# 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.