7.8.codecs — Codec registry and base classes¶
This module defines base classes for standard Python codecs (encoders anddecoders) and provides access to the internal Python codec registry whichmanages the codec and error handling lookup process.
It defines the following functions:
codecs.encode(obj[,encoding[,errors]])¶Encodesobj using the codec registered forencoding. The defaultencoding is
'ascii'.Errors may be given to set the desired error handling scheme. Thedefault error handler is
'strict'meaning that encoding errors raiseValueError(or a more codec specific subclass, such asUnicodeEncodeError). Refer toCodec Base Classes for moreinformation on codec error handling.New in version 2.4.
codecs.decode(obj[,encoding[,errors]])¶Decodesobj using the codec registered forencoding. The defaultencoding is
'ascii'.Errors may be given to set the desired error handling scheme. Thedefault error handler is
'strict'meaning that decoding errors raiseValueError(or a more codec specific subclass, such asUnicodeDecodeError). Refer toCodec Base Classes for moreinformation on codec error handling.New in version 2.4.
codecs.register(search_function)¶Register a codec search function. Search functions are expected to take oneargument, the encoding name in all lower case letters, and return a
CodecInfoobject having the following attributes:nameThe name of the encoding;encodeThe stateless encoding function;decodeThe stateless decoding function;incrementalencoderAn incremental encoder class or factory function;incrementaldecoderAn incremental decoder class or factory function;streamwriterA stream writer class or factory function;streamreaderA stream reader class or factory function.
The various functions or classes take the following arguments:
encode anddecode: These must be functions or methods which have the sameinterface as the
encode()/decode()methods of Codecinstances (seeCodec Interface). The functions/methodsare expected to work in a stateless mode.incrementalencoder andincrementaldecoder: These have to be factoryfunctions providing the following interface:
factory(errors='strict')The factory functions must return objects providing the interfaces defined bythe base classes
IncrementalEncoderandIncrementalDecoder,respectively. Incremental codecs can maintain state.streamreader andstreamwriter: These have to be factory functions providingthe following interface:
factory(stream,errors='strict')The factory functions must return objects providing the interfaces defined bythe base classes
StreamReaderandStreamWriter, 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 characterreference (for encoding only)'backslashreplace': replace with backslashed escape sequences (forencoding only)
as well as any other error handling name defined via
register_error().In case a search function cannot find a given encoding, it should return
None.
codecs.lookup(encoding)¶Looks up the codec info in the Python codec registry and returns a
CodecInfoobject as defined above.Encodings are first looked up in the registry’s cache. If not found, the list ofregistered search functions is scanned. If no
CodecInfoobject isfound, aLookupErroris raised. Otherwise, theCodecInfoobjectis stored in the cache and returned to the caller.
To simplify access to the various codecs, the module provides these additionalfunctions which uselookup() for the codec lookup:
codecs.getencoder(encoding)¶Look up the codec for the given encoding and return its encoder function.
Raises a
LookupErrorin case the encoding cannot be found.
codecs.getdecoder(encoding)¶Look up the codec for the given encoding and return its decoder function.
Raises a
LookupErrorin case the encoding cannot be found.
codecs.getincrementalencoder(encoding)¶Look up the codec for the given encoding and return its incremental encoderclass or factory function.
Raises a
LookupErrorin case the encoding cannot be found or the codecdoesn’t support an incremental encoder.New in version 2.5.
codecs.getincrementaldecoder(encoding)¶Look up the codec for the given encoding and return its incremental decoderclass or factory function.
Raises a
LookupErrorin case the encoding cannot be found or the codecdoesn’t support an incremental decoder.New in version 2.5.
codecs.getreader(encoding)¶Look up the codec for the given encoding and return its StreamReader class orfactory function.
Raises a
LookupErrorin case the encoding cannot be found.
codecs.getwriter(encoding)¶Look up the codec for the given encoding and return its StreamWriter class orfactory function.
Raises a
LookupErrorin case the encoding cannot be found.
codecs.register_error(name,error_handler)¶Register the error handling functionerror_handler under the namename.error_handler will be called during encoding and decoding in case of an error,whenname is specified as the errors parameter.
For encodingerror_handler will be called with a
UnicodeEncodeErrorinstance, which contains information about the location of the error. The errorhandler must either raise this or a different exception or return a tuple with areplacement for the unencodable part of the input and a position where encodingshould continue. The encoder will encode the replacement and continue encodingthe original input at the specified position. Negative position values will betreated as being relative to the end of the input string. If the resultingposition is out of bound anIndexErrorwill be raised.Decoding and translating works similar, except
UnicodeDecodeErrororUnicodeTranslateErrorwill be passed to the handler and that thereplacement from the error handler will be put into the output directly.
codecs.lookup_error(name)¶Return the error handler previously registered under the namename.
Raises a
LookupErrorin case the handler cannot be found.
codecs.strict_errors(exception)¶Implements the
stricterror handling: each encoding or decoding errorraises aUnicodeError.
codecs.replace_errors(exception)¶Implements the
replaceerror handling: malformed data is replaced with asuitable replacement character such as'?'in bytestrings and'\ufffd'in Unicode strings.
codecs.ignore_errors(exception)¶Implements the
ignoreerror handling: malformed data is ignored andencoding or decoding is continued without further notice.
codecs.xmlcharrefreplace_errors(exception)¶Implements the
xmlcharrefreplaceerror handling (for encoding only): theunencodable character is replaced by an appropriate XML character reference.
codecs.backslashreplace_errors(exception)¶Implements the
backslashreplaceerror handling (for encoding only): theunencodable character is replaced by a backslashed escape sequence.
To simplify working with encoded files or stream, the module also defines theseutility functions:
codecs.open(filename,mode[,encoding[,errors[,buffering]]])¶Open an encoded file using the givenmode and return a wrapped versionproviding transparent encoding/decoding. The default file mode is
'r'meaning to open the file in read mode.Note
The wrapped version will only accept the object format defined by the codecs,i.e. Unicode objects for most built-in codecs. Output is also codec-dependentand will usually be Unicode as well.
Note
Files are always opened in binary mode, even if no binary mode wasspecified. This is done to avoid data loss due to encodings using 8-bitvalues. This means that no automatic conversion of
'\n'is doneon 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 aValueErrorto be raised in case an encoding error occurs.buffering has the same meaning as for the built-in
open()function. Itdefaults to line buffered.
codecs.EncodedFile(file,input[,output[,errors]])¶Return a wrapped version of file which provides transparent encodingtranslation.
Strings written to the wrapped file are interpreted according to the giveninput encoding and then written to the original file as strings using theoutput encoding. The intermediate encoding will usually be Unicode but dependson the specified codecs.
Ifoutput is not given, it defaults toinput.
errors may be given to define the error handling. It defaults to
'strict',which causesValueErrorto be raised in case an encoding error occurs.
codecs.iterencode(iterable,encoding[,errors])¶Uses an incremental encoder to iteratively encode the input provided byiterable. This function is agenerator.errors (as well as anyother keyword argument) is passed through to the incremental encoder.
New in version 2.5.
codecs.iterdecode(iterable,encoding[,errors])¶Uses an incremental decoder to iteratively decode the input provided byiterable. This function is agenerator.errors (as well as anyother keyword argument) is passed through to the incremental decoder.
New in version 2.5.
The module also provides the following constants which are useful for readingand 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 thestream or file and in UTF-8 as a Unicode signature.
BOM_UTF16is eitherBOM_UTF16_BEorBOM_UTF16_LEdepending on the platform’snative byte order,BOMis an alias forBOM_UTF16,BOM_LEforBOM_UTF16_LEandBOM_BEforBOM_UTF16_BE. The others represent the BOM in UTF-8 and UTF-32encodings.
7.8.1.Codec Base Classes¶
Thecodecs module defines a set of base classes which define theinterface and can also be used to easily write your own codecs for use inPython.
Each codec has to define four interfaces to make it usable as codec in Python:stateless encoder, stateless decoder, stream reader and stream writer. Thestream reader and writers typically reuse the stateless encoder/decoder toimplement the file protocols.
TheCodec class defines the interface for stateless encoders/decoders.
To simplify and standardize error handling, theencode() anddecode() methods may implement different error handling schemes byproviding theerrors string argument. The following string values are definedand implemented by all standard Python codecs:
Value | Meaning |
|---|---|
| Raise |
| Ignore the character and continue with thenext. |
| Replace with a suitable replacementcharacter; Python will use the officialU+FFFD REPLACEMENT CHARACTER for the built-inUnicode codecs on decoding and ‘?’ onencoding. |
| Replace with the appropriate XML characterreference (only for encoding). |
| Replace with backslashed escape sequences(only for encoding). |
The set of allowed values can be extended viaregister_error().
7.8.1.1.Codec Objects¶
TheCodec class defines these methods which also define the functioninterfaces of the stateless encoder and decoder:
Codec.encode(input[,errors])¶Encodes the objectinput and returns a tuple (output object, length consumed).While codecs are not restricted to use with Unicode, in a Unicode context,encoding converts a Unicode object to a plain string using a particularcharacter set encoding (e.g.,
cp1252oriso-8859-1).errors defines the error handling to apply. It defaults to
'strict'handling.The method may not store state in the
Codecinstance. UseStreamWriterfor codecs which have to keep state in order to makeencoding efficient.The encoder must be able to handle zero length input and return an empty objectof the output object type in this situation.
Codec.decode(input[,errors])¶Decodes the objectinput and returns a tuple (output object, length consumed).In a Unicode context, decoding converts a plain string encoded using aparticular character set encoding to a Unicode object.
input must be an object which provides the
bf_getreadbufbuffer slot.Python strings, buffer objects and memory mapped files are examples of objectsproviding this slot.errors defines the error handling to apply. It defaults to
'strict'handling.The method may not store state in the
Codecinstance. UseStreamReaderfor codecs which have to keep state in order to makedecoding efficient.The decoder must be able to handle zero length input and return an empty objectof the output object type in this situation.
TheIncrementalEncoder andIncrementalDecoder classes providethe basic interface for incremental encoding and decoding. Encoding/decoding theinput isn’t done with one call to the stateless encoder/decoder function, butwith multiple calls to theencode()/decode() method ofthe incremental encoder/decoder. The incremental encoder/decoder keeps track ofthe encoding/decoding process during method calls.
The joined output of calls to theencode()/decode() method isthe same as if all the single inputs were joined into one, and this input wasencoded/decoded with the stateless encoder/decoder.
7.8.1.2.IncrementalEncoder Objects¶
New in version 2.5.
TheIncrementalEncoder class is used for encoding an input in multiplesteps. It defines the following methods which every incremental encoder mustdefine in order to be compatible with the Python codec registry.
- class
codecs.IncrementalEncoder([errors])¶ Constructor for an
IncrementalEncoderinstance.All incremental encoders must provide this constructor interface. They are freeto add additional keyword arguments, but only the ones defined here are used bythe Python codec registry.
The
IncrementalEncodermay implement different error handling schemesby providing theerrors keyword argument. These parameters are predefined:'strict'RaiseValueError(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.
Theerrors argument will be assigned to an attribute of the same name.Assigning to this attribute makes it possible to switch between different errorhandling strategies during the lifetime of the
IncrementalEncoderobject.The set of allowed values for theerrors argument can be extended with
register_error().encode(object[,final])¶Encodesobject (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.
7.8.1.3.IncrementalDecoder Objects¶
TheIncrementalDecoder class is used for decoding an input in multiplesteps. It defines the following methods which every incremental decoder mustdefine in order to be compatible with the Python codec registry.
- class
codecs.IncrementalDecoder([errors])¶ Constructor for an
IncrementalDecoderinstance.All incremental decoders must provide this constructor interface. They are freeto add additional keyword arguments, but only the ones defined here are used bythe Python codec registry.
The
IncrementalDecodermay implement different error handling schemesby providing theerrors keyword argument. These parameters are predefined:'strict'RaiseValueError(or a subclass); this is the default.'ignore'Ignore the character and continue with the next.'replace'Replace with a suitable replacement character.
Theerrors argument will be assigned to an attribute of the same name.Assigning to this attribute makes it possible to switch between different errorhandling strategies during the lifetime of the
IncrementalDecoderobject.The set of allowed values for theerrors argument can be extended with
register_error().decode(object[,final])¶Decodesobject (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). Iffinal istrue the decoder must decode the input completely and must flush allbuffers. If this isn’t possible (e.g. because of incomplete byte sequencesat the end of the input) it must initiate error handling just like in thestateless case (which might raise an exception).
reset()¶Reset the decoder to the initial state.
TheStreamWriter andStreamReader classes provide genericworking interfaces which can be used to implement new encoding submodules veryeasily. Seeencodings.utf_8 for an example of how this is done.
7.8.1.4.StreamWriter Objects¶
TheStreamWriter class is a subclass ofCodec and defines thefollowing methods which every stream writer must define in order to becompatible with the Python codec registry.
- class
codecs.StreamWriter(stream[,errors])¶ Constructor for a
StreamWriterinstance.All stream writers must provide this constructor interface. They are free to addadditional keyword arguments, but only the ones defined here are used by thePython codec registry.
stream must be a file-like object open for writing binary data.
The
StreamWritermay implement different error handling schemes byproviding theerrors keyword argument. These parameters are predefined:'strict'RaiseValueError(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.
Theerrors argument will be assigned to an attribute of the same name.Assigning to this attribute makes it possible to switch between different errorhandling strategies during the lifetime of the
StreamWriterobject.The set of allowed values for theerrors 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 reusingthe
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 intoa clean state that allows appending of new fresh data without having torescan the whole stream to recover state.
In addition to the above methods, theStreamWriter must also inheritall other methods and attributes from the underlying stream.
7.8.1.5.StreamReader Objects¶
TheStreamReader class is a subclass ofCodec and defines thefollowing methods which every stream reader must define in order to becompatible with the Python codec registry.
- class
codecs.StreamReader(stream[,errors])¶ Constructor for a
StreamReaderinstance.All stream readers must provide this constructor interface. They are free to addadditional keyword arguments, but only the ones defined here are used by thePython codec registry.
stream must be a file-like object open for reading (binary) data.
The
StreamReadermay implement different error handling schemes byproviding theerrors keyword argument. These parameters are defined:'strict'RaiseValueError(or a subclass); this is the default.'ignore'Ignore the character and continue with the next.'replace'Replace with a suitable replacement character.
Theerrors argument will be assigned to an attribute of the same name.Assigning to this attribute makes it possible to switch between different errorhandling strategies during the lifetime of the
StreamReaderobject.The set of allowed values for theerrors 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 thestream.
read()will never return more thanchars characters, butit might return less, if there are not enough characters available.size indicates the approximate maximum number of bytes to read from thestream for decoding purposes. The decoder can modify this setting asappropriate. The default value -1 indicates to read and decode as much aspossible.size is intended to prevent having to decode huge files inone step.
firstline indicates that it would be sufficient to only return the firstline, if there are decoding errors on later lines.
The method should use a greedy read strategy meaning that it should readas much data as is allowed within the definition of the encoding and thegiven size, e.g. if optional encoding endings or state markers areavailable on the stream, these should be read too.
Changed in version 2.4:chars argument added.
Changed in version 2.4.2:firstline argument added.
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.Ifkeepends is false line-endings will be stripped from the linesreturned.
Changed in version 2.4:keepends argument added.
readlines([sizehint[,keepends]])¶Read all lines available on the input stream and return them as a list oflines.
Line-endings are implemented using the codec’s decoder method and areincluded in the list entries ifkeepends is true.
sizehint, if given, is passed as thesize 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 isprimarily intended to be able to recover from decoding errors.
In addition to the above methods, theStreamReader must also inheritall other methods and attributes from the underlying stream.
The next two base classes are included for convenience. They are not needed bythe codec registry, but may provide useful in practice.
7.8.1.6.StreamReaderWriter Objects¶
TheStreamReaderWriter allows wrapping streams which work in both readand write modes.
The design is such that one can use the factory functions returned by thelookup() function to construct the instance.
- class
codecs.StreamReaderWriter(stream,Reader,Writer,errors)¶ Creates a
StreamReaderWriterinstance.stream must be a file-likeobject.Reader andWriter must be factory functions or classes providing theStreamReaderandStreamWriterinterface resp. Error handlingis done in the same way as defined for the stream readers and writers.
StreamReaderWriter instances define the combined interfaces ofStreamReader andStreamWriter classes. They inherit all othermethods and attributes from the underlying stream.
7.8.1.7.StreamRecoder Objects¶
TheStreamRecoder provide a frontend - backend view of encoding datawhich is sometimes useful when dealing with different encoding environments.
The design is such that one can use the factory functions returned by thelookup() function to construct the instance.
- class
codecs.StreamRecoder(stream,encode,decode,Reader,Writer,errors)¶ Creates a
StreamRecoderinstance which implements a two-way conversion:encode anddecode work on the frontend (the input toread()and outputofwrite()) whileReader andWriter work on the backend (reading andwriting to the stream).You can use these objects to do transparent direct recodings from e.g. Latin-1to UTF-8 and back.
stream must be a file-like object.
encode,decode must adhere to the
Codecinterface.Reader,Writer must be factory functions or classes providing objects of theStreamReaderandStreamWriterinterface respectively.encode anddecode are needed for the frontend translation,Reader andWriter for the backend translation. The intermediate format used isdetermined by the two sets of codecs, e.g. the Unicode codecs will use Unicodeas the intermediate encoding.
Error handling is done in the same way as defined for the stream readers andwriters.
StreamRecoder instances define the combined interfaces ofStreamReader andStreamWriter classes. They inherit all othermethods and attributes from the underlying stream.
7.8.2.Encodings and Unicode¶
Unicode strings are stored internally as sequences of code points (to be preciseasPy_UNICODE arrays). Depending on the way Python is compiled (eithervia--enable-unicode=ucs2 or--enable-unicode=ucs4, with theformer being the default)Py_UNICODE is either a 16-bit or 32-bit datatype. Once a Unicode object is used outside of CPU and memory, CPU endiannessand how these arrays are stored as bytes become an issue. Transforming aunicode object into a sequence of bytes is called encoding and recreating theunicode object from the sequence of bytes is known as decoding. There are manydifferent methods for how this transformation can be done (these methods arealso called encodings). The simplest method is to map the code points 0–255 tothe bytes0x0–0xff. This means that a unicode object that containscode points aboveU+00FF can’t be encoded with this method (which is called'latin-1' or'iso-8859-1').unicode.encode() will raise aUnicodeEncodeError that looks like this:UnicodeEncodeError:'latin-1'codeccan'tencodecharacteru'\u1234'inposition3:ordinalnotinrange(256).
There’s another group of encodings (the so called charmap encodings) that choosea different subset of all unicode code points and how these code points aremapped to the bytes0x0–0xff. To see how this is done simply opene.g.encodings/cp1252.py (which is an encoding that is used primarily onWindows). There’s a string constant with 256 characters that shows you whichcharacter is mapped to which byte value.
All of these encodings can only encode 256 of the 1114112 code pointsdefined in unicode. A simple and straightforward way that can store each Unicodecode point, is to store each code point as four consecutive bytes. There are twopossibilities: store the bytes in big endian or in little endian order. Thesetwo encodings are calledUTF-32-BE andUTF-32-LE respectively. Theirdisadvantage is that if e.g. you useUTF-32-BE on a little endian machine youwill always have to swap bytes on encoding and decoding.UTF-32 avoids thisproblem: bytes will always be in natural endianness. When these bytes are readby a CPU with a different endianness, then bytes have to be swapped though. Tobe able to detect the endianness of aUTF-16 orUTF-32 byte sequence,there’s the so called BOM (“Byte Order Mark”). This is the Unicode characterU+FEFF. This character can be prepended to everyUTF-16 orUTF-32byte sequence. The byte swapped version of this character (0xFFFE) is anillegal character that may not appear in a Unicode text. So when thefirst character in anUTF-16 orUTF-32 byte sequenceappears to be aU+FFFE the bytes have to be swapped on decoding.Unfortunately the characterU+FEFF had a second purpose asaZEROWIDTHNO-BREAKSPACE: a character that has no width and doesn’t allowa word to be split. It can e.g. be used to give hints to a ligature algorithm.With Unicode 4.0 usingU+FEFF as aZEROWIDTHNO-BREAKSPACE has beendeprecated (withU+2060 (WORDJOINER) assuming this role). NeverthelessUnicode software still must be able to handleU+FEFF in both roles: as a BOMit’s a device to determine the storage layout of the encoded bytes, and vanishesonce the byte sequence has been decoded into a Unicode string; as aZEROWIDTHNO-BREAKSPACE 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 Unicodecharacters: UTF-8. UTF-8 is an 8-bit encoding, which means there are no issueswith byte order in UTF-8. Each byte in a UTF-8 byte sequence consists of twoparts: marker bits (the most significant bits) and payload bits. The marker bitsare a sequence of zero to four1 bits followed by a0 bit. Unicode characters areencoded like this (with x being payload bits, which when concatenated give theUnicode character):
Range | Encoding |
|---|---|
| 0xxxxxxx |
| 110xxxxx 10xxxxxx |
| 1110xxxx 10xxxxxx 10xxxxxx |
| 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 anyU+FEFF character inthe decoded Unicode string (even if it’s the first character) is treated as aZEROWIDTHNO-BREAKSPACE.
Without external information it’s impossible to reliably determine whichencoding was used for encoding a Unicode string. Each charmap encoding candecode any random byte sequence. However that’s not possible with UTF-8, asUTF-8 byte sequences have a structure that doesn’t allow arbitrary bytesequences. To increase the reliability with which a UTF-8 encoding can bedetected, Microsoft invented a variant of UTF-8 (that Python 2.5 calls"utf-8-sig") for its Notepad program: Before any of the Unicode charactersis written to the file, a UTF-8 encoded BOM (which looks like this as a bytesequence:0xef,0xbb,0xbf) is written. As it’s rather improbablethat any charmap encoded file starts with these byte values (which would e.g.map to
LATIN SMALL LETTER I WITH DIAERESISRIGHT-POINTING DOUBLE ANGLE QUOTATION MARKINVERTED QUESTION MARK
in iso-8859-1), this increases the probability that autf-8-sig encoding can becorrectly guessed from the byte sequence. So here the BOM is not used to be ableto determine the byte order used for generating the byte sequence, but as asignature that helps in guessing the encoding. On encoding the utf-8-sig codecwill write0xef,0xbb,0xbf as the first three bytes to the file. Ondecodingutf-8-sig will skip those three bytes if they appear as the firstthree bytes in the file. In UTF-8, the use of the BOM is discouraged andshould generally be avoided.
7.8.3.Standard Encodings¶
Python comes with a number of codecs built-in, either implemented as C functionsor with dictionaries as mapping tables. The following table lists the codecs byname, together with a few common aliases, and the languages for which theencoding is likely used. Neither the list of aliases nor the list of languagesis meant to be exhaustive. Notice that spelling alternatives that only differ incase 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.
Many of the character sets support the same languages. They vary in individualcharacters (e.g. whether the EURO SIGN is supported or not), and in theassignment of characters to code positions. For the European languages inparticular, 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 |
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 |
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, SimplifiedChinese, 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_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_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 | 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 (BMP only) |
utf_16_le | UTF-16LE | all languages (BMP only) |
utf_7 | U7, unicode-1-1-utf-7 | all languages |
utf_8 | U8, UTF, utf8 | all languages |
utf_8_sig | all languages |
7.8.4.Python Specific Encodings¶
A number of predefined codecs are specific to Python, so their codec names haveno meaning outside Python. These are listed in the tables below based on theexpected input and output types (note that while text encodings are the mostcommon use case for codecs, the underlying codec infrastructure supportsarbitrary data transforms rather than just text encodings). For asymmetriccodecs, the stated purpose describes the encoding direction.
The following codecs provide unicode-to-str encoding1 andstr-to-unicode decoding2, similar to the Unicode textencodings.
Codec | Aliases | Purpose |
|---|---|---|
idna | ImplementsRFC 3490,see also | |
mbcs | dbcs | Windows only: Encodeoperand according to theANSI codepage (CP_ACP) |
palmos | Encoding of PalmOS 3.5 | |
punycode | ImplementsRFC 3492 | |
raw_unicode_escape | Produce a string that issuitable as raw Unicodeliteral in Python sourcecode | |
rot_13 | rot13 | Returns the Caesar-cypherencryption of the operand |
undefined | Raise an exception forall conversions. Can beused as the systemencoding if no automaticcoercion betweenbyte and Unicode stringsis desired. | |
unicode_escape | Produce a string that issuitable as Unicodeliteral in Python sourcecode | |
unicode_internal | Return the internalrepresentation of theoperand |
New in version 2.3:Theidna andpunycode encodings.
The following codecs provide str-to-str encoding and decoding2.
Codec | Aliases | Purpose | Encoder/decoder |
|---|---|---|---|
base64_codec | base64, base-64 | Convert operand tomultiline MIME base64 (theresult always includes atrailing | |
bz2_codec | bz2 | Compress the operandusing bz2 | |
hex_codec | hex | Convert operand tohexadecimalrepresentation, with twodigits per byte | |
quopri_codec | quopri, quoted-printable,quotedprintable | Convert operand to MIMEquoted printable |
|
string_escape | Produce a string that issuitable as stringliteral in Python sourcecode | ||
uu_codec | uu | Convert the operand usinguuencode | |
zlib_codec | zip, zlib | Compress the operandusing gzip |
- 1
str objects are also accepted as input in place of unicodeobjects. They are implicitly converted to unicode by decoding them usingthe default encoding. If this conversion fails, it may lead to encodingoperations raising
UnicodeDecodeError.- 2(1,2)
unicode objects are also accepted as input in place of strobjects. They are implicitly converted to str by encoding them using thedefault encoding. If this conversion fails, it may lead to decodingoperations raising
UnicodeEncodeError.
7.8.5.encodings.idna — Internationalized Domain Names in Applications¶
New in version 2.3.
This module implementsRFC 3490 (Internationalized Domain Names inApplications) andRFC 3492 (Nameprep: A Stringprep Profile forInternationalized Domain Names (IDN)). It builds upon thepunycode encodingandstringprep.
These RFCs together define a protocol to support non-ASCII characters in domainnames. A domain name containing non-ASCII characters (such aswww.Alliancefrançaise.nu) is converted into an ASCII-compatible encoding(ACE, such aswww.xn--alliancefranaise-npb.nu). The ACE form of the domainname is then used in all places where arbitrary characters are not allowed bythe protocol, such as DNS queries, HTTPHost fields, and soon. This conversion is carried out in the application; if possible invisible tothe user: The application should transparently convert Unicode domain labels toIDNA on the wire, and convert back ACE labels to Unicode before presenting themto the user.
Python supports this conversion in several ways: theidna codec performsconversion between Unicode and ACE, separating an input string into labelsbased on the separator characters defined insection 3.1 (1) ofRFC 3490and converting each label to ACE as required, and conversely separating an inputbyte string into labels based on the. separator and converting any ACElabels found into unicode. Furthermore, thesocket moduletransparently converts Unicode host names to ACE, so that applications need notbe concerned about converting host names themselves when they pass them to thesocket module. On top of that, modules that have host names as functionparameters, such ashttplib andftplib, accept Unicode host names(httplib then also transparently sends an IDNA hostname in theHost field if it sends that field at all).
When receiving host names from the wire (such as in reverse name lookup), noautomatic conversion to Unicode is performed: Applications wishing to presentsuch host names to the user should decode them to Unicode.
The moduleencodings.idna also implements the nameprep procedure, whichperforms certain normalizations on host names, to achieve case-insensitivity ofinternational domain names, and to unify similar characters. The nameprepfunctions can be used directly if desired.
encodings.idna.nameprep(label)¶Return the nameprepped version oflabel. The implementation currently assumesquery strings, so
AllowUnassignedis true.
7.8.6.encodings.utf_8_sig — UTF-8 codec with BOM signature¶
New in version 2.5.
This module implements a variant of the UTF-8 codec: On encoding a UTF-8 encodedBOM will be prepended to the UTF-8 encoded bytes. For the stateful encoder thisis only done once (on the first write to the byte stream). For decoding anoptional UTF-8 encoded BOM at the start of the data will be skipped.