6.6.1.1. 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.

6.6.1.2. 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.

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().

6.6.1.3. 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.

6.6.1.4. 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.

6.6.1.5. 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 readline() 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.

6.6.1.6. 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.

6.6.1.7. 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.