struct
— Interpreta bytes como dados binários empacotados¶
Código-fonte: Lib/struct.py
Este módulo converte entre valores Python e structs C representados como objetos bytes
do Python. Strings de formato compactas descrevem as conversões pretendidas de/para valores Python. As funções e objetos do módulo podem ser usados para duas aplicações amplamente distintas, troca de dados com fontes externas (arquivos ou conexões de rede) ou transferência de dados entre a aplicação Python e a camada C.
Nota
Quando nenhum caractere de prefixo é fornecido, o modo nativo é o padrão. Ele empacota ou descompacta dados com base na plataforma e no compilador nos quais o interpretador Python foi construído. O resultado do empacotamento de uma dada estrutura C inclui bytes de preenchimento que mantêm o alinhamento adequado para os tipos C envolvidos; similarmente, o alinhamento é levado em conta ao desempacotar. Em contraste, ao comunicar dados entre fontes externas, o programador é responsável por definir a ordenação de bytes e o preenchimento entre os elementos. Veja Byte Order, Size, and Alignment para detalhes.
Várias funções struct
(e métodos de Struct
) recebem um argumento buffer. Isso se refere a objetos que implementam Protocolo de Buffer e fornecem um buffer legível ou de leitura-gravação. Os tipos mais comuns usados para esse propósito são bytes
e bytearray
, mas muitos outros tipos que podem ser vistos como uma matriz de bytes implementam o protocolo de buffer, para que possam ser lidos/preenchidos sem cópia adicional de um objeto bytes
.
Funções e exceções¶
O módulo define as seguintes exceções e funções:
- exception struct.error¶
Exceção levantada em várias ocasiões; argumento é uma string que descreve o que está errado.
- struct.pack(format, v1, v2, ...)¶
Return a bytes object containing the values v1, v2, … packed according to the format string format. The arguments must match the values required by the format exactly.
- struct.pack_into(format, buffer, offset, v1, v2, ...)¶
Pack the values v1, v2, … according to the format string format and write the packed bytes into the writable buffer buffer starting at position offset. Note that offset is a required argument.
- struct.unpack(format, buffer)¶
Unpack from the buffer buffer (presumably packed by
pack(format, ...)
) according to the format string format. 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 bycalcsize()
.
- struct.unpack_from(format, /, buffer, offset=0)¶
Unpack from buffer starting at position offset, according to the format string format. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes, starting at position offset, must be at least the size required by the format, as reflected by
calcsize()
.
- struct.iter_unpack(format, buffer)¶
Iteratively unpack from the buffer buffer according to the format string format. This function returns an iterator which will read equally sized chunks from the buffer until all its contents have been consumed. The buffer’s size in bytes must be a multiple of the size required by the format, as reflected by
calcsize()
.Each iteration yields a tuple as specified by the format string.
Adicionado na versão 3.4.
- struct.calcsize(format)¶
Return the size of the struct (and hence of the bytes object produced by
pack(format, ...)
) corresponding to the format string format.
Format Strings¶
Format strings describe the data layout when packing and unpacking data. They are built up from format characters, which specify the type of data being packed/unpacked. In addition, special characters control the byte order, size and alignment. Each format string consists of an optional prefix character which describes the overall properties of the data and one or more format characters which describe the actual data values and padding.
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). This behavior is chosen so that the bytes of a packed struct correspond exactly to the memory layout of the corresponding C struct. Whether to use native byte ordering and padding or standard formats depends on the application.
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:
Caractere |
Ordem de bytes |
Tamanho |
Alinhamento |
---|---|---|---|
|
nativo |
nativo |
nativo |
|
nativo |
padrão |
nenhum |
|
little-endian |
padrão |
nenhum |
|
big-endian |
padrão |
nenhum |
|
rede (= big-endian) |
padrão |
nenhum |
Se o primeiro caractere não for um destes, '@'
é presumido.
Nota
The number 1023 (0x3ff
in hexadecimal) has the following byte representations:
03 ff
in big-endian (>
)ff 03
in little-endian (<
)
Python example:
>>> import struct
>>> struct.pack('>h', 1023)
b'\x03\xff'
>>> struct.pack('<h', 1023)
b'\xff\x03'
Native byte order is big-endian or little-endian, depending on the
host system. For example, Intel x86, AMD64 (x86-64), and Apple M1 are
little-endian; IBM z and many legacy architectures are big-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 Caracteres Formatados section.
Note the difference between '@'
and '='
: both use native byte order, but
the size and alignment of the latter is standardized.
The form '!'
represents the network byte order which is always big-endian
as defined in IETF RFC 1700.
There is no way to indicate non-native byte order (force byte-swapping); use the
appropriate choice of '<'
or '>'
.
Notas:
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 Exemplos.
Caracteres Formatados¶
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 '<'
, '>'
, '!'
or
'='
. When using native size, the size of the packed value is
platform-dependent.
Formatação |
Tipo em C |
Tipo em Python |
Standard size |
Notas |
---|---|---|---|---|
|
pad byte |
no value |
(7) |
|
|
char |
bytes of length 1 |
1 |
|
|
signed char |
inteiro |
1 |
(1), (2) |
|
unsigned char |
inteiro |
1 |
(2) |
|
_Bool |
bool |
1 |
(1) |
|
short |
inteiro |
2 |
(2) |
|
unsigned short |
inteiro |
2 |
(2) |
|
int |
inteiro |
4 |
(2) |
|
unsigned int |
inteiro |
4 |
(2) |
|
long |
inteiro |
4 |
(2) |
|
unsigned long |
inteiro |
4 |
(2) |
|
long long |
inteiro |
8 |
(2) |
|
unsigned long long |
inteiro |
8 |
(2) |
|
|
inteiro |
(3) |
|
|
|
inteiro |
(3) |
|
|
(6) |
ponto flutuante |
2 |
(4) |
|
float |
ponto flutuante |
4 |
(4) |
|
double |
ponto flutuante |
8 |
(4) |
|
char[] |
bytes |
(9) |
|
|
char[] |
bytes |
(8) |
|
|
void* |
inteiro |
(5) |
Alterado na versão 3.3: Added support for the 'n'
and 'N'
formats.
Alterado na versão 3.6: Added support for the 'e'
format.
Notas:
The
'?'
conversion code corresponds to the _Bool type defined by C standards since C99. In standard mode, it is represented by one byte.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.Alterado na versão 3.2: Added use of the
__index__()
method for non-integers.The
'n'
and'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.For the
'f'
,'d'
and'e'
conversion codes, the packed representation uses the IEEE 754 binary32, binary64 or binary16 format (for'f'
,'d'
or'e'
respectively), 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.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
6.1e-05
and6.5e+04
at 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.When packing,
'x'
inserts one NUL byte.The
'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 topack()
is too long (longer than the count minus 1), only the leadingcount-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 are used. Note that forunpack()
, the'p'
format character consumescount
bytes, but that the string returned can never contain more than 255 bytes.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 mapping to or from a single Python byte string, while'10c'
means 10 separate one byte character elements (e.g.,cccccccccc
) mapping to or from ten different Python byte objects. (See Exemplos for a concrete demonstration of the difference.) 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).
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.
When packing a value x
using one of the integer formats ('b'
,
'B'
, 'h'
, 'H'
, 'i'
, 'I'
, 'l'
, 'L'
,
'q'
, 'Q'
), if x
is outside the valid range for that format
then struct.error
is raised.
Alterado na versão 3.1: Previously, some of the integer formats wrapped out-of-range values and
raised DeprecationWarning
instead of struct.error
.
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.
Exemplos¶
Nota
Native byte order examples (designated by the '@'
format prefix or
lack of any prefix character) may not match what the reader’s
machine produces as
that depends on the platform and compiler.
Pack and unpack integers of three different sizes, using big endian ordering:
>>> from struct import *
>>> pack(">bhl", 1, 2, 3)
b'\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('>bhl', b'\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
>>> calcsize('>bhl')
7
Attempt to pack an integer which is too large for the defined field:
>>> pack(">h", 99999)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
struct.error: 'h' format requires -32768 <= number <= 32767
Demonstrate the difference between 's'
and 'c'
format
characters:
>>> pack("@ccc", b'1', b'2', b'3')
b'123'
>>> pack("@3s", b'123')
b'123'
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)
The ordering of format characters may have an impact on size in native
mode since padding is implicit. In standard mode, the user is
responsible for inserting any desired padding.
Note in
the first pack
call below that three NUL bytes were added after the
packed '#'
to align the following integer on a four-byte boundary.
In this example, the output was produced on a little endian machine:
>>> pack('@ci', b'#', 0x12131415)
b'#\x00\x00\x00\x15\x14\x13\x12'
>>> pack('@ic', 0x12131415, b'#')
b'\x15\x14\x13\x12#'
>>> calcsize('@ci')
8
>>> calcsize('@ic')
5
The following format 'llh0l'
results in two pad bytes being added
at the end, assuming the platform’s longs are aligned on 4-byte boundaries:
>>> pack('@llh0l', 1, 2, 3)
b'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'
Applications¶
Two main applications for the struct
module exist, data
interchange between Python and C code within an application or another
application compiled using the same compiler (native formats), and
data interchange between applications using agreed upon data layout
(standard formats). Generally speaking, the format strings
constructed for these two domains are distinct.
Native Formats¶
When constructing format strings which mimic native layouts, the
compiler and machine architecture determine byte ordering and padding.
In such cases, the @
format character should be used to specify
native byte ordering and data sizes. Internal pad bytes are normally inserted
automatically. It is possible that a zero-repeat format code will be
needed at the end of a format string to round up to the correct
byte boundary for proper alignment of consecutive chunks of data.
Consider these two simple examples (on a 64-bit, little-endian machine):
>>> calcsize('@lhl')
24
>>> calcsize('@llh')
18
Data is not padded to an 8-byte boundary at the end of the second format string without the use of extra padding. A zero-repeat format code solves that problem:
>>> calcsize('@llh0l')
24
The 'x'
format code can be used to specify the repeat, but for
native formats it is better to use a zero-repeat format like '0l'
.
By default, native byte ordering and alignment is used, but it is
better to be explicit and use the '@'
prefix character.
Standard Formats¶
When exchanging data beyond your process such as networking or storage,
be precise. Specify the exact byte order, size, and alignment. Do
not assume they match the native order of a particular machine.
For example, network byte order is big-endian, while many popular CPUs
are little-endian. By defining this explicitly, the user need not
care about the specifics of the platform their code is running on.
The first character should typically be <
or >
(or !
). Padding is the responsibility of the programmer. The
zero-repeat format character won’t work. Instead, the user must
explicitly add 'x'
pad bytes where needed. Revisiting the
examples from the previous section, we have:
>>> calcsize('<qh6xq')
24
>>> pack('<qh6xq', 1, 2, 3) == pack('@lhl', 1, 2, 3)
True
>>> calcsize('@llh')
18
>>> pack('@llh', 1, 2, 3) == pack('<qqh', 1, 2, 3)
True
>>> calcsize('<qqh6x')
24
>>> calcsize('@llh0l')
24
>>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
True
The above results (executed on a 64-bit machine) aren’t guaranteed to match when executed on different machines. For example, the examples below were executed on a 32-bit machine:
>>> calcsize('<qqh6x')
24
>>> calcsize('@llh0l')
12
>>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
False
Classes¶
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 module-level functions with the same format since the format string is only compiled once.Nota
The compiled versions of the most recent format strings passed to the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single
Struct
instance.Compiled Struct objects support the following methods and attributes:
- pack(v1, v2, ...)¶
Identical to the
pack()
function, using the compiled format. (len(result)
will equalsize
.)
- pack_into(buffer, offset, v1, v2, ...)¶
Identical to the
pack_into()
function, using the compiled format.
- unpack(buffer)¶
Identical to the
unpack()
function, using the compiled format. The buffer’s size in bytes must equalsize
.
- unpack_from(buffer, offset=0)¶
Identical to the
unpack_from()
function, using the compiled format. The buffer’s size in bytes, starting at position offset, must be at leastsize
.
- iter_unpack(buffer)¶
Identical to the
iter_unpack()
function, using the compiled format. The buffer’s size in bytes must be a multiple ofsize
.Adicionado na versão 3.4.
- format¶
The format string used to construct this Struct object.