4.2.2 编码和UNICODE
字符串是把编码表0x0--0x10FFFF里的字符组成一串的序列。当字符串在不同的CPU和不同架构的内存里保存时，字节顺序和怎么样保存这些字符就成为了一个问题。因此提供很多编解码的组件，把一个字符串对象序列化成一些字节数组就叫做编码，而从从这些字节数组再构造成字符串对象，就叫做解码。在Python有不同类型的编码和解码对象，就叫做文本编码处理。
最简单的文本编码就是latin-1或叫做iso-8859-1编码了，它简单地把0-255映射为字节0x0-0xFF，如果超过出U+00FF的值后不能处理，就会抛出异常UnicodeEncoderError，详细这样：UnicodeEncodeError: 'latin-1' codec can't encode character '\u1234' in position 3: ordinal not in range(256)。
另外一种编码就是采用映射编码，主要采用字典查找的方式。比如不同的UNICODE的字符集都可以映射在0x0-0xFF里面，为了理解起来方便，可以查看最简单的编码文件，它在库目录encodings/cp1252.py里。可以通过256个字符来查找它对应在那一个字节编码里。具体的代码如下：
""" Python Character Mapping Codec cp1252 generated from 'MAPPINGS/VENDORS/MICSFT/WINDOWS/CP1252.TXT' with gencodec.py.
"""#"
import codecs
### Codec APIs
class Codec(codecs.Codec):
def encode(self,input,errors='strict'):
return codecs.charmap_encode(input,errors,encoding_table)
def decode(self,input,errors='strict'):
return codecs.charmap_decode(input,errors,decoding_table)
class IncrementalEncoder(codecs.IncrementalEncoder):
def encode(self, input, final=False):
return codecs.charmap_encode(input,self.errors,encoding_table)[0]
class IncrementalDecoder(codecs.IncrementalDecoder):
def decode(self, input, final=False):
return codecs.charmap_decode(input,self.errors,decoding_table)[0]
class StreamWriter(Codec,codecs.StreamWriter):
pass
class StreamReader(Codec,codecs.StreamReader):
pass
### encodings module API
def getregentry():
return codecs.CodecInfo(
name='cp1252',
encode=Codec().encode,
decode=Codec().decode,
incrementalencoder=IncrementalEncoder,
incrementaldecoder=IncrementalDecoder,
streamreader=StreamReader,
streamwriter=StreamWriter,
)
### Decoding Table
decoding_table = (
'\x00' # 0x00 -> NULL
'\x01' # 0x01 -> START OF HEADING
'\x02' # 0x02 -> START OF TEXT
'\x03' # 0x03 -> END OF TEXT
'\x04' # 0x04 -> END OF TRANSMISSION
'\x05' # 0x05 -> ENQUIRY
'\x06' # 0x06 -> ACKNOWLEDGE
'\x07' # 0x07 -> BELL
'\x08' # 0x08 -> BACKSPACE
'\t' # 0x09 -> HORIZONTAL TABULATION
'\n' # 0x0A -> LINE FEED
'\x0b' # 0x0B -> VERTICAL TABULATION
'\x0c' # 0x0C -> FORM FEED
'\r' # 0x0D -> CARRIAGE RETURN
'\x0e' # 0x0E -> SHIFT OUT
'\x0f' # 0x0F -> SHIFT IN
'\x10' # 0x10 -> DATA LINK ESCAPE
'\x11' # 0x11 -> DEVICE CONTROL ONE
'\x12' # 0x12 -> DEVICE CONTROL TWO
'\x13' # 0x13 -> DEVICE CONTROL THREE
'\x14' # 0x14 -> DEVICE CONTROL FOUR
'\x15' # 0x15 -> NEGATIVE ACKNOWLEDGE
'\x16' # 0x16 -> SYNCHRONOUS IDLE
'\x17' # 0x17 -> END OF TRANSMISSION BLOCK
'\x18' # 0x18 -> CANCEL
'\x19' # 0x19 -> END OF MEDIUM
'\x1a' # 0x1A -> SUBSTITUTE
'\x1b' # 0x1B -> ESCAPE
'\x1c' # 0x1C -> FILE SEPARATOR
'\x1d' # 0x1D -> GROUP SEPARATOR
'\x1e' # 0x1E -> RECORD SEPARATOR
'\x1f' # 0x1F -> UNIT SEPARATOR
' ' # 0x20 -> SPACE
'!' # 0x21 -> EXCLAMATION MARK
'"' # 0x22 -> QUOTATION MARK
'#' # 0x23 -> NUMBER SIGN
'$' # 0x24 -> DOLLAR SIGN
'%' # 0x25 -> PERCENT SIGN
'&' # 0x26 -> AMPERSAND
"'" # 0x27 -> APOSTROPHE
'(' # 0x28 -> LEFT PARENTHESIS
')' # 0x29 -> RIGHT PARENTHESIS
'*' # 0x2A -> ASTERISK
'+' # 0x2B -> PLUS SIGN
',' # 0x2C -> COMMA
'-' # 0x2D -> HYPHEN-MINUS
'.' # 0x2E -> FULL STOP
'/' # 0x2F -> SOLIDUS
'0' # 0x30 -> DIGIT ZERO
'1' # 0x31 -> DIGIT ONE
'2' # 0x32 -> DIGIT TWO
'3' # 0x33 -> DIGIT THREE
'4' # 0x34 -> DIGIT FOUR
'5' # 0x35 -> DIGIT FIVE
'6' # 0x36 -> DIGIT SIX
'7' # 0x37 -> DIGIT SEVEN
'8' # 0x38 -> DIGIT EIGHT
'9' # 0x39 -> DIGIT NINE
':' # 0x3A -> COLON
';' # 0x3B -> SEMICOLON
'<' # 0x3C -> LESS-THAN SIGN
'=' # 0x3D -> EQUALS SIGN
'>' # 0x3E -> GREATER-THAN SIGN
'?' # 0x3F -> QUESTION MARK
'@' # 0x40 -> COMMERCIAL AT
'A' # 0x41 -> LATIN CAPITAL LETTER A
'B' # 0x42 -> LATIN CAPITAL LETTER B
'C' # 0x43 -> LATIN CAPITAL LETTER C
'D' # 0x44 -> LATIN CAPITAL LETTER D
'E' # 0x45 -> LATIN CAPITAL LETTER E
'F' # 0x46 -> LATIN CAPITAL LETTER F
'G' # 0x47 -> LATIN CAPITAL LETTER G
'H' # 0x48 -> LATIN CAPITAL LETTER H
'I' # 0x49 -> LATIN CAPITAL LETTER I
'J' # 0x4A -> LATIN CAPITAL LETTER J
'K' # 0x4B -> LATIN CAPITAL LETTER K
'L' # 0x4C -> LATIN CAPITAL LETTER L
'M' # 0x4D -> LATIN CAPITAL LETTER M
'N' # 0x4E -> LATIN CAPITAL LETTER N
'O' # 0x4F -> LATIN CAPITAL LETTER O
'P' # 0x50 -> LATIN CAPITAL LETTER P
'Q' # 0x51 -> LATIN CAPITAL LETTER Q
'R' # 0x52 -> LATIN CAPITAL LETTER R
'S' # 0x53 -> LATIN CAPITAL LETTER S
'T' # 0x54 -> LATIN CAPITAL LETTER T
'U' # 0x55 -> LATIN CAPITAL LETTER U
'V' # 0x56 -> LATIN CAPITAL LETTER V
'W' # 0x57 -> LATIN CAPITAL LETTER W
'X' # 0x58 -> LATIN CAPITAL LETTER X
'Y' # 0x59 -> LATIN CAPITAL LETTER Y
'Z' # 0x5A -> LATIN CAPITAL LETTER Z
'[' # 0x5B -> LEFT SQUARE BRACKET
'\\' # 0x5C -> REVERSE SOLIDUS
']' # 0x5D -> RIGHT SQUARE BRACKET
'^' # 0x5E -> CIRCUMFLEX ACCENT
'_' # 0x5F -> LOW LINE
'`' # 0x60 -> GRAVE ACCENT
'a' # 0x61 -> LATIN SMALL LETTER A
'b' # 0x62 -> LATIN SMALL LETTER B
'c' # 0x63 -> LATIN SMALL LETTER C
'd' # 0x64 -> LATIN SMALL LETTER D
'e' # 0x65 -> LATIN SMALL LETTER E
'f' # 0x66 -> LATIN SMALL LETTER F
'g' # 0x67 -> LATIN SMALL LETTER G
'h' # 0x68 -> LATIN SMALL LETTER H
'i' # 0x69 -> LATIN SMALL LETTER I
'j' # 0x6A -> LATIN SMALL LETTER J
'k' # 0x6B -> LATIN SMALL LETTER K
'l' # 0x6C -> LATIN SMALL LETTER L
'm' # 0x6D -> LATIN SMALL LETTER M
'n' # 0x6E -> LATIN SMALL LETTER N
'o' # 0x6F -> LATIN SMALL LETTER O
'p' # 0x70 -> LATIN SMALL LETTER P
'q' # 0x71 -> LATIN SMALL LETTER Q
'r' # 0x72 -> LATIN SMALL LETTER R
's' # 0x73 -> LATIN SMALL LETTER S
't' # 0x74 -> LATIN SMALL LETTER T
'u' # 0x75 -> LATIN SMALL LETTER U
'v' # 0x76 -> LATIN SMALL LETTER V
'w' # 0x77 -> LATIN SMALL LETTER W
'x' # 0x78 -> LATIN SMALL LETTER X
'y' # 0x79 -> LATIN SMALL LETTER Y
'z' # 0x7A -> LATIN SMALL LETTER Z
'{' # 0x7B -> LEFT CURLY BRACKET
'|' # 0x7C -> VERTICAL LINE
'}' # 0x7D -> RIGHT CURLY BRACKET
'~' # 0x7E -> TILDE
'\x7f' # 0x7F -> DELETE
'\u20ac' # 0x80 -> EURO SIGN
'\ufffe' # 0x81 -> UNDEFINED
'\u201a' # 0x82 -> SINGLE LOW-9 QUOTATION MARK
'\u0192' # 0x83 -> LATIN SMALL LETTER F WITH HOOK
'\u201e' # 0x84 -> DOUBLE LOW-9 QUOTATION MARK
'\u2026' # 0x85 -> HORIZONTAL ELLIPSIS
'\u2020' # 0x86 -> DAGGER
'\u2021' # 0x87 -> DOUBLE DAGGER
'\u02c6' # 0x88 -> MODIFIER LETTER CIRCUMFLEX ACCENT
'\u2030' # 0x89 -> PER MILLE SIGN
'\u0160' # 0x8A -> LATIN CAPITAL LETTER S WITH CARON
'\u2039' # 0x8B -> SINGLE LEFT-POINTING ANGLE QUOTATION MARK
'\u0152' # 0x8C -> LATIN CAPITAL LIGATURE OE
'\ufffe' # 0x8D -> UNDEFINED
'\u017d' # 0x8E -> LATIN CAPITAL LETTER Z WITH CARON
'\ufffe' # 0x8F -> UNDEFINED
'\ufffe' # 0x90 -> UNDEFINED
'\u2018' # 0x91 -> LEFT SINGLE QUOTATION MARK
'\u2019' # 0x92 -> RIGHT SINGLE QUOTATION MARK
'\u201c' # 0x93 -> LEFT DOUBLE QUOTATION MARK
'\u201d' # 0x94 -> RIGHT DOUBLE QUOTATION MARK
'\u2022' # 0x95 -> BULLET
'\u2013' # 0x96 -> EN DASH
'\u2014' # 0x97 -> EM DASH
'\u02dc' # 0x98 -> SMALL TILDE
'\u2122' # 0x99 -> TRADE MARK SIGN
'\u0161' # 0x9A -> LATIN SMALL LETTER S WITH CARON
'\u203a' # 0x9B -> SINGLE RIGHT-POINTING ANGLE QUOTATION MARK
'\u0153' # 0x9C -> LATIN SMALL LIGATURE OE
'\ufffe' # 0x9D -> UNDEFINED
'\u017e' # 0x9E -> LATIN SMALL LETTER Z WITH CARON
'\u0178' # 0x9F -> LATIN CAPITAL LETTER Y WITH DIAERESIS
'\xa0' # 0xA0 -> NO-BREAK SPACE
'\xa1' # 0xA1 -> INVERTED EXCLAMATION MARK
'\xa2' # 0xA2 -> CENT SIGN
'\xa3' # 0xA3 -> POUND SIGN
'\xa4' # 0xA4 -> CURRENCY SIGN
'\xa5' # 0xA5 -> YEN SIGN
'\xa6' # 0xA6 -> BROKEN BAR
'\xa7' # 0xA7 -> SECTION SIGN
'\xa8' # 0xA8 -> DIAERESIS
'\xa9' # 0xA9 -> COPYRIGHT SIGN
'\xaa' # 0xAA -> FEMININE ORDINAL INDICATOR
'\xab' # 0xAB -> LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
'\xac' # 0xAC -> NOT SIGN
'\xad' # 0xAD -> SOFT HYPHEN
'\xae' # 0xAE -> REGISTERED SIGN
'\xaf' # 0xAF -> MACRON
'\xb0' # 0xB0 -> DEGREE SIGN
'\xb1' # 0xB1 -> PLUS-MINUS SIGN
'\xb2' # 0xB2 -> SUPERSCRIPT TWO
'\xb3' # 0xB3 -> SUPERSCRIPT THREE
'\xb4' # 0xB4 -> ACUTE ACCENT
'\xb5' # 0xB5 -> MICRO SIGN
'\xb6' # 0xB6 -> PILCROW SIGN
'\xb7' # 0xB7 -> MIDDLE DOT
'\xb8' # 0xB8 -> CEDILLA
'\xb9' # 0xB9 -> SUPERSCRIPT ONE
'\xba' # 0xBA -> MASCULINE ORDINAL INDICATOR
'\xbb' # 0xBB -> RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
'\xbc' # 0xBC -> VULGAR FRACTION ONE QUARTER
'\xbd' # 0xBD -> VULGAR FRACTION ONE HALF
'\xbe' # 0xBE -> VULGAR FRACTION THREE QUARTERS
'\xbf' # 0xBF -> INVERTED QUESTION MARK
'\xc0' # 0xC0 -> LATIN CAPITAL LETTER A WITH GRAVE
'\xc1' # 0xC1 -> LATIN CAPITAL LETTER A WITH ACUTE
'\xc2' # 0xC2 -> LATIN CAPITAL LETTER A WITH CIRCUMFLEX
'\xc3' # 0xC3 -> LATIN CAPITAL LETTER A WITH TILDE
'\xc4' # 0xC4 -> LATIN CAPITAL LETTER A WITH DIAERESIS
'\xc5' # 0xC5 -> LATIN CAPITAL LETTER A WITH RING ABOVE
'\xc6' # 0xC6 -> LATIN CAPITAL LETTER AE
'\xc7' # 0xC7 -> LATIN CAPITAL LETTER C WITH CEDILLA
'\xc8' # 0xC8 -> LATIN CAPITAL LETTER E WITH GRAVE
'\xc9' # 0xC9 -> LATIN CAPITAL LETTER E WITH ACUTE
'\xca' # 0xCA -> LATIN CAPITAL LETTER E WITH CIRCUMFLEX
'\xcb' # 0xCB -> LATIN CAPITAL LETTER E WITH DIAERESIS
'\xcc' # 0xCC -> LATIN CAPITAL LETTER I WITH GRAVE
'\xcd' # 0xCD -> LATIN CAPITAL LETTER I WITH ACUTE
'\xce' # 0xCE -> LATIN CAPITAL LETTER I WITH CIRCUMFLEX
'\xcf' # 0xCF -> LATIN CAPITAL LETTER I WITH DIAERESIS
'\xd0' # 0xD0 -> LATIN CAPITAL LETTER ETH
'\xd1' # 0xD1 -> LATIN CAPITAL LETTER N WITH TILDE
'\xd2' # 0xD2 -> LATIN CAPITAL LETTER O WITH GRAVE
'\xd3' # 0xD3 -> LATIN CAPITAL LETTER O WITH ACUTE
'\xd4' # 0xD4 -> LATIN CAPITAL LETTER O WITH CIRCUMFLEX
'\xd5' # 0xD5 -> LATIN CAPITAL LETTER O WITH TILDE
'\xd6' # 0xD6 -> LATIN CAPITAL LETTER O WITH DIAERESIS
'\xd7' # 0xD7 -> MULTIPLICATION SIGN
'\xd8' # 0xD8 -> LATIN CAPITAL LETTER O WITH STROKE
'\xd9' # 0xD9 -> LATIN CAPITAL LETTER U WITH GRAVE
'\xda' # 0xDA -> LATIN CAPITAL LETTER U WITH ACUTE
'\xdb' # 0xDB -> LATIN CAPITAL LETTER U WITH CIRCUMFLEX
'\xdc' # 0xDC -> LATIN CAPITAL LETTER U WITH DIAERESIS
'\xdd' # 0xDD -> LATIN CAPITAL LETTER Y WITH ACUTE
'\xde' # 0xDE -> LATIN CAPITAL LETTER THORN
'\xdf' # 0xDF -> LATIN SMALL LETTER SHARP S
'\xe0' # 0xE0 -> LATIN SMALL LETTER A WITH GRAVE
'\xe1' # 0xE1 -> LATIN SMALL LETTER A WITH ACUTE
'\xe2' # 0xE2 -> LATIN SMALL LETTER A WITH CIRCUMFLEX
'\xe3' # 0xE3 -> LATIN SMALL LETTER A WITH TILDE
'\xe4' # 0xE4 -> LATIN SMALL LETTER A WITH DIAERESIS
'\xe5' # 0xE5 -> LATIN SMALL LETTER A WITH RING ABOVE
'\xe6' # 0xE6 -> LATIN SMALL LETTER AE
'\xe7' # 0xE7 -> LATIN SMALL LETTER C WITH CEDILLA
'\xe8' # 0xE8 -> LATIN SMALL LETTER E WITH GRAVE
'\xe9' # 0xE9 -> LATIN SMALL LETTER E WITH ACUTE
'\xea' # 0xEA -> LATIN SMALL LETTER E WITH CIRCUMFLEX
'\xeb' # 0xEB -> LATIN SMALL LETTER E WITH DIAERESIS
'\xec' # 0xEC -> LATIN SMALL LETTER I WITH GRAVE
'\xed' # 0xED -> LATIN SMALL LETTER I WITH ACUTE
'\xee' # 0xEE -> LATIN SMALL LETTER I WITH CIRCUMFLEX
'\xef' # 0xEF -> LATIN SMALL LETTER I WITH DIAERESIS
'\xf0' # 0xF0 -> LATIN SMALL LETTER ETH
'\xf1' # 0xF1 -> LATIN SMALL LETTER N WITH TILDE
'\xf2' # 0xF2 -> LATIN SMALL LETTER O WITH GRAVE
'\xf3' # 0xF3 -> LATIN SMALL LETTER O WITH ACUTE
'\xf4' # 0xF4 -> LATIN SMALL LETTER O WITH CIRCUMFLEX
'\xf5' # 0xF5 -> LATIN SMALL LETTER O WITH TILDE
'\xf6' # 0xF6 -> LATIN SMALL LETTER O WITH DIAERESIS
'\xf7' # 0xF7 -> DIVISION SIGN
'\xf8' # 0xF8 -> LATIN SMALL LETTER O WITH STROKE
'\xf9' # 0xF9 -> LATIN SMALL LETTER U WITH GRAVE
'\xfa' # 0xFA -> LATIN SMALL LETTER U WITH ACUTE
'\xfb' # 0xFB -> LATIN SMALL LETTER U WITH CIRCUMFLEX
'\xfc' # 0xFC -> LATIN SMALL LETTER U WITH DIAERESIS
'\xfd' # 0xFD -> LATIN SMALL LETTER Y WITH ACUTE
'\xfe' # 0xFE -> LATIN SMALL LETTER THORN
'\xff' # 0xFF -> LATIN SMALL LETTER Y WITH DIAERESIS
)
### Encoding table
encoding_table=codecs.charmap_build(decoding_table)
所有上面这些编码只能表示码表1114112个字符中的256个字符，为了表示所有字符，最简单直接的方式就是使用四个字节来保存一个字符。使用四个字节来表示就出现两种表示方式：按大端顺序或按小端顺序保存，这两种方式就叫UTF-32-BE和UTF-32-LE。使用32位表示方式最大缺点是在不同架构的电脑上需要相互转换字节的顺序，才可以进行编码和解码处理。UTF-32采用平台架构默认顺序来保存，如果平台是大端模式则自动采用大端保存。当在不同字节顺序架构的CPU上处理时，就需要根据UTF-16或者UTF-32的前缀进行交换处理。而为了这个原因需要定义的前缀就叫做BOM（Byte Order Mark)，一般情况下使用一个UNICODE字符来表示BOM，它就是U+FEFF，用这个字符可以表示UTF-16或者UTF-32的字节顺序。如果第一个字符读取出现是0xFFFE，这是一个非法的字符，那么意味着后面所有UNICODE保存的字符，都需要进行交换之后的顺序才是正确的字节顺序。不过，在UNICODE已经定义字符U+FEFF作为ZERO WIDTH NO-BREAK SPACE，在UNICODE 4.0里已经取消它的定义，专门用来做字节顺序的识别作用，采用U+2060来代替那个意思。尽管如些，许多UNICODE处理软件都已经作了这样的处理：检测到作为BOM作用时，就识别U+FEFF作为字节顺序标记；当作为字符串时，就作为ZERO WIDTH NO-BREAK SPACE的意义处理。
在这里还有一种不关心字节顺序的编码方式：UTF-8。由于UTF-8采用一个字节8位来编码，就没有字节排序的问题了。每个UTF-8的字节都包括两部分：标记位（最高位）和数据位。标记位采用0-4个1的编码，UNICODE编码如下表(x表示数据1位)：
范围 |
编码 |
U-00000000 ... U-0000007F |
0xxxxxxx |
U-00000080 ... U-000007FF |
110xxxxx 10xxxxxx |
U-00000800 ... U-0000FFFF |
1110xxxx 10xxxxxx 10xxxxxx |
U-00010000 ... U-0010FFFF |
11110xxx 10xxxxxx 10xxxxxx 10xxxxxx |
UNICODE字符的最低位，就在最右的X位里。
因此在UTF-8的编码中，UNICODE字符串表示，就不再需要BOM字符U+FEFF来标识了，即第一个字符是ZERO WIDTH NO-BREAK SPACE字符，也可以识别出来。不过，在表示什么编码方面，还是需要一个标记来区分是UTF-8编码，还是UTF-32编码。为了这个目的，微软发明一个UTF-8的标记（utf-8-sig），在UTF-8编码的字符写到文件之前，先写入三个标记字符：0xef, 0xbb, 0xbf。当读取文件时，可以首三个字符进行识别，如果这三个字符出现就是UTF-8编码，并跳过这三个字符。
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