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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)
|
| 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.
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.
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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