數據解析時,python可以相互轉換各種數據類型。最近在斯坦福公開課《密碼學》網站上面做題發現,我對數據轉換很不熟悉,寫下日誌記下用法。
導航
數字 | 字符串 | 字節碼 | |
---|---|---|---|
到數字 | 進制轉換 | 字符轉整數 | 字節串轉整數 |
到字符串 | str() | 字符串編碼解碼 | decode(‘hex’) |
到字節碼 | 數字轉字符串 | 字符串轉字節串 | no |
還有常見的單個字符轉換
函數 | 功能 | 記憶口訣 | 備註 |
---|---|---|---|
chr | 數字轉成對應的ascii字符 | chr長得很像char,因此轉成char | 範圍爲0~255 |
ord | 單個字符轉對應ascii序號 | digit爲最後一個字母 |
進制轉換
10進制轉16進制:
hex(16) ==> 0x10
16進制轉10進制:
int(STRING,BASE)將字符串STRING轉成十進制int,其中STRING的基是base。該函數的第一個參數是字符串
int('0x10', 16) ==> 16
類似的還有八進制oct(), 二進制bin()
16進制字符串轉成二進制
hex_str='00fe' bin(int('1'+hex_str, 16))[3:] #含有前導0 # 結果 '0000000011111110' bin(int(hex_str, 16))[2:] #忽略前導0 # 結果 '11111110'
二進制字符串轉成16進制字符串
bin_str='0b0111000011001100' hex(int(bin_str,2)) # 結果 '0x70cc'
字符to整數
10進制字符串:
int('10') ==> 10
16進制字符串:
int('10', 16) ==> 16 # 或者 int('0x10', 16) ==> 16
字節串to整數
使用網絡數據包常用的struct,兼容C語言的數據結構
struct中支持的格式如下表
Format | C-Type | Python-Type | 字節數 | 備註 |
---|---|---|---|---|
x | pad byte | no value | 1 | |
c | char | string of length 1 | 1 | |
b | signed char | integer | 1 | |
B | unsigned char | integer | 1 | |
? | _Bool | bool | 1 | |
h | short | integer | 2 | |
H | unsigned short | integer | 2 | |
i | int | integer | 4 | |
I | unsigned int | integer or long | 4 | |
l | long | integer | 4 | |
L | unsigned long | long | 4 | |
q | long long | long | 8 | 僅支持64bit機器 |
Q | unsigned long long | long | 8 | 僅支持64bit機器 |
f | float | float | 4 | |
d | double | float | 8 | |
s | char[] | string | 1 | |
p | char[] | string | 1(與機器有關) | 作爲指針 |
P | void * | long | 4 | 作爲指針 |
對齊方式:放在第一個fmt位置
CHARACTER | BYTE ORDER | SIZE | ALIGNMENT |
---|---|---|---|
@ | native | native | native |
= | native | standard | none |
< | little-endian | standard | none |
> | big-endian | standard | none |
! | network (= big-endian) | standard | none |
轉義爲short型整數:
struct.unpack('<hh', bytes(b'\x01\x00\x00\x00')) ==> (1, 0)
轉義爲long型整數:
struct.unpack('<L', bytes(b'\x01\x00\x00\x00')) ==> (1,)
整數to字節串
轉爲兩個字節:
struct.pack('<HH', 1,2) ==> b'\x01\x00\x02\x00'
轉爲四個字節:
struct.pack('<LL', 1,2) ==> b'\x01\x00\x00\x00\x02\x00\x00\x00'
整數to字符串
直接用函數
str(100)
字符串to字節串
我用c++實現的encode(hex)和decode(hex)
decode和encode區別
decode函數是重新解碼,把CT字符串所顯示的69dda8455c7dd425【每隔兩個字符】解碼成十六進制字符\x69\xdd\xa8\x45\x5c\x7d\xd4\x25
CT='69dda8455c7dd425' print "%r"%CT.decode('hex')
encode函數是重新編碼,把CT字符串所顯示的69dda8455c7dd425【每個字符】編碼成acsii值,ascii值爲十六進制顯示,佔兩位。執行下列結果顯示36396464613834353563376464343235等價於將CT第一個字符’6’編碼爲0x36h 第二個字符’9’編碼爲0x39h
CT='69dda8455c7dd425' print "%r"%CT.encode('hex')
可以理解爲:decode解碼,字符串變短一半,encode編碼,字符串變爲兩倍長度
decode(‘ascii’)解碼爲字符串Unicode格式。輸出帶有’u’
encode(‘ascii’),編碼爲Unicode格式,其實python默認處理字符串存儲就是Unicode,輸出結果估計和原來的字符串一樣。
字符串編碼爲字節碼:
'12abc'.encode('ascii') ==> b'12abc'
數字或字符數組:
bytes([1,2, ord('1'),ord('2')]) ==> b'\x01\x0212'
16進制字符串:
bytes().fromhex('010210') ==> b'\x01\x02\x10'
16進制字符串:
bytes(map(ord, '\x01\x02\x31\x32')) ==> b'\x01\x0212'
16進制數組:
bytes([0x01,0x02,0x31,0x32]) ==> b'\x01\x0212'
字節串to字符串
字節碼解碼爲字符串:
bytes(b'\x31\x32\x61\x62').decode('ascii') ==> 12ab
字節串轉16進製表示,夾帶ascii:
str(bytes(b'\x01\x0212'))[2:-1] ==> \x01\x0212
字節串轉16進製表示,固定兩個字符表示:
str(binascii.b2a_hex(b'\x01\x0212'))[2:-1] ==> 01023132
字節串轉16進制數組:
[hex(x) for x in bytes(b'\x01\x0212')] ==> ['0x1', '0x2', '0x31', '0x32']
問題:什麼時候字符串前面加上’r’、’b’、’r’,其實官方文檔有寫。我認爲在Python2中,r和b是等效的。
The Python 2.x documentation:
A prefix of ‘b’ or ‘B’ is ignored in Python 2; it indicates that the literal should become a bytes literal in Python 3 (e.g. when code is automatically converted with 2to3). A ‘u’ or ‘b’ prefix may be followed by an ‘r’ prefix.
‘b’字符加在字符串前面,對於python2會被忽略。加上’b’目的僅僅爲了兼容python3,讓python3以bytes數據類型(0~255)存放這個字符、字符串。
The Python 3.3 documentation states:
Bytes literals are always prefixed with ‘b’ or ‘B’; they produce an instance of the bytes type instead of the str type. They may only contain ASCII characters; bytes with a numeric value of 128 or greater must be expressed with escapes.
數據類型byte總是以’b’爲前綴,該數據類型僅爲ascii。
下面是stackflow上面一個回答。我覺得不錯,拿出來跟大家分享
In Python 2.x
Pre-3.0 versions of Python lacked this kind of distinction between text and binary data. Instead, there was:
unicode = u’…’ literals = sequence of Unicode characters = 3.x str
str = ‘…’ literals = sequences of confounded bytes/characters
Usually text, encoded in some unspecified encoding.
But also used to represent binary data like struct.pack output.
Python 3.x makes a clear distinction between the types:
str = ‘…’ literals = a sequence of Unicode characters (UTF-16 or UTF-32, depending on how Python was compiled)
bytes = b’…’ literals = a sequence of octets (integers between 0 and 255)
CPP實現encode
就是做個筆記,畢竟在做題Cryptography時候用c++寫字符串的處理很蛋疼!爲了防止再次造輪子,記下來。
#include <cstring> //用到strlen函數 static unsigned char ByteMap[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8','9', 'a', 'b', 'c', 'd', 'e', 'f' }; unsigned char hex_2_dec(unsigned char c){ if(c >= '0' && c <= '9') return c - '0'; if(c >= 'a' && c <= 'f') return c - 'a' + 10; } void str_encode(unsigned char *src, unsigned char *dest, int len_of_src) { // 使用注意:dest_len >= 2*len_src +1,最後一位是存放'\0'。 int t1; for (int i = 0; i < len_of_src; ++i) { t1 = (int) src[i]; dest[2 * i] = ByteMap[t1 / 16]; dest[2 * i + 1] = ByteMap[t1 % 16]; } dest[2 * len_of_src] = 0; //必須填充最後一個爲'\0' } void str_decode(unsigned char *src,unsigned char *dest){ int len_of_src=strlen((char *)src); unsigned char t1; for(int i=1;i<=len_of_src;i+=2){ t1=hex_2_dec(src[i-1]); t1= 16*t1 + hex_2_dec(src[i]); dest[i/2]=t1; } }