7.2. codecs — Registre des codecs et classes de base associées

Code source : Lib/codecs.py

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Ce module définit les classes de base pour les codecs (encodeurs et décodeurs) standards Python et fournit l’interface avec le registre des codecs internes à Python, qui gère le processus de recherche de codecs et de gestion des erreurs. La plupart des codecs sont des encodeurs de texte, qui encode du texte vers des séquences d’octets (type bytes de Python) mais il existe aussi des codecs qui encodent du texte vers du texte et des bytes vers des bytes. Les codecs personnalisés peuvent encoder et décoder des types arbitraires, mais l’utilisation de certaines fonctionnalités du module est restreinte aux encodeurs de texte ou aux codecs qui encodent vers bytes.

Le module définit les fonctions suivantes pour encoder et décoder à l’aide de n’importe quel codec :

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

Encode obj en utilisant le codec enregistré pour encoding.

Vous pouvez spécifier errors pour définir la façon de gérer les erreurs. Le gestionnaire d’erreurs par défaut est 'strict', ce qui veut dire qu’une erreur lors de l’encodage lève ValueError (ou une sous-classe spécifique du codec, telle que UnicodeEncodeError). Référez-vous aux Classes de base de codecs pour plus d’informations sur la gestion des erreurs par les codecs.

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

Décode obj en utilisant le codec enregistré pour encoding.

Vous pouvez spécifier errors pour définir la façon de gérer les erreurs. Le gestionnaire d’erreurs par défaut est 'strict', ce qui veut dire qu’une erreur lors du décodage lève ValueError (ou une sous-classe spécifique du codec, telle que UnicodeDecodeError). Référez-vous aux Classes de base de codecs pour plus d’informations sur la gestion des erreurs par les codecs.

Les détails complets de chaque codec peuvent être examinés directement :

codecs.lookup(encoding)

Recherche les informations relatives au codec dans le registre des codecs de Python et renvoie l’objet CodecInfo tel que défini ci-dessous.

Les encodeurs sont recherchés en priorité dans le cache du registre. S’ils n’y sont pas, la liste des fonctions de recherche enregistrées est passée en revue. Si aucun objet CodecInfo n’est trouvé, une LookupError est levée. Sinon, l’objet CodecInfo est mis en cache et renvoyé vers l’appelant.

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

Les détails d’un codec trouvé dans le registre des codecs. Les arguments du constructeur sont stockés dans les attributs éponymes :

name

Le nom de l’encodeur.

encode

decode

Les fonctions d’encodage et de décodage. Ces fonctions ou méthodes doivent avoir la même interface que les méthodes encode() et decode() des instances de Codec (voir Interface des codecs). Les fonctions et méthodes sont censées fonctionner sans état interne.

incrementalencoder

incrementaldecoder

Classes d’encodeurs et de décodeurs incrémentaux ou fonctions usines. Elles doivent avoir respectivement les mêmes interfaces que celles définies par les classes de base IncrementalEncoder et IncrementalDecoder. Les codecs incrémentaux peuvent conserver des états internes.

streamwriter

streamreader

Classes d’écriture et de lecture de flux ou fonctions usines. Elles doivent avoir les mêmes interfaces que celles définies par les classes de base StreamWriter et StreamReader, respectivement. Les codecs de flux peuvent conserver un état interne.

Pour simplifier l’accès aux différents composants du codec, le module fournit les fonctions supplémentaires suivantes qui utilisent lookup() pour la recherche du codec :

codecs.getencoder(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa fonction d’encodage.

Lève une LookupError si l’encodage encoding n’est pas trouvé.

codecs.getdecoder(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa fonction de décodage.

Lève une LookupError si l’encodage encoding n’est pas trouvé.

codecs.getincrementalencoder(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa classe d’encodage incrémental ou la fonction usine.

Lève une LookupError si l’encodage encoding n’est pas trouvé ou si le codec ne gère pas l’encodage incrémental.

codecs.getincrementaldecoder(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa classe de décodage incrémental ou la fonction usine.

Lève une LookupError si l’encodage encoding n’est pas trouvé ou si le codec ne gère pas le décodage incrémental.

codecs.getreader(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa classe StreamReader ou la fonction usine.

Lève une LookupError si l’encodage encoding n’est pas trouvé.

codecs.getwriter(encoding)

Recherche le codec pour l’encodage encoding et renvoie sa classe StreamWriter ou la fonction usine.

Lève une LookupError si l’encodage encoding n’est pas trouvé.

Les codecs personnalisés sont mis à disposition en enregistrant une fonction de recherche de codecs adaptée :

codecs.register(search_function)

Enregistre une fonction de recherche de codec. Il convient qu’une fonction de recherche prenne un argument, le nom de l’encodage écrit en lettres minuscules, et renvoie un objet CodecInfo. Si la fonction de recherche ne trouve pas un encodage donné, il convient qu’elle renvoie `` None``.

Note

l’enregistrement d’une fonction de recherche n’est actuellement pas réversible, ce qui peut entraîner des problèmes dans certains cas, par exemple pour les tests unitaires ou le rechargement de module.

Alors qu’il est recommandé d’utiliser la fonction native open() et le module associé io pour travailler avec des fichiers texte encodés, le présent module fournit des fonctions et classes utilitaires supplémentaires qui permettent l’utilisation d’une plus large gamme de codecs si vous travaillez avec des fichiers binaires :

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

Ouvre un fichier encodé en utilisant le mode donné et renvoie une instance de StreamReaderWriter, permettant un encodage-décodage transparent. Le mode de fichier par défaut est 'r', ce qui signifie que le fichier est ouvert en lecture.

Note

les fichiers encodés sous-jacents sont toujours ouverts en mode binaire. Aucune conversion automatique de '\n' n’est effectuée à la lecture ou à l’écriture. L’argument mode peut être n’importe quel mode binaire acceptable pour la fonction native open() ; le 'b' est automatiquement ajouté.

encoding spécifie l’encodage à utiliser pour le fichier. Tout encodage qui encode et décode des octets (type bytes) est autorisé et les types de données pris en charge par les méthodes relatives aux fichiers dépendent du codec utilisé.

errors peut être spécifié pour définir la gestion des erreurs. La valeur par défaut est 'strict', ce qui lève une ValueError en cas d’erreur lors du codage.

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')

Renvoie une instance de StreamRecoder, version encapsulée de file qui fournit un transcodage transparent. Le fichier original est fermé quand la version encapsulée est fermée.

Les données écrites dans un fichier encapsulant sont décodées en fonction du data_encoding spécifié puis écrites vers le fichier original en tant que bytes en utilisant file_encoding. Les octets lus dans le fichier original sont décodés conformément à file_encoding et le résultat est encodé en utilisant data_encoding.

Si file_encoding n’est pas spécifié, la valeur par défaut est data_encoding.

errors peut être spécifié pour définir la gestion des erreurs. La valeur par défaut est 'strict', ce qui lève une ValueError en cas d’erreur lors du codage.

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

Utilise un encodeur incrémental pour encoder de manière itérative l’entrée fournie par iterator. Cette fonction est un générateur. L’argument errors (ainsi que tout autre argument passé par mot-clé) est transmis à l’encodeur incrémental.

Cette fonction nécessite que le codec accepte les objets texte (classe str) en entrée. Par conséquent, il ne prend pas en charge les encodeurs bytes vers bytes tels que base64_codec.

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

Utilise un décodeur incrémental pour décoder de manière itérative l’entrée fournie par iterator. Cette fonction est un générateur. L’argument errors (ainsi que tout autre argument passé par mot-clé) est transmis au décodeur incrémental.

Cette fonction requiert que le codec accepte les objets bytes en entrée. Par conséquent, elle ne prend pas en charge les encodeurs de texte vers texte tels que rot_13, bien que rot_13 puisse être utilisé de manière équivalente avec iterencode().

Le module fournit également les constantes suivantes qui sont utiles pour lire et écrire les fichiers dépendants de la plateforme :

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

Ces constantes définissent diverses séquences d’octets, les marques d’ordre d’octets (BOM pour byte order mark en anglais) Unicode pour plusieurs encodages. Elles sont utilisées dans les flux de données UTF-16 et UTF-32 pour indiquer l’ordre des octets utilisé, et dans UTF-8 comme signature Unicode. BOM_UTF16 vaut soit BOM_UTF16_BE, soit BOM_UTF16_LE selon le boutisme natif de la plateforme, BOM est un alias pour BOM_UTF16, BOM_LE pour BOM_UTF16_LE et BOM_BE pour BOM_UTF16_BE. Les autres sont les marques BOM dans les encodages UTF-8 et UTF-32.

7.2.1. Classes de base de codecs

Le module codecs définit un ensemble de classes de base qui spécifient les interfaces pour travailler avec des objets codecs et qui peuvent également être utilisées comme base pour des implémentations de codecs personnalisés.

Chaque codec doit définir quatre interfaces pour être utilisable comme codec en Python : codeur sans état, décodeur sans état, lecteur de flux et écrivain de flux. Le lecteur et l’écrivain de flux réutilisent généralement l’encodeur-décodeur sans état pour implémenter les protocoles de fichiers. Les auteurs de codecs doivent également définir comment le codec gère les erreurs d’encodage et de décodage.

7.2.1.1. Gestionnaires d’erreurs

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

Valeur	Signification
'strict'	Raise UnicodeError (or a subclass); this is the default. Implemented in strict_errors().
'ignore'	Ignore the malformed data and continue without further notice. Implemented in ignore_errors().

Les gestionnaires d’erreurs suivants ne s’appliquent que pour les encodeurs de texte :

Valeur	Signification
'replace'	Replace with a suitable replacement marker; Python will use the official U+FFFD REPLACEMENT CHARACTER for the built-in codecs on decoding, and “?” on encoding. Implemented in replace_errors().
'xmlcharrefreplace'	Replace with the appropriate XML character reference (only for encoding). Implemented in xmlcharrefreplace_errors().
'backslashreplace'	Remplace avec une séquence échappée par des antislashs. Implémenté dans backslashreplace_errors().
'namereplace'	Replace with \N{...} escape sequences (only for encoding). Implemented in namereplace_errors().
'surrogateescape'	On decoding, replace byte with individual surrogate code ranging from U+DC80 to U+DCFF. This code will then be turned back into the same byte when the 'surrogateescape' error handler is used when encoding the data. (See PEP 383 for more.)

En plus, le gestionnaire d’erreurs suivant est spécifique aux codecs suivants :

Valeur	Codecs	Signification
'surrogatepass'	utf-8, utf-16, utf-32, utf-16-be, utf-16-le, utf-32-be, utf-32-le	Allow encoding and decoding of surrogate codes. These codecs normally treat the presence of surrogates as an error.

Nouveau dans la version 3.1: les gestionnaires d’erreurs 'surrogateescape' et 'surrogatepass'.

Modifié dans la version 3.4: le gestionnaire d’erreurs 'surrogatepass' fonctionne maintenant avec les codecs utf-16* et utf-32*.

Nouveau dans la version 3.5: le gestionnaire d’erreurs 'namereplace'.

Modifié dans la version 3.5: le gestionnaire d’erreurs 'backslashreplace' fonctionne maintenant pour le décodage et la traduction.

L’ensemble des valeurs autorisées peut être étendu en enregistrant un nouveau gestionnaire d’erreurs nommé :

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.

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 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.

Nouveau dans la version 3.5.

7.2.1.2. 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])

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

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.

7.2.1.3. 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.

7.2.1.3.1. 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 Gestionnaires d’erreurs 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()

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).

setstate(state)

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

7.2.1.3.2. 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 Gestionnaires d’erreurs 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)

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

7.2.1.4. 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.

7.2.1.4.1. 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 Gestionnaires d’erreurs 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.

7.2.1.4.2. 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 Gestionnaires d’erreurs 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.

The size argument indicates the approximate maximum number of encoded bytes or code points to read for decoding. The decoder can modify this setting as appropriate. The default value -1 indicates to read and decode as much as possible. This parameter is intended to prevent having to decode huge files in one step.

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.

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.

7.2.1.4.3. 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.

7.2.1.4.4. 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.

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

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.

7.2.2. Encodings and Unicode

Strings are stored internally as sequences of code points in range 0x0–0x10FFFF. (See PEP 393 for more details about the implementation.) Once a string object is used outside of CPU and memory, endianness and how these arrays are stored as bytes become an issue. As with other codecs, serialising a string into a sequence of bytes is known as encoding, and recreating the string from the sequence of bytes is known as decoding. There are a variety of different text serialisation codecs, which are collectivity referred to as text encodings.

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

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 (case insensitive) aliases: utf-8, utf8, latin-1, latin1, iso-8859-1, iso8859-1, mbcs (Windows only), ascii, us-ascii, utf-16, utf16, utf-32, utf32, and the same using underscores instead of dashes. Using alternative aliases for these encodings may result in slower execution.

Modifié dans la version 3.6: 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	Anglais
big5	big5-tw, csbig5	Chinois Traditionnel
big5hkscs	big5-hkscs, hkscs	Chinois Traditionnel
cp037	IBM037, IBM039	Anglais
cp273	273, IBM273, csIBM273	Allemand Nouveau dans la version 3.4.
cp424	EBCDIC-CP-HE, IBM424	Hébreux
cp437	437, IBM437	Anglais
cp500	EBCDIC-CP-BE, EBCDIC-CP-CH, IBM500	Europe de l’ouest
cp720		Arabe
cp737		Grec
cp775	IBM775	Langues Baltiques
cp850	850, IBM850	Europe de l’ouest
cp852	852, IBM852	Europe centrale et Europe de l’Est
cp855	855, IBM855	Bulgare, Biélorusse, Macédonien, Russe, Serbe
cp856		Hébreux
cp857	857, IBM857	Turc
cp858	858, IBM858	Europe de l’ouest
cp860	860, IBM860	Portugais
cp861	861, CP-IS, IBM861	Islandais
cp862	862, IBM862	Hébreux
cp863	863, IBM863	Canadien
cp864	IBM864	Arabe
cp865	865, IBM865	Danish, Norwegian
cp866	866, IBM866	Russe
cp869	869, CP-GR, IBM869	Grec
cp874		Thai
cp875		Grec
cp932	932, ms932, mskanji, ms-kanji	Japanese
cp949	949, ms949, uhc	Korean
cp950	950, ms950	Chinois Traditionnel
cp1006		Urdu
cp1026	ibm1026	Turc
cp1125	1125, ibm1125, cp866u, ruscii	Ukrainian Nouveau dans la version 3.4.
cp1140	ibm1140	Europe de l’ouest
cp1250	windows-1250	Europe centrale et Europe de l’Est
cp1251	windows-1251	Bulgare, Biélorusse, Macédonien, Russe, Serbe
cp1252	windows-1252	Europe de l’ouest
cp1253	windows-1253	Grec
cp1254	windows-1254	Turc
cp1255	windows-1255	Hébreux
cp1256	windows-1256	Arabe
cp1257	windows-1257	Langues Baltiques
cp1258	windows-1258	Vietnamese
cp65001		Windows uniquement : Windows UTF-8 (CP_UTF8) Nouveau dans la 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	Europe de l’Ouest
iso8859_2	iso-8859-2, latin2, L2	Europe centrale et Europe de l’Est
iso8859_3	iso-8859-3, latin3, L3	Esperanto, Maltese
iso8859_4	iso-8859-4, latin4, L4	Langues Baltiques
iso8859_5	iso-8859-5, cyrillic	Bulgare, Biélorusse, Macédonien, Russe, Serbe
iso8859_6	iso-8859-6, arabic	Arabe
iso8859_7	iso-8859-7, greek, greek8	Grec
iso8859_8	iso-8859-8, hebrew	Hébreux
iso8859_9	iso-8859-9, latin5, L5	Turc
iso8859_10	iso-8859-10, latin6, L6	Nordic languages
iso8859_11	iso-8859-11, thai	Thai languages
iso8859_13	iso-8859-13, latin7, L7	Langues Baltiques
iso8859_14	iso-8859-14, latin8, L8	Celtic languages
iso8859_15	iso-8859-15, latin9, L9	Europe de l’ouest
iso8859_16	iso-8859-16, latin10, L10	South-Eastern Europe
johab	cp1361, ms1361	Korean
koi8_r		Russe
koi8_t		Tajik Nouveau dans la version 3.5.
koi8_u		Ukrainian
kz1048	kz_1048, strk1048_2002, rk1048	Kazakh Nouveau dans la version 3.5.
mac_cyrillic	maccyrillic	Bulgare, Biélorusse, Macédonien, Russe, Serbe
mac_greek	macgreek	Grec
mac_iceland	maciceland	Islandais
mac_latin2	maclatin2, maccentraleurope	Europe centrale et Europe de l’Est
mac_roman	macroman, macintosh	Europe de l’ouest
mac_turkish	macturkish	Turc
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

Modifié dans la version 3.4: 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.

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.

7.2.4.1. 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	Objectif
idna		Implements RFC 3490, see also encodings.idna. Only errors='strict' is supported.
mbcs	ansi, dbcs	Windows only: Encode operand according to the ANSI codepage (CP_ACP)
oem		Windows only: Encode operand according to the OEM codepage (CP_OEMCP) Nouveau dans la version 3.6.
palmos		Encoding of PalmOS 3.5
punycode		Implements RFC 3492. Stateful codecs are not supported.
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		Encoding suitable as the contents of a Unicode literal in ASCII-encoded Python source code, except that quotes are not escaped. Decodes from Latin-1 source code. Beware that Python source code actually uses UTF-8 by default.
unicode_internal		Return the internal representation of the operand. Stateful codecs are not supported. Obsolète depuis la version 3.3: This representation is obsoleted by PEP 393.

7.2.4.2. Binary Transforms

The following codecs provide binary transforms: bytes-like object to bytes mappings. They are not supported by bytes.decode() (which only produces str output).

Codec	Aliases	Objectif	Encoder / decoder
base64_codec 1	base64, base_64	Convert operand to multiline MIME base64 (the result always includes a trailing '\n') Modifié dans la version 3.4: accepts any bytes-like object as input for encoding and decoding	base64.encodebytes() / base64.decodebytes()
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, quotedprintable, quoted_printable	Convert operand to MIME quoted printable	quopri.encode() with quotetabs=True / quopri.decode()
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

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

Nouveau dans la version 3.2: Restoration of the binary transforms.

Modifié dans la version 3.4: Restoration of the aliases for the binary transforms.

7.2.4.3. Text Transforms

The following codec provides a text transform: a str to str mapping. It is not supported by str.encode() (which only produces bytes output).

Codec	Aliases	Objectif
rot_13	rot13	Returns the Caesar-cypher encryption of the operand

Nouveau dans la version 3.2: Restoration of the rot_13 text transform.

Modifié dans la version 3.4: Restoration of the rot13 alias.

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 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.

Modifié dans la version 3.3: Support any error handler.

Modifié dans la 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.