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# 7.2. `codecs` — Codec registry and base classes

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='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.

`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)
  - `'surrogateescape'`: on decoding, replace with code points in the Unicode Private Use Area ranging from U+DC80 to U+DCFF. These private code points will then be turned back into the same bytes when the `surrogateescape` error handler is used when encoding the data. (See PEP 383 for more.)

  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.

  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.

To simplify access to the various codecs, 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.

`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 replacement may be either `str` or `bytes`. If the replacement is bytes, the encoder will simply copy them into the output buffer. If the replacement is a string, the encoder will encode the replacement. Encoding continues on 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)

  Return the error handler previously registered under the name name.

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

`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.

  Note

  The wrapped version’s methods will accept and return strings only. Bytes arguments will be rejected.

  Note

  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 `b'\n'` is done on reading and writing.

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

  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 has the same meaning as for the built-in `open()` function. It defaults to line buffered.

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

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

  Bytes written to the wrapped file are interpreted according to the given data_encoding and then written to the original file as bytes using the file_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. errors (as well as any other keyword argument) is passed through to the incremental encoder.

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

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

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 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.

## 7.2.1. Codec Base Classes

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:

Value | Meaning
--- | ---
`'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).
`'surrogateescape'` | Replace byte with surrogate U+DCxx, as defined in PEP 383.

In addition, the following error handlers are specific to a single codec:

Value | Codec | Meaning
--- | --- | ---
`'surrogatepass'` | utf-8 | Allow encoding and decoding of surrogate codes in UTF-8.

New in version 3.1: The `'surrogateescape'` and `'surrogatepass'` error handlers.

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

### 7.2.1.1. 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). Encoding converts a string object to a bytes object 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.

  The method may not store state in the `Codec` instance. Use `StreamCodec` for codecs which have to keep state in order to make encoding/decoding 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])

  Decodes the object input and returns a tuple (output object, length consumed). Decoding converts a bytes object encoded using a particular character set encoding to a string object.

  input must be a bytes object or one which provides the read-only character buffer interface – for example, buffer objects and memory mapped files.

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

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

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.

### 7.2.1.2. 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])

  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. 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.

  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.

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

  `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('', final=True)` to reset the encoder and to get the output.

`IncrementalEncoder.``getstate`()

  Return the current state of the encoder which must be an integer. The implementation should make sure that `0` is the most common state. (States that are more complicated than integers can be converted into an integer by marshaling/pickling the state and encoding the bytes of the resulting string into an integer).

`IncrementalEncoder.``setstate`(state)

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

### 7.2.1.3. 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])

  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. 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.

  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.

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

  `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)

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

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.

### 7.2.1.4. 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])

  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.

  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.

  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.

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

  `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).

  `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.

### 7.2.1.5. 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])

  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.

  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.

  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.

    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.

    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.

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

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

  `reset`()

    Resets the codec buffers used for keeping state.

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

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

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

### 7.2.1.6. StreamReaderWriter Objects

The `StreamReaderWriter` 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)

  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.

### 7.2.1.7. StreamRecoder Objects

The `StreamRecoder` provide a frontend - backend view of encoding data 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)

  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.

  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.

## 7.2.2. Encodings and Unicode

Strings are stored internally as sequences of codepoints in range `0 - 10FFFF` (see PEP 393 for more details about the implementation). Once a string object is used outside of CPU and memory, CPU endianness and how these arrays are stored as bytes become an issue. Transforming a string object into a sequence of bytes is called encoding and recreating the string 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 codepoints 0-255 to the bytes `0x0`-`0xff`. This means that a string object that contains codepoints above `U+00FF` can’t be encoded with this method (which is called `'latin-1'` or `'iso-8859-1'`). `str.encode()` will raise a `UnicodeEncodeError` that looks like this: `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 codepoints 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 codepoints defined in Unicode. A simple and straightforward way that can store each Unicode code point, is to store each codepoint 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

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.

## 7.2.3. 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: Some common encodings can bypass the codecs lookup machinery to improve performance. These optimization opportunities are only recognized by CPython for a limited set of aliases: utf-8, utf8, latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, utf-16, and utf-32. Using alternative spellings for these encodings may result in slower execution.

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 a 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
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
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
cp65001 |  | Windows only: Windows UTF-8 (`CP_UTF8`) New in version 3.3.
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 | West 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_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_u |  | Ukrainian
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 | all languages
utf_8_sig |  | all languages

## 7.2.4. Python Specific Encodings

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

Codec | Aliases | 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
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 Deprecated since version 3.3.

The following codecs provide bytes-like object to `bytes` mappings.

Codec | Purpose | Encoder/decoder
--- | --- | ---
base64_codec [1] | Convert operand to MIME base64 (the result always includes a trailing `'\n'`) | `base64.b64encode()`, `base64.b64decode()`
bz2_codec | Compress the operand using bz2 | `bz2.compress()`, `bz2.decompress()`
hex_codec | Convert operand to hexadecimal representation, with two digits per byte | `base64.b16encode()`, `base64.b16decode()`
quopri_codec | Convert operand to MIME quoted printable | `quopri.encodestring()`, `quopri.decodestring()`
uu_codec | Convert the operand using uuencode | `uu.encode()`, `uu.decode()`
zlib_codec | Compress the operand using gzip | `zlib.compress()`, `zlib.decompress()`

[1] | Rather than accepting any bytes-like object, `'base64_codec'` accepts only `bytes` and `bytearray` for encoding and only `bytes`, `bytearray`, and ASCII-only instances of `str` for decoding

The following codecs provide `str` to `str` mappings.

Codec | Purpose
--- | ---
rot_13 | Returns the Caesar-cypher encryption of the operand

New in version 3.2: bytes-to-bytes and str-to-str codecs.

## 7.2.5. `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 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 `http.client` and `ftplib`, accept Unicode host names (`http.client` 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.

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.

## 7.2.6. `encodings.mbcs` — Windows ANSI codepage

Encode operand according to the ANSI codepage (CP_ACP).

Availability: Windows only.

Changed in version 3.3: Support any error handler.

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

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

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.

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