# `codecs` --- Codec registry and base classes

Source code: Lib/codecs.py

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. Most standard codecs are text encodings, which encode text to bytes, but there are also codecs provided that encode text to text, and bytes to bytes. Custom codecs may encode and decode between arbitrary types, but some module features are restricted to use specifically with text encodings, or with codecs that encode to `bytes`.

The module defines the following functions for encoding and decoding with any codec:

`codecs.``encode`(obj, encoding='utf-8', errors='strict')

  Encodes obj using the codec registered for encoding.

  Errors may be given to set the desired error handling scheme. The default error handler is `'strict'` meaning that encoding errors raise `ValueError` (or a more codec specific subclass, such as `UnicodeEncodeError`). Refer to Codec Base Classes for more information on codec error handling.

`codecs.``decode`(obj, encoding='utf-8', errors='strict')

  Decodes obj using the codec registered for encoding.

  Errors may be given to set the desired error handling scheme. The default error handler is `'strict'` meaning that decoding errors raise `ValueError` (or a more codec specific subclass, such as `UnicodeDecodeError`). Refer to Codec Base Classes for more information on codec error handling.

The full details for each codec can also be looked up directly:

`codecs.``lookup`(encoding)

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

  Encodings are first looked up in the registry's cache. If not found, the list of registered search functions is scanned. If no `CodecInfo` object is found, a `LookupError` is raised. Otherwise, the `CodecInfo` object is stored in the cache and returned to the caller.

class `codecs.``CodecInfo`(encode, decode, streamreader=None, streamwriter=None, incrementalencoder=None, incrementaldecoder=None, name=None)

  Codec details when looking up the codec registry. The constructor arguments are stored in attributes of the same name:

  `name`

    The name of the encoding.

  `encode`
  `decode`

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

  `incrementalencoder`
  `incrementaldecoder`

    Incremental encoder and decoder classes or factory functions. These have to provide the interface defined by the base classes `IncrementalEncoder` and `IncrementalDecoder`, respectively. Incremental codecs can maintain state.

  `streamwriter`
  `streamreader`

    Stream writer and reader classes or factory functions. These have to provide the interface defined by the base classes `StreamWriter` and `StreamReader`, respectively. Stream codecs can maintain state.

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

`codecs.``getencoder`(encoding)

  Look up the codec for the given encoding and return its encoder function.

  Raises a `LookupError` in case the encoding cannot be found.

`codecs.``getdecoder`(encoding)

  Look up the codec for the given encoding and return its decoder function.

  Raises a `LookupError` in case the encoding cannot be found.

`codecs.``getincrementalencoder`(encoding)

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

  Raises a `LookupError` in case the encoding cannot be found or the codec doesn't support an incremental encoder.

`codecs.``getincrementaldecoder`(encoding)

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

  Raises a `LookupError` in case the encoding cannot be found or the codec doesn't support an incremental decoder.

`codecs.``getreader`(encoding)

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

  Raises a `LookupError` in case the encoding cannot be found.

`codecs.``getwriter`(encoding)

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

  Raises a `LookupError` in case the encoding cannot be found.

Custom codecs are made available by registering a suitable codec search function:

`codecs.``register`(search_function)

  Register a codec search function. Search functions are expected to take one argument, being the encoding name in all lower case letters, and return a `CodecInfo` object. In case a search function cannot find a given encoding, it should return `None`.

  備註

  Search function registration is not currently reversible, which may cause problems in some cases, such as unit testing or module reloading.

While the builtin `open()` and the associated `io` module are the recommended approach for working with encoded text files, this module provides additional utility functions and classes that allow the use of a wider range of codecs when working with binary files:

`codecs.``open`(filename, mode='r', encoding=None, errors='strict', buffering=-1)

  Open an encoded file using the given mode and return an instance of `StreamReaderWriter`, providing transparent encoding/decoding. The default file mode is `'r'`, meaning to open the file in read mode.

  備註

  Underlying encoded files are always opened in binary mode. No automatic conversion of `'\n'` is done on reading and writing. The mode argument may be any binary mode acceptable to the built-in `open()` function; the `'b'` is automatically added.

  encoding specifies the encoding which is to be used for the file. Any encoding that encodes to and decodes from bytes is allowed, and the data types supported by the file methods depend on the codec used.

  errors may be given to define the error handling. It defaults to `'strict'` which causes a `ValueError` to be raised in case an encoding error occurs.

  buffering 的含义与内置 `open()` 函数中的相同。 默认值 -1 表示将使用默认的缓冲区大小。

`codecs.``EncodedFile`(file, data_encoding, file_encoding=None, errors='strict')

  Return a `StreamRecoder` instance, a wrapped version of file which provides transparent transcoding. The original file is closed when the wrapped version is closed.

  Data written to the wrapped file is decoded according to the given data_encoding and then written to the original file as bytes using file_encoding. Bytes read from the original file are decoded according to file_encoding, and the result is encoded using data_encoding.

  If file_encoding is not given, it defaults to data_encoding.

  errors may be given to define the error handling. It defaults to `'strict'`, which causes `ValueError` to be raised in case an encoding error occurs.

`codecs.``iterencode`(iterator, encoding, errors='strict', **kwargs)

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

  This function requires that the codec accept text `str` objects to encode. Therefore it does not support bytes-to-bytes encoders such as `base64_codec`.

`codecs.``iterdecode`(iterator, encoding, errors='strict', **kwargs)

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

  This function requires that the codec accept `bytes` objects to decode. Therefore it does not support text-to-text encoders such as `rot_13`, although `rot_13` may be used equivalently with `iterencode()`.

The module also provides the following constants which are useful for reading and writing to platform dependent files:

`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 byte sequences, being Unicode byte order marks (BOMs) for several encodings. They are used in UTF-16 and UTF-32 data streams to indicate the byte order used, 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.

## Codec Base Classes

The `codecs` module defines a set of base classes which define the interfaces for working with codec objects, and can also be used as the basis for custom codec implementations.

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. Codec authors also need to define how the codec will handle encoding and decoding errors.

### Error Handlers

为了简化和标准化错误处理，编解码器可以通过接受 errors 字符串参数来实现不同的错误处理方案。 所有标准的 Python 编解码器都定义并实现了以下字符串值：

Value | Meaning
--- | ---
`'strict'` | 引发 `UnicodeError` (或其子类)；这是默认的方案。 在 `strict_errors()` 中实现。
`'ignore'` | 忽略错误格式的数据并且不加进一步通知就继续执行。 在 `ignore_errors()` 中实现。

The following error handlers are only applicable to text encodings:

Value | Meaning
--- | ---
`'replace'` | 使用适当的替换标记进行替换；Python 内置编解码器将在解码时使用官方 `U+FFFD` 替换字符，而在编码时使用 '?' 。 在 `replace_errors()` 中实现。
`'xmlcharrefreplace'` | 使用适当的 XML 字符引用进行替换（仅在编码时）。 在 `xmlcharrefreplace_errors()` 中实现。
`'backslashreplace'` | Replace with backslashed escape sequences. Implemented in `backslashreplace_errors()`.
`'namereplace'` | 使用 `\N{...}` 转义序列进行替换（仅在编码时）。 在 `namereplace_errors()` 中实现。
`'surrogateescape'` | 在解码时，将字节替换为 `U+DC80` 至 `U+DCFF` 范围内的单个代理代码。 当在编码数据时使用 `'surrogateescape'` 错误处理方案时，此代理将被转换回相同的字节。 （请参阅 PEP 383 了解详情。）

In addition, the following error handler is specific to the given codecs:

Value | Codecs | Meaning
--- | --- | ---
`'surrogatepass'` | utf-8, utf-16, utf-32, utf-16-be, utf-16-le, utf-32-be, utf-32-le | 允许编码和解码代理代码。 这些编解码器通常会将出现的代理代码视为错误。

3.1 版新加入: The `'surrogateescape'` and `'surrogatepass'` error handlers.

3.4 版更變: The `'surrogatepass'` error handlers now works with utf-16* and utf-32* codecs.

3.5 版新加入: The `'namereplace'` error handler.

3.5 版更變: The `'backslashreplace'` error handlers now works with decoding and translating.

The set of allowed values can be extended by registering a new named error handler:

`codecs.``register_error`(name, error_handler)

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

  对于编码操作，将会调用 error_handler 并传入一个 `UnicodeEncodeError` 实例，其中包含有关错误位置的信息。 错误处理程序必须引发此异常或别的异常，或者也可以返回一个元组，其中包含输入的不可编码部分的替换对象，以及应当继续进行编码的位置。 替换对象可以为 `str` 或 `bytes` 类型。 如果替换对象为字节串，编码器将简单地将其复制到输出缓冲区。 如果替换对象为字符串，编码器将对替换对象进行编码。 对原始输入的编码操作会在指定位置继续进行。 负的位置值将被视为相对于输入字符串的末尾。 如果结果位置超出范围则将引发 `IndexError`。

  Decoding and translating works similarly, 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.

Previously registered error handlers (including the standard error handlers) can be looked up by name:

`codecs.``lookup_error`(name)

  Return the error handler previously registered under the name name.

  Raises a `LookupError` in case the handler cannot be found.

The following standard error handlers are also made available as module level functions:

`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 (for text encodings only): substitutes `'?'` for encoding errors (to be encoded by the codec), and `'\ufffd'` (the Unicode replacement character) for decoding errors.

`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 with text encodings only): the unencodable character is replaced by an appropriate XML character reference.

`codecs.``backslashreplace_errors`(exception)

  Implements the `'backslashreplace'` error handling (for text encodings only): malformed data is replaced by a backslashed escape sequence.

`codecs.``namereplace_errors`(exception)

  Implements the `'namereplace'` error handling (for encoding with text encodings only): the unencodable character is replaced by a `\N{...}` escape sequence.

  3.5 版新加入.

### Stateless Encoding and Decoding

The base `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). For instance, text encoding converts a string object to a bytes object using a particular character set encoding (e.g., `cp1252` or `iso-8859-1`).

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

  The method may not store state in the `Codec` instance. Use `StreamWriter` for codecs which have to keep state in order to make encoding efficient.

  The encoder must be able to handle zero length input and return an empty object of the output object type in this situation.

`Codec.``decode`(input[, errors])

  解码 input 对象并返回一个元组 (输出对象, 消耗长度)。 例如，text encoding 的解码操作会使用特定的字符集编码格式将字节串对象转换为字符串对象。

  For text encodings and bytes-to-bytes codecs, input must be a bytes object or one which provides the read-only buffer interface -- for example, buffer objects and memory mapped files.

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

  The method may not store state in the `Codec` instance. Use `StreamReader` for codecs which have to keep state in order to make decoding efficient.

  The decoder must be able to handle zero length input and return an empty object of the output object type in this situation.

### Incremental Encoding and Decoding

The `IncrementalEncoder` and `IncrementalDecoder` classes provide the basic interface for incremental encoding and decoding. Encoding/decoding the input isn't done with one call to the stateless encoder/decoder function, but with multiple calls to the `encode()`/`decode()` method of the incremental encoder/decoder. The incremental encoder/decoder keeps track of the encoding/decoding process during method calls.

The joined output of calls to the `encode()`/`decode()` method is the same as if all the single inputs were joined into one, and this input was encoded/decoded with the stateless encoder/decoder.

#### IncrementalEncoder Objects

The `IncrementalEncoder` class is used for encoding an input in multiple steps. It defines the following methods which every incremental encoder must define in order to be compatible with the Python codec registry.

class `codecs.``IncrementalEncoder`(errors='strict')

  Constructor for an `IncrementalEncoder` instance.

  All incremental encoders must provide this constructor interface. They are free to add additional keyword arguments, but only the ones defined here are used by the Python codec registry.

  The `IncrementalEncoder` may implement different error handling schemes by providing the errors keyword argument. See Error Handlers for possible values.

  The errors argument will be assigned to an attribute of the same name. Assigning to this attribute makes it possible to switch between different error handling strategies during the lifetime of the `IncrementalEncoder` object.

  `encode`(object[, final])

    Encodes object (taking the current state of the encoder into account) and returns the resulting encoded object. If this is the last call to `encode()` final must be true (the default is false).

  `reset`()

    Reset the encoder to the initial state. The output is discarded: call `.encode(object, final=True)`, passing an empty byte or text string if necessary, to reset the encoder and to get the output.

  `getstate`()

    返回编码器的当前状态，该值必须为一个整数。 实现应当确保 `0` 是最常见的状态。 （比整数更复杂的状态表示可以通过编组/选择状态并将结果字符串的字节数据编码为整数来转换为一个整数值）。

  `setstate`(state)

    Set the state of the encoder to state. state must be an encoder state returned by `getstate()`.

#### IncrementalDecoder Objects

The `IncrementalDecoder` class is used for decoding an input in multiple steps. It defines the following methods which every incremental decoder must define in order to be compatible with the Python codec registry.

class `codecs.``IncrementalDecoder`(errors='strict')

  Constructor for an `IncrementalDecoder` instance.

  All incremental decoders must provide this constructor interface. They are free to add additional keyword arguments, but only the ones defined here are used by the Python codec registry.

  The `IncrementalDecoder` may implement different error handling schemes by providing the errors keyword argument. See Error Handlers for possible values.

  The errors argument will be assigned to an attribute of the same name. Assigning to this attribute makes it possible to switch between different error handling strategies during the lifetime of the `IncrementalDecoder` object.

  `decode`(object[, final])

    Decodes object (taking the current state of the decoder into account) and returns the resulting decoded object. If this is the last call to `decode()` final must be true (the default is false). If final is true the decoder must decode the input completely and must flush all buffers. If this isn't possible (e.g. because of incomplete byte sequences at the end of the input) it must initiate error handling just like in the stateless case (which might raise an exception).

  `reset`()

    Reset the decoder to the initial state.

  `getstate`()

    Return the current state of the decoder. This must be a tuple with two items, the first must be the buffer containing the still undecoded input. The second must be an integer and can be additional state info. (The implementation should make sure that `0` is the most common additional state info.) If this additional state info is `0` it must be possible to set the decoder to the state which has no input buffered and `0` as the additional state info, so that feeding the previously buffered input to the decoder returns it to the previous state without producing any output. (Additional state info that is more complicated than integers can be converted into an integer by marshaling/pickling the info and encoding the bytes of the resulting string into an integer.)

  `setstate`(state)

    将解码器的状态设为 state。 state 必须为 `getstate()` 所返回的一个解码器状态。

### Stream Encoding and Decoding

The `StreamWriter` and `StreamReader` classes provide generic working interfaces which can be used to implement new encoding submodules very easily. See `encodings.utf_8` for an example of how this is done.

#### StreamWriter Objects

The `StreamWriter` class is a subclass of `Codec` and defines the following methods which every stream writer must define in order to be compatible with the Python codec registry.

class `codecs.``StreamWriter`(stream, errors='strict')

  Constructor for a `StreamWriter` instance.

  All stream writers must provide this constructor interface. They are free to add additional keyword arguments, but only the ones defined here are used by the Python codec registry.

  The stream argument must be a file-like object open for writing text or binary data, as appropriate for the specific codec.

  The `StreamWriter` may implement different error handling schemes by providing the errors keyword argument. See Error Handlers for the standard error handlers the underlying stream codec may support.

  The errors argument will be assigned to an attribute of the same name. Assigning to this attribute makes it possible to switch between different error handling strategies during the lifetime of the `StreamWriter` object.

  `write`(object)

    Writes the object's contents encoded to the stream.

  `writelines`(list)

    Writes the concatenated list of strings to the stream (possibly by reusing the `write()` method). The standard bytes-to-bytes codecs do not support this method.

  `reset`()

    Flushes and resets the codec buffers used for keeping state.

    Calling this method should ensure that the data on the output is put into a clean state that allows appending of new fresh data without having to rescan the whole stream to recover state.

In addition to the above methods, the `StreamWriter` must also inherit all other methods and attributes from the underlying stream.

#### StreamReader Objects

The `StreamReader` class is a subclass of `Codec` and defines the following methods which every stream reader must define in order to be compatible with the Python codec registry.

class `codecs.``StreamReader`(stream, errors='strict')

  Constructor for a `StreamReader` instance.

  All stream readers must provide this constructor interface. They are free to add additional keyword arguments, but only the ones defined here are used by the Python codec registry.

  The stream argument must be a file-like object open for reading text or binary data, as appropriate for the specific codec.

  The `StreamReader` may implement different error handling schemes by providing the errors keyword argument. See Error Handlers for the standard error handlers the underlying stream codec may support.

  The errors argument will be assigned to an attribute of the same name. Assigning to this attribute makes it possible to switch between different error handling strategies during the lifetime of the `StreamReader` object.

  The set of allowed values for the errors argument can be extended with `register_error()`.

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

    Decodes data from the stream and returns the resulting object.

    The chars argument indicates the number of decoded code points or bytes to return. The `read()` method will never return more data than requested, but it might return less, if there is not enough available.

    size 参数指明要读取并解码的已编码字节或码位的最大数量近似值。 解码器可以适当地修改此设置。 默认值 -1 表示尽可能多地读取并解码。 此形参的目的是防止一次性解码过于巨大的文件。

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

    The method should use a greedy read strategy meaning that it should read as much data as is allowed within the definition of the encoding and the given size, e.g. if optional encoding endings or state markers are available on the stream, these should be read too.

  `readline`([size[, keepends]])

    Read one line from the input stream and return the decoded data.

    size, if given, is passed as size argument to the stream's `read()` method.

    If keepends is false line-endings will be stripped from the lines returned.

  `readlines`([sizehint[, keepends]])

    Read all lines available on the input stream and return them as a list of lines.

    行结束符会使用编解码器的 `decode()` 方法来实现，并且如果 keepends 为真值则会将其包含在列表条目中。

    sizehint, if given, is passed as the size argument to the stream's `read()` method.

  `reset`()

    Resets the codec buffers used for keeping state.

    请注意不应当对流进行重定位。 使用此方法的主要目的是为了能够从解码错误中恢复。

In addition to the above methods, the `StreamReader` must also inherit all other methods and attributes from the underlying stream.

#### StreamReaderWriter Objects

The `StreamReaderWriter` is a convenience class that allows wrapping streams which work in both read and write modes.

The design is such that one can use the factory functions returned by the `lookup()` function to construct the instance.

class `codecs.``StreamReaderWriter`(stream, Reader, Writer, errors='strict')

  Creates a `StreamReaderWriter` instance. stream must be a file-like object. Reader and Writer must be factory functions or classes providing the `StreamReader` and `StreamWriter` interface resp. Error handling is done in the same way as defined for the stream readers and writers.

`StreamReaderWriter` instances define the combined interfaces of `StreamReader` and `StreamWriter` classes. They inherit all other methods and attributes from the underlying stream.

#### StreamRecoder Objects

The `StreamRecoder` translates data from one encoding to another, which is sometimes useful when dealing with different encoding environments.

The design is such that one can use the factory functions returned by the `lookup()` function to construct the instance.

class `codecs.``StreamRecoder`(stream, encode, decode, Reader, Writer, errors='strict')

  Creates a `StreamRecoder` instance which implements a two-way conversion: encode and decode work on the frontend — the data visible to code calling `read()` and `write()`, while Reader and Writer work on the backend — the data in stream.

  你可以使用这些对象来进行透明转码，例如从 Latin-1 转为 UTF-8 以及反向转换。

  The stream argument must be a file-like object.

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

  Error handling is done in the same way as defined for the stream readers and writers.

`StreamRecoder` instances define the combined interfaces of `StreamReader` and `StreamWriter` classes. They inherit all other methods and attributes from the underlying stream.

## Encodings and Unicode

字符串在系统内部存储为 `0x0`--`0x10FFFF` 范围内的码位序列。 （请参阅 PEP 393 了解有关实现的详情。） 一旦字符串对象要在 CPU 和内存以外使用，字节的大小端顺序和字节数组的存储方式就成为一个关键问题。 如同使用其他编解码器一样，将字符串序列化为字节序列被称为 编码，而从字节序列重建字符串被称为 解码。 存在许多不同的文本序列化编解码器，它们被统称为 文本编码。

The simplest text encoding (called `'latin-1'` or `'iso-8859-1'`) maps the code points 0--255 to the bytes `0x0`--`0xff`, which means that a string object that contains code points above `U+00FF` can't be encoded with this codec. Doing so will raise a `UnicodeEncodeError` that looks like the following (although the details of the error message may differ): `UnicodeEncodeError: 'latin-1' codec can't encode character '\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 string; as a `ZERO WIDTH NO-BREAK SPACE` it's a normal character that will be decoded like any other.

There's another encoding that is able to encoding the full range of Unicode characters: UTF-8. UTF-8 is an 8-bit encoding, which means there are no issues with byte order in UTF-8. Each byte in a UTF-8 byte sequence consists of two parts: marker bits (the most significant bits) and payload bits. The marker bits are a sequence of zero to four `1` bits followed by a `0` bit. Unicode characters are encoded like this (with x being payload bits, which when concatenated give the Unicode character):

Range | Encoding
--- | ---
`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

The least significant bit of the Unicode character is the rightmost x bit.

As UTF-8 is an 8-bit encoding no BOM is required and any `U+FEFF` character in the decoded 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 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

对于 iso-8859-1 编码格式来说），这提升了根据字节序列来正确猜测 `utf-8-sig` 编码格式的成功率。 所以在这里 BOM 的作用并不是帮助确定生成字节序列所使用的字节顺序，而是作为帮助猜测编码格式的记号。 在进行编码时 utf-8-sig 编解码器将把 `0xef`, `0xbb`, `0xbf` 作为头三个字节写入文件。 在进行解码时 `utf-8-sig` 将跳过这三个字节，如果它们作为文件的头三个字节出现的话。 在 UTF-8 中并不推荐使用 BOM，通常应当避免它们的出现。

## Standard Encodings

Python comes with a number of codecs built-in, either implemented as C functions or with dictionaries as mapping tables. The following table lists the codecs by name, together with a few common aliases, and the languages for which the encoding is likely used. Neither the list of aliases nor the list of languages is meant to be exhaustive. Notice that spelling alternatives that only differ in case or use a hyphen instead of an underscore are also valid aliases; therefore, e.g. `'utf-8'` is a valid alias for the `'utf_8'` codec.

CPython implementation detail: 有些常见编码格式可以绕过编解码器查找机制来提升性能。 这些优化机会对于 CPython 来说仅能通过一组有限的别名（大小写不敏感）来识别：utf-8, utf8, latin-1, latin1, iso-8859-1, iso8859-1, mbcs (Windows 专属), ascii, us-ascii, utf-16, utf16, utf-32, utf32, 也包括使用下划线替代连字符的的形式。 使用这些编码格式的其他别名可能会导致更慢的执行速度。

3.6 版更變: Optimization opportunity recognized for us-ascii.

Many of the character sets support the same languages. They vary in individual characters (e.g. whether the EURO SIGN is supported or not), and in the assignment of characters to code positions. For the European languages in particular, the following variants typically exist:

- an ISO 8859 codeset

- a Microsoft Windows code page, which is typically derived from an 8859 codeset, but replaces control characters with additional graphic characters

- an IBM EBCDIC code page

- an IBM PC code page, which is ASCII compatible

Codec | Aliases | Languages
--- | --- | ---
ascii | 646, us-ascii | English
big5 | big5-tw, csbig5 | Traditional Chinese
big5hkscs | big5-hkscs, hkscs | Traditional Chinese
cp037 | IBM037, IBM039 | English
cp273 | 273, IBM273, csIBM273 | German 3.4 版新加入.
cp424 | EBCDIC-CP-HE, IBM424 | Hebrew
cp437 | 437, IBM437 | English
cp500 | EBCDIC-CP-BE, EBCDIC-CP-CH, IBM500 | Western Europe
cp720 |  | Arabic
cp737 |  | Greek
cp775 | IBM775 | Baltic languages
cp850 | 850, IBM850 | Western Europe
cp852 | 852, IBM852 | Central and Eastern Europe
cp855 | 855, IBM855 | Bulgarian, Byelorussian, Macedonian, Russian, Serbian
cp856 |  | Hebrew
cp857 | 857, IBM857 | Turkish
cp858 | 858, IBM858 | Western Europe
cp860 | 860, IBM860 | Portuguese
cp861 | 861, CP-IS, IBM861 | Icelandic
cp862 | 862, IBM862 | Hebrew
cp863 | 863, IBM863 | Canadian
cp864 | IBM864 | Arabic
cp865 | 865, IBM865 | Danish, Norwegian
cp866 | 866, IBM866 | Russian
cp869 | 869, CP-GR, IBM869 | Greek
cp874 |  | Thai
cp875 |  | Greek
cp932 | 932, ms932, mskanji, ms-kanji | Japanese
cp949 | 949, ms949, uhc | Korean
cp950 | 950, ms950 | Traditional Chinese
cp1006 |  | Urdu
cp1026 | ibm1026 | Turkish
cp1125 | 1125, ibm1125, cp866u, ruscii | Ukrainian 3.4 版新加入.
cp1140 | ibm1140 | Western Europe
cp1250 | windows-1250 | Central and Eastern Europe
cp1251 | windows-1251 | Bulgarian, Byelorussian, Macedonian, Russian, Serbian
cp1252 | windows-1252 | Western Europe
cp1253 | windows-1253 | Greek
cp1254 | windows-1254 | Turkish
cp1255 | windows-1255 | Hebrew
cp1256 | windows-1256 | Arabic
cp1257 | windows-1257 | Baltic languages
cp1258 | windows-1258 | Vietnamese
euc_jp | eucjp, ujis, u-jis | Japanese
euc_jis_2004 | jisx0213, eucjis2004 | Japanese
euc_jisx0213 | eucjisx0213 | Japanese
euc_kr | euckr, korean, ksc5601, ks_c-5601, ks_c-5601-1987, ksx1001, ks_x-1001 | Korean
gb2312 | chinese, csiso58gb231280, euc-cn, euccn, eucgb2312-cn, gb2312-1980, gb2312-80, iso-ir-58 | Simplified Chinese
gbk | 936, cp936, ms936 | Unified Chinese
gb18030 | gb18030-2000 | Unified Chinese
hz | hzgb, hz-gb, hz-gb-2312 | Simplified Chinese
iso2022_jp | csiso2022jp, iso2022jp, iso-2022-jp | Japanese
iso2022_jp_1 | iso2022jp-1, iso-2022-jp-1 | Japanese
iso2022_jp_2 | iso2022jp-2, iso-2022-jp-2 | Japanese, Korean, Simplified Chinese, Western Europe, Greek
iso2022_jp_2004 | iso2022jp-2004, iso-2022-jp-2004 | Japanese
iso2022_jp_3 | iso2022jp-3, iso-2022-jp-3 | Japanese
iso2022_jp_ext | iso2022jp-ext, iso-2022-jp-ext | Japanese
iso2022_kr | csiso2022kr, iso2022kr, iso-2022-kr | Korean
latin_1 | iso-8859-1, iso8859-1, 8859, cp819, latin, latin1, L1 | Western Europe
iso8859_2 | iso-8859-2, latin2, L2 | Central and Eastern Europe
iso8859_3 | iso-8859-3, latin3, L3 | Esperanto, Maltese
iso8859_4 | iso-8859-4, latin4, L4 | Baltic languages
iso8859_5 | iso-8859-5, cyrillic | Bulgarian, Byelorussian, Macedonian, Russian, Serbian
iso8859_6 | iso-8859-6, arabic | Arabic
iso8859_7 | iso-8859-7, greek, greek8 | Greek
iso8859_8 | iso-8859-8, hebrew | Hebrew
iso8859_9 | iso-8859-9, latin5, L5 | Turkish
iso8859_10 | iso-8859-10, latin6, L6 | Nordic languages
iso8859_11 | iso-8859-11, thai | Thai languages
iso8859_13 | iso-8859-13, latin7, L7 | Baltic languages
iso8859_14 | iso-8859-14, latin8, L8 | Celtic languages
iso8859_15 | iso-8859-15, latin9, L9 | Western Europe
iso8859_16 | iso-8859-16, latin10, L10 | South-Eastern Europe
johab | cp1361, ms1361 | Korean
koi8_r |  | Russian
koi8_t |  | Tajik 3.5 版新加入.
koi8_u |  | Ukrainian
kz1048 | kz_1048, strk1048_2002, rk1048 | Kazakh 3.5 版新加入.
mac_cyrillic | maccyrillic | Bulgarian, Byelorussian, Macedonian, Russian, Serbian
mac_greek | macgreek | Greek
mac_iceland | maciceland | Icelandic
mac_latin2 | maclatin2, maccentraleurope | Central and Eastern Europe
mac_roman | macroman, macintosh | Western Europe
mac_turkish | macturkish | Turkish
ptcp154 | csptcp154, pt154, cp154, cyrillic-asian | Kazakh
shift_jis | csshiftjis, shiftjis, sjis, s_jis | Japanese
shift_jis_2004 | shiftjis2004, sjis_2004, sjis2004 | Japanese
shift_jisx0213 | shiftjisx0213, sjisx0213, s_jisx0213 | Japanese
utf_32 | U32, utf32 | all languages
utf_32_be | UTF-32BE | all languages
utf_32_le | UTF-32LE | all languages
utf_16 | U16, utf16 | all languages
utf_16_be | UTF-16BE | all languages
utf_16_le | UTF-16LE | all languages
utf_7 | U7, unicode-1-1-utf-7 | all languages
utf_8 | U8, UTF, utf8, cp65001 | all languages
utf_8_sig |  | all languages

3.4 版更變: The utf-16* and utf-32* encoders no longer allow surrogate code points (`U+D800`--`U+DFFF`) to be encoded. The utf-32* decoders no longer decode byte sequences that correspond to surrogate code points.

3.8 版更變: `cp65001` 现在是 `utf_8` 的一个别名。

## Python Specific Encodings

有一些预定义编解码器是 Python 专属的，因此它们在 Python 之外没有意义。 这些编解码器按其所预期的输入和输出类型在下表中列出（请注意虽然文本编码是编解码器最常见的使用场景，但下层的编解码器架构支持任意数据转换而不仅是文本编码）。 对于非对称编解码器，该列描述的含义是编码方向。

### Text Encodings

The following codecs provide `str` to `bytes` encoding and bytes-like object to `str` decoding, similar to the Unicode text encodings.

Codec | Aliases | Meaning
--- | --- | ---
idna |  | 实现 RFC 3490，另请参阅 `encodings.idna` 。仅支持 `errors='strict'` 。
mbcs | ansi, dbcs | Windows 专属：根据 ANSI 代码页（CP_ACP）对操作数进行编码。
oem |  | Windows 专属：根据 OEM 代码页（CP_OEMCP）对操作数进行编码。 3.6 版新加入.
palmos |  | PalmOS 3.5 的编码格式
punycode |  | 实现 RFC 3492。 不支持有状态编解码器。
raw_unicode_escape |  | Latin-1 encoding with `\uXXXX` and `\UXXXXXXXX` for other code points. Existing backslashes are not escaped in any way. It is used in the Python pickle protocol.
undefined |  | Raise an exception for all conversions, even empty strings. The error handler is ignored.
unicode_escape |  | 适合用于以 ASCII 编码的 Python 源代码中的 Unicode 字面值内容的编码格式，但引号不会被转义。 对 Latin-1 源代码进行解码。 请注意 Python 源代码实际上默认使用 UTF-8。

3.8 版更變: "unicode_internal" 编解码器已被移除。

### Binary Transforms

以下编解码器提供了二进制转换: bytes-like object 到 `bytes` 的映射。 它们不被 `bytes.decode()` 所支持（该方法只生成 `str` 类型的输出）。

Codec | Aliases | Meaning | Encoder / decoder
--- | --- | --- | ---
base64_codec 1 | base64, base_64 | 将操作数转换为多行 MIME base64 (结果总是包含一个末尾的 `'\n'`) 3.4 版更變: accepts any bytes-like object as input for encoding and decoding | `base64.encodebytes()` / `base64.decodebytes()`
bz2_codec | bz2 | 使用bz2压缩操作数 | `bz2.compress()` / `bz2.decompress()`
hex_codec | hex | 将操作数转换为十六进制表示，每个字节有两位数 | `binascii.b2a_hex()` / `binascii.a2b_hex()`
quopri_codec | quopri, quotedprintable, quoted_printable | 将操作数转换为 MIME 带引号的可打印数据 | `quopri.encode()` with `quotetabs=True` / `quopri.decode()`
uu_codec | uu | 使用uuencode转换操作数 | `uu.encode()` / `uu.decode()`
zlib_codec | zip, zlib | 使用gzip压缩操作数 | `zlib.compress()` / `zlib.decompress()`

1

  In addition to bytes-like objects, `'base64_codec'` also accepts ASCII-only instances of `str` for decoding

3.2 版新加入: Restoration of the binary transforms.

3.4 版更變: Restoration of the aliases for the binary transforms.

### Text Transforms

以下编解码器提供了文本转换: `str` 到 `str` 的映射。 它不被 `str.encode()` 所支持（该方法只生成 `bytes` 类型的输出）。

Codec | Aliases | Meaning
--- | --- | ---
rot_13 | rot13 | 返回操作数的凯撒密码加密结果

3.2 版新加入: Restoration of the `rot_13` text transform.

3.4 版更變: Restoration of the `rot13` alias.

## `encodings.idna` --- Internationalized Domain Names in Applications

This module implements RFC 3490 (Internationalized Domain Names in Applications) and RFC 3492 (Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)). It builds upon the `punycode` encoding and `stringprep`.

These RFCs together define a protocol to support non-ASCII characters in domain names. A domain name containing non-ASCII characters (such as `www.Alliancefrançaise.nu`) is converted into an ASCII-compatible encoding (ACE, such as `www.xn--alliancefranaise-npb.nu`). The ACE form of the domain name is then used in all places where arbitrary characters are not allowed by the protocol, such as DNS queries, HTTP Host fields, and so on. This conversion is carried out in the application; if possible invisible to the user: The application should transparently convert Unicode domain labels to IDNA on the wire, and convert back ACE labels to Unicode before presenting them to the user.

Python 以多种方式支持这种转换: `idna` 编解码器执行 Unicode 和 ACE 之间的转换，基于在 section 3.1 of RFC 3490 中定义的分隔字符将输入字符串拆分为标签，再根据需要将每个标签转换为 ACE，相反地又会基于 `.` 分隔符将输入字节串拆分为标签，再将找到的任何 ACE 标签转换为 Unicode。 此外，`socket` 模块可透明地将 Unicode 主机名转换为 ACE，以便应用在将它们传给 socket 模块时无须自行转换主机名。 除此之外，许多包含以主机名作为函数参数的模块例如 `http.client` 和 `ftplib` 都接受 Unicode 主机名（并且 `http.client` 也会在 Host 字段中透明地发送 IDNA 主机名，如果它需要发送该字段的话）。

当从线路接收主机名时（例如反向名称查找），到 Unicode 的转换不会自动被执行：希望向用户提供此种主机名的应用应当将它们解码为 Unicode。

The module `encodings.idna` also implements the nameprep procedure, which performs certain normalizations on host names, to achieve case-insensitivity of international domain names, and to unify similar characters. The nameprep functions can be used directly if desired.

`encodings.idna.``nameprep`(label)

  Return the nameprepped version of label. The implementation currently assumes query strings, so `AllowUnassigned` is true.

`encodings.idna.``ToASCII`(label)

  Convert a label to ASCII, as specified in RFC 3490. `UseSTD3ASCIIRules` is assumed to be false.

`encodings.idna.``ToUnicode`(label)

  Convert a label to Unicode, as specified in RFC 3490.

## `encodings.mbcs` --- Windows ANSI codepage

此模块实现ANSI代码页（CP_ACP）。

Availability: 仅Windows可用

3.3 版更變: Support any error handler.

3.2 版更變: Before 3.2, the errors argument was ignored; `'replace'` was always used to encode, and `'ignore'` to decode.

## `encodings.utf_8_sig` --- UTF-8 codec with BOM signature

此模块实现了 UTF-8 编解码器的一个变种：在编码时将把 UTF-8 已编码 BOM 添加到 UTF-8 编码字节数据的开头。 对于有状态编码器此操作只执行一次（当首次写入字节流时）。 在解码时将跳过数据开头作为可选项的 UTF-8 已编码 BOM。