7.8. codecs — 编解码器注册和相关基类

This module defines base classes for standard Python codecs (encoders and decoders) and provides access to the internal Python codec registry which manages the codec and error handling lookup process.

It defines the following functions:

codecs.encode(obj[, encoding[, errors]])

Encodes obj using the codec registered for encoding. The default encoding is 'ascii'.

可以给定 Errors 以设置所需要的错误处理方案。 默认的错误处理方案 'strict' 表示编码错误将引发 ValueError (或更特定编解码器相关的子类，例如 UnicodeEncodeError)。 请参阅 编解码器基类 了解有关编解码器错误处理的更多信息。

2.4 新版功能.

codecs.decode(obj[, encoding[, errors]])

Decodes obj using the codec registered for encoding. The default encoding is 'ascii'.

可以给定 Errors 以设置所需要的错误处理方案。 默认的错误处理方案 'strict' 表示编码错误将引发 ValueError (或更特定编解码器相关的子类，例如 UnicodeDecodeError)。 请参阅 编解码器基类 了解有关编解码器错误处理的更多信息。

2.4 新版功能.

codecs.register(search_function)

Register a codec search function. Search functions are expected to take one argument, the encoding name in all lower case letters, and return a CodecInfo object having the following attributes:

• name The name of the encoding;

• encode The stateless encoding function;

• decode The stateless decoding function;

• incrementalencoder An incremental encoder class or factory function;

• incrementaldecoder An incremental decoder class or factory function;

• streamwriter A stream writer class or factory function;

• streamreader A stream reader class or factory function.

The various functions or classes take the following arguments:

encode and decode: These must be functions or methods which have the same interface as the encode()/decode() methods of Codec instances (see Codec Interface). The functions/methods are expected to work in a stateless mode.

incrementalencoder and incrementaldecoder: These have to be factory functions providing the following interface:

factory(errors='strict')

The factory functions must return objects providing the interfaces defined by the base classes IncrementalEncoder and IncrementalDecoder, respectively. Incremental codecs can maintain state.

streamreader and streamwriter: These have to be factory functions providing the following interface:

factory(stream, errors='strict')

The factory functions must return objects providing the interfaces defined by the base classes StreamReader and StreamWriter, respectively. Stream codecs can maintain state.

Possible values for errors are

• 'strict': raise an exception in case of an encoding error

• 'replace': replace malformed data with a suitable replacement marker, such as '?' or '\ufffd'

• 'ignore': ignore malformed data and continue without further notice

• 'xmlcharrefreplace': replace with the appropriate XML character reference (for encoding only)

• 'backslashreplace': replace with backslashed escape sequences (for encoding only)

as well as any other error handling name defined via register_error().

In case a search function cannot find a given encoding, it should return None.

codecs.lookup(encoding)

Looks up the codec info in the Python codec registry and returns a CodecInfo object as defined above.

首先将会在注册表缓存中查找编码，如果未找到，则会扫描注册的搜索函数列表。 如果没有找到 CodecInfo 对象，则将引发 LookupError。 否则，CodecInfo 对象将被存入缓存并返回给调用者。

To simplify access to the various codecs, the module provides these additional functions which use lookup() for the codec lookup:

codecs.getencoder(encoding)

查找给定编码的编解码器并返回其编码器函数。

在编码无法找到时将引发 LookupError。

codecs.getdecoder(encoding)

查找给定编码的编解码器并返回其解码器函数。

在编码无法找到时将引发 LookupError。

codecs.getincrementalencoder(encoding)

查找给定编码的编解码器并返回其增量式编码器类或工厂函数。

在编码无法找到或编解码器不支持增量式编码器时将引发 LookupError。

2.5 新版功能.

codecs.getincrementaldecoder(encoding)

查找给定编码的编解码器并返回其增量式解码器类或工厂函数。

在编码无法找到或编解码器不支持增量式解码器时将引发 LookupError。

2.5 新版功能.

codecs.getreader(encoding)

Look up the codec for the given encoding and return its StreamReader class or factory function.

在编码无法找到时将引发 LookupError。

codecs.getwriter(encoding)

Look up the codec for the given encoding and return its StreamWriter class or factory function.

在编码无法找到时将引发 LookupError。

codecs.register_error(name, error_handler)

Register the error handling function error_handler under the name name. error_handler will be called during encoding and decoding in case of an error, when name is specified as the errors parameter.

For encoding error_handler will be called with a UnicodeEncodeError instance, which contains information about the location of the error. The error handler must either raise this or a different exception or return a tuple with a replacement for the unencodable part of the input and a position where encoding should continue. The encoder will encode the replacement and continue encoding the original input at the specified position. Negative position values will be treated as being relative to the end of the input string. If the resulting position is out of bound an IndexError will be raised.

Decoding and translating works similar, except UnicodeDecodeError or UnicodeTranslateError will be passed to the handler and that the replacement from the error handler will be put into the output directly.

codecs.lookup_error(name)

返回之前在名称 name 之下注册的错误处理方案。

在处理方案无法找到时将引发 LookupError。

codecs.strict_errors(exception)

Implements the strict error handling: each encoding or decoding error raises a UnicodeError.

codecs.replace_errors(exception)

Implements the replace error handling: malformed data is replaced with a suitable replacement character such as '?' in bytestrings and '\ufffd' in Unicode strings.

codecs.ignore_errors(exception)

Implements the ignore error handling: malformed data is ignored and encoding or decoding is continued without further notice.

codecs.xmlcharrefreplace_errors(exception)

Implements the xmlcharrefreplace error handling (for encoding only): the unencodable character is replaced by an appropriate XML character reference.

codecs.backslashreplace_errors(exception)

Implements the backslashreplace error handling (for encoding only): the unencodable character is replaced by a backslashed escape sequence.

To simplify working with encoded files or stream, the module also defines these utility functions:

codecs.open(filename, mode[, encoding[, errors[, buffering]]])

Open an encoded file using the given mode and return a wrapped version providing transparent encoding/decoding. The default file mode is 'r' meaning to open the file in read mode.

注解

The wrapped version will only accept the object format defined by the codecs, i.e. Unicode objects for most built-in codecs. Output is also codec-dependent and will usually be Unicode as well.

注解

Files are always opened in binary mode, even if no binary mode was specified. This is done to avoid data loss due to encodings using 8-bit values. This means that no automatic conversion of '\n' is done on reading and writing.

encoding specifies the encoding which is to be used for the file.

可以指定 errors 来定义错误处理方案。 默认值 'strict' 表示在出现编码错误时引发 ValueError。

buffering 的含义与内置 open() 函数中的相同。 默认为行缓冲。

codecs.EncodedFile(file, input[, output[, errors]])

Return a wrapped version of file which provides transparent encoding translation.

Strings written to the wrapped file are interpreted according to the given input encoding and then written to the original file as strings using the output encoding. The intermediate encoding will usually be Unicode but depends on the specified codecs.

If output is not given, it defaults to input.

可以指定 errors 来定义错误处理方案。 默认值 'strict' 表示在出现编码错误时引发 ValueError。

codecs.iterencode(iterable, encoding[, errors])

Uses an incremental encoder to iteratively encode the input provided by iterable. This function is a generator. errors (as well as any other keyword argument) is passed through to the incremental encoder.

2.5 新版功能.

codecs.iterdecode(iterable, encoding[, errors])

Uses an incremental decoder to iteratively decode the input provided by iterable. This function is a generator. errors (as well as any other keyword argument) is passed through to the incremental decoder.

2.5 新版功能.

本模块还提供了以下常量，适用于读取和写入依赖于平台的文件：

codecs.BOM

codecs.BOM_BE

codecs.BOM_LE

codecs.BOM_UTF8

codecs.BOM_UTF16

codecs.BOM_UTF16_BE

codecs.BOM_UTF16_LE

codecs.BOM_UTF32

codecs.BOM_UTF32_BE

codecs.BOM_UTF32_LE

These constants define various encodings of the Unicode byte order mark (BOM) used in UTF-16 and UTF-32 data streams to indicate the byte order used in the stream or file and in UTF-8 as a Unicode signature. BOM_UTF16 is either BOM_UTF16_BE or BOM_UTF16_LE depending on the platform’s native byte order, BOM is an alias for BOM_UTF16, BOM_LE for BOM_UTF16_LE and BOM_BE for BOM_UTF16_BE. The others represent the BOM in UTF-8 and UTF-32 encodings.

7.8.1. 编解码器基类

The codecs module defines a set of base classes which define the interface and can also be used to easily write your own codecs for use in Python.

Each codec has to define four interfaces to make it usable as codec in Python: stateless encoder, stateless decoder, stream reader and stream writer. The stream reader and writers typically reuse the stateless encoder/decoder to implement the file protocols.

The Codec class defines the interface for stateless encoders/decoders.

To simplify and standardize error handling, the encode() and decode() methods may implement different error handling schemes by providing the errors string argument. The following string values are defined and implemented by all standard Python codecs:

值	含义
'strict'	Raise UnicodeError (or a subclass); this is the default.
'ignore'	Ignore the character and continue with the next.
'replace'	Replace with a suitable replacement character; Python will use the official U+FFFD REPLACEMENT CHARACTER for the built-in Unicode codecs on decoding and ‘?’ on encoding.
'xmlcharrefreplace'	Replace with the appropriate XML character reference (only for encoding).
'backslashreplace'	Replace with backslashed escape sequences (only for encoding).

The set of allowed values can be extended via register_error().

7.8.1.1. Codec Objects

The Codec class defines these methods which also define the function interfaces of the stateless encoder and decoder:

Codec.encode(input[, errors])

Encodes the object input and returns a tuple (output object, length consumed). While codecs are not restricted to use with Unicode, in a Unicode context, encoding converts a Unicode object to a plain string using a particular character set encoding (e.g., cp1252 or iso-8859-1).

errors defines the error handling to apply. It defaults to 'strict' handling.

此方法不一定会在 Codec 实例中保存状态。 可使用必须保存状态的 StreamWriter 作为编解码器以便高效地进行编码。

编码器必须能够处理零长度的输入并在此情况下返回输出对象类型的空对象。

Codec.decode(input[, errors])

Decodes the object input and returns a tuple (output object, length consumed). In a Unicode context, decoding converts a plain string encoded using a particular character set encoding to a Unicode object.

input must be an object which provides the bf_getreadbuf buffer slot. Python strings, buffer objects and memory mapped files are examples of objects providing this slot.

errors defines the error handling to apply. It defaults to 'strict' handling.

此方法不一定会在 Codec 实例中保存状态。 可使用必须保存状态的 StreamReader 作为编解码器以便高效地进行解码。

解码器必须能够处理零长度的输入并在此情况下返回输出对象类型的空对象。

IncrementalEncoder 和 IncrementalDecoder 类提供了增量式编码和解码的基本接口。 对输入的编码/解码不是通过对无状态编码器/解码器的一次调用，而是通过对增量式编码器/解码器的 encode()/decode() 方法的多次调用。 增量式编码器/解码器会在方法调用期间跟踪编码/解码过程。

调用 encode()/decode() 方法后的全部输出相当于将所有通过无状态编码器/解码器进行编码/解码的单个输入连接在一起所得到的输出。

7.8.1.2. IncrementalEncoder 对象

2.5 新版功能.

IncrementalEncoder 类用来对一个输入进行分步编码。 它定义了以下方法，每个增量式编码器都必须定义这些方法以便与 Python 编解码器注册表相兼容。

class codecs.IncrementalEncoder([errors])

IncrementalEncoder 实例的构造器。

所有增量式编码器必须提供此构造器接口。 它们可以自由地添加额外的关键字参数，但只有在这里定义的参数才会被 Python 编解码器注册表所使用。

The IncrementalEncoder may implement different error handling schemes by providing the errors keyword argument. These parameters are predefined:

• 'strict' Raise ValueError (or a subclass); this is the default.

• 'ignore' Ignore the character and continue with the next.

• 'replace' Replace with a suitable replacement character

• 'xmlcharrefreplace' Replace with the appropriate XML character reference

• 'backslashreplace' Replace with backslashed escape sequences.

errors 参数将被赋值给一个同名的属性。 通过对此属性赋值就可以在 IncrementalEncoder 对象的生命期内在不同的错误处理策略之间进行切换。

errors 参数所允许的值集合可以使用 register_error() 来扩展。

encode(object[, final])

编码 object (会将编码器的当前状态纳入考虑) 并返回已编码的结果对象。 如果这是对 encode() 的最终调用则 final 必须为真值（默认为假值）。

reset()

Reset the encoder to the initial state.

7.8.1.3. IncrementalDecoder 对象

IncrementalDecoder 类用来对一个输入进行分步解码。 它定义了以下方法，每个增量式解码器都必须定义这些方法以便与 Python 编解码器注册表相兼容。

class codecs.IncrementalDecoder([errors])

IncrementalDecoder 实例的构造器。

所有增量式解码器必须提供此构造器接口。 它们可以自由地添加额外的关键字参数，但只有在这里定义的参数才会被 Python 编解码器注册表所使用。

The IncrementalDecoder may implement different error handling schemes by providing the errors keyword argument. These parameters are predefined:

• 'strict' Raise ValueError (or a subclass); this is the default.

• 'ignore' Ignore the character and continue with the next.

• 'replace' Replace with a suitable replacement character.

errors 参数将被赋值给一个同名的属性。 通过对此属性赋值就可以在 IncrementalDecoder 对象的生命期内在不同的错误处理策略之间进行切换。

errors 参数所允许的值集合可以使用 register_error() 来扩展。

decode(object[, final])

解码 object (会将解码器的当前状态纳入考虑) 并返回已解码的结果对象。 如果这是对 decode() 的最终调用则 final 必须为真值（默认为假值）。 如果 final 为真值则解码器必须对输入进行完全解码并且必须 刷新所有缓冲区。 如果这无法做到（例如由于在输入结束时字节串序列不完整）则它必须像在无状态的情况下那样初始化错误处理（这可能引发一个异常）。

reset()

将解码器重置为初始状态。

StreamWriter 和 StreamReader 类提供了一些泛用工作接口，可被用来非常方便地实现新的编码格式子模块。 请参阅 encodings.utf_8 中的示例了解如何做到这一点。

7.8.1.4. StreamWriter 对象

StreamWriter 类是 Codec 的子类，它定义了以下方法，每个流式写入器都必须定义这些方法以便与 Python 编解码器注册表相兼容。

class codecs.StreamWriter(stream[, errors])

StreamWriter 实例的构造器。

所有流式写入器必须提供此构造器接口。 它们可以自由地添加额外的关键字参数，但只有在这里定义的参数才会被 Python 编解码器注册表所使用。

stream must be a file-like object open for writing binary data.

The StreamWriter may implement different error handling schemes by providing the errors keyword argument. These parameters are predefined:

• 'strict' Raise ValueError (or a subclass); this is the default.

• 'ignore' Ignore the character and continue with the next.

• 'replace' Replace with a suitable replacement character

• 'xmlcharrefreplace' Replace with the appropriate XML character reference

• 'backslashreplace' Replace with backslashed escape sequences.

errors 参数将被赋值给一个同名的属性。 通过对此属性赋值就可以在 StreamWriter 对象的生命期内在不同的错误处理策略之间进行切换。

errors 参数所允许的值集合可以使用 register_error() 来扩展。

write(object)

将编码后的对象内容写入到流。

writelines(list)

Writes the concatenated list of strings to the stream (possibly by reusing the write() method).

reset()

刷新并重置用于保持状态的编解码器缓冲区。

调用此方法应当确保在干净的状态下放入输出数据，以允许直接添加新的干净数据而无须重新扫描整个流来恢复状态。

除了上述的方法，StreamWriter 还必须继承来自下层流的所有其他方法和属性。

7.8.1.5. StreamReader 对象

StreamReader 类是 Codec 的子类，它定义了以下方法，每个流式读取器都必须定义这些方法以便与 Python 编解码器注册表相兼容。

class codecs.StreamReader(stream[, errors])

StreamReader 实例的构造器。

所有流式读取器必须提供此构造器接口。 它们可以自由地添加额外的关键字参数，但只有在这里定义的参数才会被 Python 编解码器注册表所使用。

stream must be a file-like object open for reading (binary) data.

The StreamReader may implement different error handling schemes by providing the errors keyword argument. These parameters are defined:

• 'strict' Raise ValueError (or a subclass); this is the default.

• 'ignore' Ignore the character and continue with the next.

• 'replace' Replace with a suitable replacement character.

errors 参数将被赋值给一个同名的属性。 通过对此属性赋值就可以在 StreamReader 对象的生命期内在不同的错误处理策略之间进行切换。

errors 参数所允许的值集合可以使用 register_error() 来扩展。

read([size[, chars[, firstline]]])

解码来自流的数据并返回结果对象。

chars indicates the number of characters to read from the stream. read() will never return more than chars characters, but it might return less, if there are not enough characters available.

size indicates the approximate maximum number of bytes to read from the stream for decoding purposes. The decoder can modify this setting as appropriate. The default value -1 indicates to read and decode as much as possible. size is intended to prevent having to decode huge files in one step.

firstline indicates that it would be sufficient to only return the first line, if there are decoding errors on later lines.

此方法应当使用“贪婪”读取策略，这意味着它应当在编码格式定义和给定大小所允许的情况下尽可能多地读取数据，例如，如果在流上存在可选的编码结束或状态标记，这些内容也应当被读取。

在 2.4 版更改: chars argument added.

在 2.4.2 版更改: firstline argument added.

readline([size[, keepends]])

从输入流读取一行并返回解码后的数据。

如果给定了 size，则将其作为 size 参数传递给流的 read() 方法。

如果 keepends 为假值，则行结束符将从返回的行中去除。

在 2.4 版更改: keepends argument added.

readlines([sizehint[, keepends]])

从输入流读取所有行并将其作为一个行列表返回。

Line-endings are implemented using the codec’s decoder method and are included in the list entries if keepends is true.

如果给定了 sizehint，则将其作为 size 参数传递给流的 read() 方法。

reset()

重置用于保持状态的编解码器缓冲区。

Note that no stream repositioning should take place. This method is primarily intended to be able to recover from decoding errors.

除了上述的方法，StreamReader 还必须继承来自下层流的所有其他方法和属性。

The next two base classes are included for convenience. They are not needed by the codec registry, but may provide useful in practice.

7.8.1.6. StreamReaderWriter 对象

The StreamReaderWriter allows wrapping streams which work in both read and write modes.

其设计使得开发者可以使用 lookup() 函数所返回的工厂函数来构造实例。

class codecs.StreamReaderWriter(stream, Reader, Writer, errors)

创建一个 StreamReaderWriter 实例。 stream 必须为一个文件类对象。 Reader 和 Writer 必须为分别提供了 StreamReader 和 StreamWriter 接口的工厂函数或类。 错误处理通过与流式读取器和写入器所定义的相同方式来完成。

StreamReaderWriter 实例定义了 StreamReader 和 StreamWriter 类的组合接口。 它们还继承了来自下层流的所有其他方法和属性。

7.8.1.7. StreamRecoder 对象

The StreamRecoder provide a frontend - backend view of encoding data which is sometimes useful when dealing with different encoding environments.

其设计使得开发者可以使用 lookup() 函数所返回的工厂函数来构造实例。

class codecs.StreamRecoder(stream, encode, decode, Reader, Writer, errors)

Creates a StreamRecoder instance which implements a two-way conversion: encode and decode work on the frontend (the input to read() and output of write()) while Reader and Writer work on the backend (reading and writing to the stream).

You can use these objects to do transparent direct recodings from e.g. Latin-1 to UTF-8 and back.

stream must be a file-like object.

encode, decode must adhere to the Codec interface. Reader, Writer must be factory functions or classes providing objects of the StreamReader and StreamWriter interface respectively.

encode and decode are needed for the frontend translation, Reader and Writer for the backend translation. The intermediate format used is determined by the two sets of codecs, e.g. the Unicode codecs will use Unicode as the intermediate encoding.

错误处理通过与流式读取器和写入器所定义的相同方式来完成。

StreamRecoder 实例定义了 StreamReader 和 StreamWriter 类的组合接口。 它们还继承了来自下层流的所有其他方法和属性。

7.8.2. 编码格式与 Unicode

Unicode strings are stored internally as sequences of code points (to be precise as Py_UNICODE arrays). Depending on the way Python is compiled (either via --enable-unicode=ucs2 or --enable-unicode=ucs4, with the former being the default) Py_UNICODE is either a 16-bit or 32-bit data type. Once a Unicode object is used outside of CPU and memory, CPU endianness and how these arrays are stored as bytes become an issue. Transforming a unicode object into a sequence of bytes is called encoding and recreating the unicode object from the sequence of bytes is known as decoding. There are many different methods for how this transformation can be done (these methods are also called encodings). The simplest method is to map the code points 0–255 to the bytes 0x0–0xff. This means that a unicode object that contains code points above U+00FF can’t be encoded with this method (which is called 'latin-1' or 'iso-8859-1'). unicode.encode() will raise a UnicodeEncodeError that looks like this: UnicodeEncodeError: 'latin-1' codec can't encode character u'\u1234' in position 3: ordinal not in range(256).

There’s another group of encodings (the so called charmap encodings) that choose a different subset of all unicode code points and how these code points are mapped to the bytes 0x0–0xff. To see how this is done simply open e.g. encodings/cp1252.py (which is an encoding that is used primarily on Windows). There’s a string constant with 256 characters that shows you which character is mapped to which byte value.

All of these encodings can only encode 256 of the 1114112 code points defined in unicode. A simple and straightforward way that can store each Unicode code point, is to store each code point as four consecutive bytes. There are two possibilities: store the bytes in big endian or in little endian order. These two encodings are called UTF-32-BE and UTF-32-LE respectively. Their disadvantage is that if e.g. you use UTF-32-BE on a little endian machine you will always have to swap bytes on encoding and decoding. UTF-32 avoids this problem: bytes will always be in natural endianness. When these bytes are read by a CPU with a different endianness, then bytes have to be swapped though. To be able to detect the endianness of a UTF-16 or UTF-32 byte sequence, there’s the so called BOM (“Byte Order Mark”). This is the Unicode character U+FEFF. This character can be prepended to every UTF-16 or UTF-32 byte sequence. The byte swapped version of this character (0xFFFE) is an illegal character that may not appear in a Unicode text. So when the first character in an UTF-16 or UTF-32 byte sequence appears to be a U+FFFE the bytes have to be swapped on decoding. Unfortunately the character U+FEFF had a second purpose as a ZERO WIDTH NO-BREAK SPACE: a character that has no width and doesn’t allow a word to be split. It can e.g. be used to give hints to a ligature algorithm. With Unicode 4.0 using U+FEFF as a ZERO WIDTH NO-BREAK SPACE has been deprecated (with U+2060 (WORD JOINER) assuming this role). Nevertheless Unicode software still must be able to handle U+FEFF in both roles: as a BOM it’s a device to determine the storage layout of the encoded bytes, and vanishes once the byte sequence has been decoded into a Unicode string; as a ZERO WIDTH NO-BREAK SPACE it’s a normal character that will be decoded like any other.

还有另一种编码格式能够对所有的 Unicode 字符进行编码：UTF-8。 UTF-8 是一种 8 位编码，这意味着在 UTF-8 中没有字节顺序问题。 UTF-8 字节序列中的每个字节由两部分组成：标志位（最重要的位）和内容位。 标志位是由零至四个值为 1 的二进制位加一个值为 0 的二进制位构成的序列。 Unicode 字符会按以下形式进行编码（其中 x 为内容位，当拼接为一体时将给出对应的 Unicode 字符）：

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 字符最不重要的一个位就是最右侧的二进制位 x。

As UTF-8 is an 8-bit encoding no BOM is required and any U+FEFF character in the decoded Unicode string (even if it’s the first character) is treated as a ZERO WIDTH NO-BREAK SPACE.

Without external information it’s impossible to reliably determine which encoding was used for encoding a Unicode string. Each charmap encoding can decode any random byte sequence. However that’s not possible with UTF-8, as UTF-8 byte sequences have a structure that doesn’t allow arbitrary byte sequences. To increase the reliability with which a UTF-8 encoding can be detected, Microsoft invented a variant of UTF-8 (that Python 2.5 calls "utf-8-sig") for its Notepad program: Before any of the Unicode characters is written to the file, a UTF-8 encoded BOM (which looks like this as a byte sequence: 0xef, 0xbb, 0xbf) is written. As it’s rather improbable that any charmap encoded file starts with these byte values (which would e.g. map to

LATIN SMALL LETTER I WITH DIAERESIS

RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK

INVERTED QUESTION MARK

in iso-8859-1), this increases the probability that a utf-8-sig encoding can be correctly guessed from the byte sequence. So here the BOM is not used to be able to determine the byte order used for generating the byte sequence, but as a signature that helps in guessing the encoding. On encoding the utf-8-sig codec will write 0xef, 0xbb, 0xbf as the first three bytes to the file. On decoding utf-8-sig will skip those three bytes if they appear as the first three bytes in the file. In UTF-8, the use of the BOM is discouraged and should generally be avoided.

7.8.3. 标准编码

Python 自带了许多内置的编解码器，它们的实现或者是通过 C 函数，或者是通过映射表。 以下表格是按名称排序的编解码器列表，并提供了一些常见别名以及编码格式通常针对的语言。 别名和语言列表都不是详尽无遗的。 请注意仅有大小写区别或使用连字符替代下划线的拼写形式也都是有效的别名；因此，'utf-8' 是 'utf_8' 编解码器的有效别名。

许多字符集都支持相同的语言。 它们在个别字符（例如是否支持 EURO SIGN 等）以及给字符所分配的码位方面存在差异。 特别是对于欧洲语言来说，通常存在以下几种变体：

• 某个 ISO 8859 编码集

• 某个 Microsoft Windows 编码页，通常是派生自某个 8859 编码集，但会用附加的图形字符来替换控制字符。

• 某个 IBM EBCDIC 编码页

• 某个 IBM PC 编码页，通常会兼容 ASCII

编码	别名	语言
ascii	646, us-ascii	英语
big5	big5-tw, csbig5	繁体中文
big5hkscs	big5-hkscs, hkscs	繁体中文
cp037	IBM037, IBM039	英语
cp424	EBCDIC-CP-HE, IBM424	希伯来语
cp437	437, IBM437	英语
cp500	EBCDIC-CP-BE, EBCDIC-CP-CH, IBM500	西欧
cp720		阿拉伯语
cp737		希腊语
cp775	IBM775	波罗的海语言
cp850	850, IBM850	西欧
cp852	852, IBM852	中欧和东欧
cp855	855, IBM855	保加利亚语，白俄罗斯语，马其顿语，俄语，塞尔维亚语
cp856		希伯来语
cp857	857, IBM857	土耳其语
cp858	858, IBM858	西欧
cp860	860, IBM860	葡萄牙语
cp861	861, CP-IS, IBM861	冰岛语
cp862	862, IBM862	希伯来语
cp863	863, IBM863	加拿大语
cp864	IBM864	阿拉伯语
cp865	865, IBM865	丹麦语/挪威语
cp866	866, IBM866	俄语
cp869	869, CP-GR, IBM869	希腊语
cp874		泰语
cp875		希腊语
cp932	932, ms932, mskanji, ms-kanji	日语
cp949	949, ms949, uhc	韩语
cp950	950, ms950	繁体中文
cp1006		乌尔都语
cp1026	ibm1026	土耳其语
cp1140	ibm1140	西欧
cp1250	windows-1250	中欧和东欧
cp1251	windows-1251	保加利亚语，白俄罗斯语，马其顿语，俄语，塞尔维亚语
cp1252	windows-1252	西欧
cp1253	windows-1253	希腊语
cp1254	windows-1254	土耳其语
cp1255	windows-1255	希伯来语
cp1256	windows-1256	阿拉伯语
cp1257	windows-1257	波罗的海语言
cp1258	windows-1258	越南语
euc_jp	eucjp, ujis, u-jis	日语
euc_jis_2004	jisx0213, eucjis2004	日语
euc_jisx0213	eucjisx0213	日语
euc_kr	euckr, korean, ksc5601, ks_c-5601, ks_c-5601-1987, ksx1001, ks_x-1001	韩语
gb2312	chinese, csiso58gb231280, euc- cn, euccn, eucgb2312-cn, gb2312-1980, gb2312-80, iso- ir-58	简体中文
gbk	936, cp936, ms936	统一汉语
gb18030	gb18030-2000	统一汉语
hz	hzgb, hz-gb, hz-gb-2312	简体中文
iso2022_jp	csiso2022jp, iso2022jp, iso-2022-jp	日语
iso2022_jp_1	iso2022jp-1, iso-2022-jp-1	日语
iso2022_jp_2	iso2022jp-2, iso-2022-jp-2	日语，韩语，简体中文，西欧，希腊语
iso2022_jp_2004	iso2022jp-2004, iso-2022-jp-2004	日语
iso2022_jp_3	iso2022jp-3, iso-2022-jp-3	日语
iso2022_jp_ext	iso2022jp-ext, iso-2022-jp-ext	日语
iso2022_kr	csiso2022kr, iso2022kr, iso-2022-kr	韩语
latin_1	iso-8859-1, iso8859-1, 8859, cp819, latin, latin1, L1	West Europe
iso8859_2	iso-8859-2, latin2, L2	中欧和东欧
iso8859_3	iso-8859-3, latin3, L3	世界语，马耳他语
iso8859_4	iso-8859-4, latin4, L4	波罗的海语言
iso8859_5	iso-8859-5, cyrillic	保加利亚语，白俄罗斯语，马其顿语，俄语，塞尔维亚语
iso8859_6	iso-8859-6, arabic	阿拉伯语
iso8859_7	iso-8859-7, greek, greek8	希腊语
iso8859_8	iso-8859-8, hebrew	希伯来语
iso8859_9	iso-8859-9, latin5, L5	土耳其语
iso8859_10	iso-8859-10, latin6, L6	北欧语言
iso8859_11	iso-8859-11, thai	泰语
iso8859_13	iso-8859-13, latin7, L7	波罗的海语言
iso8859_14	iso-8859-14, latin8, L8	凯尔特语
iso8859_15	iso-8859-15, latin9, L9	西欧
iso8859_16	iso-8859-16, latin10, L10	东南欧
johab	cp1361, ms1361	韩语
koi8_r		俄语
koi8_u		乌克兰语
mac_cyrillic	maccyrillic	保加利亚语，白俄罗斯语，马其顿语，俄语，塞尔维亚语
mac_greek	macgreek	希腊语
mac_iceland	maciceland	冰岛语
mac_latin2	maclatin2, maccentraleurope	中欧和东欧
mac_roman	macroman	西欧
mac_turkish	macturkish	土耳其语
ptcp154	csptcp154, pt154, cp154, cyrillic-asian	哈萨克语
shift_jis	csshiftjis, shiftjis, sjis, s_jis	日语
shift_jis_2004	shiftjis2004, sjis_2004, sjis2004	日语
shift_jisx0213	shiftjisx0213, sjisx0213, s_jisx0213	日语
utf_32	U32, utf32	所有语言
utf_32_be	UTF-32BE	所有语言
utf_32_le	UTF-32LE	所有语言
utf_16	U16, utf16	所有语言
utf_16_be	UTF-16BE	all languages (BMP only)
utf_16_le	UTF-16LE	all languages (BMP only)
utf_7	U7, unicode-1-1-utf-7	所有语言
utf_8	U8, UTF, utf8	所有语言
utf_8_sig		所有语言

7.8.4. Python 专属的编码格式

A number of predefined codecs are specific to Python, so their codec names have no meaning outside Python. These are listed in the tables below based on the expected input and output types (note that while text encodings are the most common use case for codecs, the underlying codec infrastructure supports arbitrary data transforms rather than just text encodings). For asymmetric codecs, the stated purpose describes the encoding direction.

The following codecs provide unicode-to-str encoding 1 and str-to-unicode decoding 2, similar to the Unicode text encodings.

编码	别名	Purpose
idna		Implements RFC 3490, see also encodings.idna
mbcs	dbcs	Windows only: Encode operand according to the ANSI codepage (CP_ACP)
palmos		Encoding of PalmOS 3.5
punycode		Implements RFC 3492
raw_unicode_escape		Produce a string that is suitable as raw Unicode literal in Python source code
rot_13	rot13	Returns the Caesar-cypher encryption of the operand
undefined		Raise an exception for all conversions. Can be used as the system encoding if no automatic coercion between byte and Unicode strings is desired.
unicode_escape		Produce a string that is suitable as Unicode literal in Python source code
unicode_internal		Return the internal representation of the operand

2.3 新版功能: The idna and punycode encodings.

The following codecs provide str-to-str encoding and decoding 2.

编码	别名	Purpose	Encoder/decoder
base64_codec	base64, base-64	Convert operand to multiline MIME base64 (the result always includes a trailing '\n')	base64.encodestring(), base64.decodestring()
bz2_codec	bz2	Compress the operand using bz2	bz2.compress(), bz2.decompress()
hex_codec	hex	Convert operand to hexadecimal representation, with two digits per byte	binascii.b2a_hex(), binascii.a2b_hex()
quopri_codec	quopri, quoted-printable, quotedprintable	Convert operand to MIME quoted printable	quopri.encode() with quotetabs=True, quopri.decode()
string_escape		Produce a string that is suitable as string literal in Python source code
uu_codec	uu	Convert the operand using uuencode	uu.encode(), uu.decode()
zlib_codec	zip, zlib	Compress the operand using gzip	zlib.compress(), zlib.decompress()

1

str objects are also accepted as input in place of unicode objects. They are implicitly converted to unicode by decoding them using the default encoding. If this conversion fails, it may lead to encoding operations raising UnicodeDecodeError.

2(1,2)

unicode objects are also accepted as input in place of str objects. They are implicitly converted to str by encoding them using the default encoding. If this conversion fails, it may lead to decoding operations raising UnicodeEncodeError.

7.8.5. encodings.idna — 应用程序中的国际化域名

2.3 新版功能.

此模块实现了 RFC 3490 (应用程序中的国际化域名) 和 RFC 3492 (Nameprep: 用于国际化域名 (IDN) 的 Stringprep 配置文件)。 它是在 punycode 编码格式和 stringprep 的基础上构建的。

这些 RFC 共同定义了一个在域名中支持非 ASCII 字符的协议。 一个包含非 ASCII 字符的域名 (例如 www.Alliancefrançaise.nu) 会被转换为兼容 ASCII 的编码格式 (简称 ACE，例如 www.xn--alliancefranaise-npb.nu)。 随后此域名的 ACE 形式可以用于所有由于特定协议而不允许使用任意字符的场合，例如 DNS 查询，HTTP Host 字段等等。 此转换是在应用中进行的；如有可能将对用户可见：应用应当透明地将 Unicode 域名标签转换为线上的 IDNA，并在 ACE 标签被呈现给用户之前将其转换回 Unicode。

Python supports this conversion in several ways: the idna codec performs conversion between Unicode and ACE, separating an input string into labels based on the separator characters defined in section 3.1 (1) of RFC 3490 and converting each label to ACE as required, and conversely separating an input byte string into labels based on the . separator and converting any ACE labels found into unicode. Furthermore, the socket module transparently converts Unicode host names to ACE, so that applications need not be concerned about converting host names themselves when they pass them to the socket module. On top of that, modules that have host names as function parameters, such as httplib and ftplib, accept Unicode host names (httplib then also transparently sends an IDNA hostname in the Host field if it sends that field at all).

When receiving host names from the wire (such as in reverse name lookup), no automatic conversion to Unicode is performed: Applications wishing to present such host names to the user should decode them to Unicode.

encodings.idna 模块还实现了 nameprep 过程，该过程会对主机名执行特定的规范化操作，以实现国际域名的大小写不敏感特性与合并相似的字符。 如果有需要可以直接使用 nameprep 函数。

encodings.idna.nameprep(label)

返回 label 经过名称处理操作的版本。 该实现目前基于查询字符串，因此 AllowUnassigned 为真值。

encodings.idna.ToASCII(label)

将标签转换为 ASCII，规则定义见 RFC 3490。 UseSTD3ASCIIRules 预设为假值。

encodings.idna.ToUnicode(label)

将标签转换为 Unicode，规则定义见 RFC 3490。

7.8.6. encodings.utf_8_sig — 带BOM签名的UTF-8编解码器

2.5 新版功能.

This module implements a variant of the UTF-8 codec: On encoding a UTF-8 encoded BOM will be prepended to the UTF-8 encoded bytes. For the stateful encoder this is only done once (on the first write to the byte stream). For decoding an optional UTF-8 encoded BOM at the start of the data will be skipped.