python struct模块

import struct  # a module


# Functions to convert between Python values and C structs.
# Python bytes objects are used to hold the data representing the C struct
# and also as format strings (explained below) to describe the layout of data in the C struct.
# @: native order, size & alignment (default)
# =: native order, std. size & alignment
# <: little-endian, std. size & alignment
# >: big-endian, std. size & alignment
# !: same as >

# x: pad byte (no data); c:char; b:signed byte; B:unsigned byte;
# ?: _Bool (requires C99; if not available, char is used instead)
# h:short; H:unsigned short; i:int; I:unsigned int;
# l:long; L:unsigned long; f:float; d:double; e:half-float.Special cases (preceding decimal count indicates length):
# s:string (array of char); p: pascal string (with count byte).Special cases (only available in native format):
# n:ssize_t; N:size_t;
# P:an integer type that is wide enough to hold a pointer.Special case (not in native mode unless 'long long' in platform C):
# q:long long; Q:unsigned long long
# class Struct(object) Struct(fmt) --> compiled struct object
# Return a new Struct object which writes and reads binary data according to the format string fmt

# iter_unpack(...) S.iter_unpack(buffer) -> iterator(v1, v2, ...)
#
# pack_into(...) S.pack_into(buffer, offset, v1, v2, ...)
# unpack_from(...) S.unpack_from(buffer, offset=0) -> (v1, v2, ...)
#
# pack(...) S.pack(v1, v2, ...) -> bytes
# unpack(...) S.unpack(buffer) -> (v1, v2, ...)


import struct


# 1、struct.pack
a = 20
b = 400

str_ = struct.pack(">ii", a, b)  # 转换后的str虽然是字符串类型，但相当于其他语言中的字节流（字节数组），可以在网络上传输
print('length:', len(str_))
print('type:', type(str_))
print(str_)
print(repr(str_))

# length: 8
# type: <class 'bytes'>
# b'x00x00x00x14x00x00x01x90'
# b'x00x00x00x14x00x00x01x90'


str_ = struct.pack("<ii", a, b)  # 转换后的str虽然是字符串类型，但相当于其他语言中的字节流（字节数组），可以在网络上传输
print('length:', len(str_))
print('type:', type(str_))
print(str_)
print(repr(str_))

# length: 8
# type: <class 'bytes'>
# b'x14x00x00x00x90x01x00x00'
# b'x14x00x00x00x90x01x00x00'


# 2、struct.unpack
a1, a2 = struct.unpack("<ii", str_)  # 返回元祖
print(a1, a2)   # 20，400


# 3、struct.calcsize用于计算格式字符串所对应的结果的长度
print(struct.calcsize('>ii'))  # 8


# 4、struct.pack_into, struct.unpack_from
from ctypes import create_string_buffer

buf = create_string_buffer(12)
print(repr(buf.raw))   # b'x00x00x00x00x00x00x00x00x00x00x00x00'

struct.pack_into(">iii", buf, 0, 1, 2, -1)  # 0是offset
print(repr(buf.raw))   # b'x01x00x00x00x02x00x00x00xffxffxffxff'

print(struct.unpack_from('>iii', buf, 0))  # (1, 2, -1)

# 5、struct.iter_unpack
ret = struct.iter_unpack('>iii', buf)
print(ret)     # <unpack_iterator object at 0x0000000001D02E18>
for r in ret:
    print(r)   # (1, 2, -1)


# 6、练习
print(struct.pack('ci', b'*', 0x12131415))   # b'*x00x00x00x15x14x13x12'
print(struct.pack('ic', 0x12131415, b'*'))   # b'x15x14x13x12*'

record = b'raymond   x32x12x08x01x08'
print(struct.unpack('<10sHHb', record))      # (b'raymond   ', 4658, 264, 8)

字符	字节顺序	大小	对齐
@	本机	本机	本机
=	本机	标准	无
<	小端	标准	无
>	大端	标准	无
!	网络（等于大端）	标准	无

格式	C类型	Python类型	标准大小	提示
x	填充字节	没有值
c	char	长度为1的字节	1
b	signed char	integer	1	1，3
B	unsigned char	integer	1	3
?	_Bool	bool	1	1
h	short	integer	2	3
H	unsigned short	integer	2	3
i	int	integer	4	3
I	unsigned int	integer	4	3
l	long	integer	4	3
L	unsigned long	integer	4	3
q	long long	integer	8	2，3
Q	unsigned long long	integer	8	2，3
n	ssize_t	integer		4
N	size_t	integet		4
e	7	float	2	5
f	float	float	4	5
d	double	float	8	5
s	char[]	bytes
p	char[]	bytes
P	void *	integer		6