteger 
qlonglonginteger(3)
Qunsignedlonglonginteger(3)
ffloatfloat 
ddoublefloat 
schar[]bytes(1)
pchar[]bytes(1)
Pvoid*integer 

Notes:

  1. Thec,s andp conversion codes operate onbytesobjects, but packing with such codes also supportsstr objects,which are encoded using UTF-8.
  2. The'?' conversion code corresponds to the_Bool type defined byC99. If this type is not available, it is simulated using achar. Instandard mode, it is always represented by one byte.
  3. The'q' and'Q' conversion codes are available in native mode only ifthe platform C compiler supports Clonglong, 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 mustnot contain whitespace though.

For the's' format character, the count is interpreted as the length of thebytes, 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 tomake it fit. For unpacking, the resulting bytes object always has exactly thespecified number of bytes. As a special case,'0s' means a single, emptystring (while'0c' means 0 characters).

The'p' format character encodes a “Pascal string”, meaning a shortvariable-length string stored in a fixed number of bytes. The count is the totalnumber of bytes stored. The first byte stored is the length of the string, or255, whichever is smaller. The bytes of the string follow. If the stringpassed in topack() is too long (longer than the count minus 1), only theleading count-1 bytes of the string are stored. If the string is shorter thancount-1, it is padded with null bytes so that exactly count bytes in all areused. Note that forunpack(), the'p' format character consumes countbytes, but that the string returned can never contain more than 255 bytes.

For the'?' format character, the return value is eitherTrue orFalse. 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, andany non-zero value will be True when unpacking.

By default, C numbers are represented in the machine’s native format and byteorder, and properly aligned by skipping pad bytes if necessary (according to therules used by the C compiler).

Alternatively, the first character of the format string can be used to indicatethe byte order, size and alignment of the packed data, according to thefollowing table:

CharacterByte orderSize and alignment
@nativenative
=nativestandard
<little-endianstandard
>big-endianstandard
!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 DECprocessors are little-endian.

Native size and alignment are determined using the C compiler’ssizeof expression. This is always combined with native byte order.

Standard size and alignment are as follows: no alignment is required for anytype (so you have to use pad bytes);short is 2 bytes;int andlong are 4 bytes;longlong (__int64 on Windows) is 8bytes;float anddouble are 32-bit and 64-bit IEEE floatingpoint numbers, respectively._Bool is 1 byte.

Note the difference between'@' and'=': both use native byte order, butthe size and alignment of the latter is standardized.

The form'!' is available for those poor souls who claim they can’t rememberwhether network byte order is big-endian or little-endian.

There is no way to indicate non-native byte order (force byte-swapping); use theappropriate 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 byteorder character'=' chooses to use little- or big-endian ordering based onthe 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-endianmachine):

>>>fromstructimport*>>>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 aparticular type, end the format with the code for that type with a repeat countof zero. For example, the format'llh0l' specifies two pad bytes at theend, assuming longs are aligned on 4-byte boundaries. This only works whennative size and alignment are in effect; standard size and alignment does notenforce any alignment.

Unpacked fields can be named by assigning them to variables or by wrappingthe result in a named tuple:

>>>record=b'raymond\x32\x12\x08\x01\x08'>>>name,serialnum,school,gradelevel=unpack('<10sHHb',record)>>>fromcollectionsimportnamedtuple>>>Student=namedtuple('Student','name serialnum school gradelevel')>>>Student._make(unpack('<10sHHb',record))Student(name=b'raymond   ', serialnum=4658, school=264, gradelevel=8)

See also

Modulearray
Packed binary storage of homogeneous data.
Modulexdrlib
Packing and unpacking of XDR data.

Struct Objects

Thestruct module also defines the following type:

classstruct.Struct(format)

Return a new Struct object which writes and reads binary data according to theformat stringformat. Creating a Struct object once and calling its methodsis more efficient than calling thestruct functions with the same formatsince the format string only needs to be compiled once.

Compiled Struct objects support the following methods and attributes:

pack(v1,v2,...)
Identical to thepack() function, using the compiled format.(len(result) will equalself.size.)
pack_into(buffer,offset,v1,v2,...)
Identical to thepack_into() function, using the compiled format.
unpack(bytes)
Identical to theunpack() function, using the compiled format.(len(bytes) must equalself.size).
unpack_from(buffer[,offset=0])
Identical to theunpack_from() function, using the compiled format.(len(buffer[offset:]) must be at leastself.size).
format
The format string used to construct this Struct object.
size
The calculated size of the struct (and hence of the bytes) correspondingtoformat.

Table Of Contents

Previous topic

re — Regular expression operations

Next topic

difflib — Helpers for computing deltas

This Page

Quick search

Enter search terms or a module, class or function name.

Navigation

©Copyright 1990-2009, Python Software Foundation. Last updated on Feb 14, 2009. Created usingSphinx 0.6.

[8]ページ先頭

©2009-2025 Movatter.jp