12.3. "bz2" — Compression compatible with **bzip2**
***************************************************

2.3 版新加入.

This module provides a comprehensive interface for the bz2 compression
library. It implements a complete file interface, one-shot
(de)compression functions, and types for sequential (de)compression.

Here is a summary of the features offered by the bz2 module:

* "BZ2File" class implements a complete file interface, including
  "readline()", "readlines()", "writelines()", "seek()", etc;

* "BZ2File" class implements emulated "seek()" support;

* "BZ2File" class implements universal newline support;

* "BZ2File" class offers an optimized line iteration using the
  readahead algorithm borrowed from file objects;

* Sequential (de)compression supported by "BZ2Compressor" and
  "BZ2Decompressor" classes;

* One-shot (de)compression supported by "compress()" and
  "decompress()" functions;

* Thread safety uses individual locking mechanism.

備註: Handling of multi-stream bzip2 files is not supported.
  Modules such as bz2file let you overcome this.


12.3.1. (De)compression of files
================================

Handling of compressed files is offered by the "BZ2File" class.

class bz2.BZ2File(filename[, mode[, buffering[, compresslevel]]])

   Open a bz2 file. Mode can be either "'r'" or "'w'", for reading
   (default) or writing. When opened for writing, the file will be
   created if it doesn’t exist, and truncated otherwise. If
   *buffering* is given, "0" means unbuffered, and larger numbers
   specify the buffer size; the default is "0". If *compresslevel* is
   given, it must be a number between "1" and "9"; the default is "9".
   Add a "'U'" to mode to open the file for input in *universal
   newlines* mode. Any line ending in the input file will be seen as a
   "'\n'" in Python.  Also, a file so opened gains the attribute
   "newlines"; the value for this attribute is one of "None" (no
   newline read yet), "'\r'", "'\n'", "'\r\n'" or a tuple containing
   all the newline types seen. Universal newlines are available only
   when reading. Instances support iteration in the same way as normal
   "file" instances.

   "BZ2File" supports the "with" statement.

   2.7 版更變: Support for the "with" statement was added.

   備註: This class does not support input files containing multiple
     streams (such as those produced by the **pbzip2** tool). When
     reading such an input file, only the first stream will be
     accessible. If you require support for multi-stream files,
     consider using the third-party "bz2file" module (available from
     PyPI). This module provides a backport of Python 3.3’s "BZ2File"
     class, which does support multi-stream files.

   close()

      Close the file. Sets data attribute "closed" to true. A closed
      file cannot be used for further I/O operations. "close()" may be
      called more than once without error.

   read([size])

      Read at most *size* uncompressed bytes, returned as a string. If
      the *size* argument is negative or omitted, read until EOF is
      reached.

   readline([size])

      Return the next line from the file, as a string, retaining
      newline. A non-negative *size* argument limits the maximum
      number of bytes to return (an incomplete line may be returned
      then). Return an empty string at EOF.

   readlines([size])

      Return a list of lines read. The optional *size* argument, if
      given, is an approximate bound on the total number of bytes in
      the lines returned.

   xreadlines()

      For backward compatibility. "BZ2File" objects now include the
      performance optimizations previously implemented in the
      "xreadlines" module.

      2.3 版後已棄用: This exists only for compatibility with the
      method by this name on "file" objects, which is deprecated.  Use
      "for line in file" instead.

   seek(offset[, whence])

      Move to new file position. Argument *offset* is a byte count.
      Optional argument *whence* defaults to "os.SEEK_SET" or "0"
      (offset from start of file; offset should be ">= 0"); other
      values are "os.SEEK_CUR" or "1" (move relative to current
      position; offset can be positive or negative), and "os.SEEK_END"
      or "2" (move relative to end of file; offset is usually
      negative, although many platforms allow seeking beyond the end
      of a file).

      Note that seeking of bz2 files is emulated, and depending on the
      parameters the operation may be extremely slow.

   tell()

      Return the current file position, an integer (may be a long
      integer).

   write(data)

      Write string *data* to file. Note that due to buffering,
      "close()" may be needed before the file on disk reflects the
      data written.

   writelines(sequence_of_strings)

      Write the sequence of strings to the file. Note that newlines
      are not added. The sequence can be any iterable object producing
      strings. This is equivalent to calling write() for each string.


12.3.2. Sequential (de)compression
==================================

Sequential compression and decompression is done using the classes
"BZ2Compressor" and "BZ2Decompressor".

class bz2.BZ2Compressor([compresslevel])

   Create a new compressor object. This object may be used to compress
   data sequentially. If you want to compress data in one shot, use
   the "compress()" function instead. The *compresslevel* parameter,
   if given, must be a number between "1" and "9"; the default is "9".

   compress(data)

      Provide more data to the compressor object. It will return
      chunks of compressed data whenever possible. When you’ve
      finished providing data to compress, call the "flush()" method
      to finish the compression process, and return what is left in
      internal buffers.

   flush()

      Finish the compression process and return what is left in
      internal buffers. You must not use the compressor object after
      calling this method.

class bz2.BZ2Decompressor

   Create a new decompressor object. This object may be used to
   decompress data sequentially. If you want to decompress data in one
   shot, use the "decompress()" function instead.

   decompress(data)

      Provide more data to the decompressor object. It will return
      chunks of decompressed data whenever possible. If you try to
      decompress data after the end of stream is found, "EOFError"
      will be raised. If any data was found after the end of stream,
      it’ll be ignored and saved in "unused_data" attribute.


12.3.3. One-shot (de)compression
================================

One-shot compression and decompression is provided through the
"compress()" and "decompress()" functions.

bz2.compress(data[, compresslevel])

   Compress *data* in one shot. If you want to compress data
   sequentially, use an instance of "BZ2Compressor" instead. The
   *compresslevel* parameter, if given, must be a number between "1"
   and "9"; the default is "9".

bz2.decompress(data)

   Decompress *data* in one shot. If you want to decompress data
   sequentially, use an instance of "BZ2Decompressor" instead.
