bz2 —- 对 bzip2 压缩算法的支持

源代码：Lib/bz2.py

此模块提供了使用 bzip2 压缩算法压缩和解压数据的一套完整的接口。

bz2 模块包含：

• 用于读写压缩文件的 open() 函数和 BZ2File 类。

• 用于增量压缩和解压的 BZ2Compressor 和 BZ2Decompressor 类。

• 用于一次性压缩和解压的 compress() 和 decompress() 函数。

此模块中的所有类都能安全地从多个线程访问。

文件压缩和解压

• bz2.open(filename, mode='r', compresslevel=9, encoding=None, errors=None, newline=None)
• 以二进制或文本模式打开 bzip2 压缩文件，返回一个 file object。

和 BZ2File 的构造函数类似，filename 参数可以是一个实际的文件名（str 或 bytes 对象），或是已有的可供读取或写入的文件对象。

mode 参数可设为二进制模式的 'r'、'rb'、'w'、'wb'、'x'、'xb'、'a' 或 'ab'，或者文本模式的 'rt'、'wt'、'xt' 或 'at'。默认是 'rb'。

compresslevel 参数是 1 到 9 的整数，和 BZ2File 的构造函数一样。

For binary mode, this function is equivalent to the BZ2Fileconstructor: BZ2File(filename, mode, compresslevel=compresslevel). Inthis case, the encoding, errors and newline arguments must not beprovided.

For text mode, a BZ2File object is created, and wrapped in anio.TextIOWrapper instance with the specified encoding, errorhandling behavior, and line ending(s).

3.3 新版功能.

在 3.4 版更改: 添加了 'x' (仅创建) 模式。

在 3.6 版更改: 接受一个 类路径对象。

• class bz2.BZ2File(filename, mode='r', buffering=None, compresslevel=9)
• 用二进制模式打开 bzip2 压缩文件。

If filename is a str or bytes object, open the named filedirectly. Otherwise, filename should be a file object, which willbe used to read or write the compressed data.

The mode argument can be either 'r' for reading (default), 'w' foroverwriting, 'x' for exclusive creation, or 'a' for appending. Thesecan equivalently be given as 'rb', 'wb', 'xb' and 'ab'respectively.

If filename is a file object (rather than an actual file name), a mode of'w' does not truncate the file, and is instead equivalent to 'a'.

The buffering argument is ignored. Its use is deprecated since Python 3.0.

If mode is 'w' or 'a', compresslevel can be an integer between1 and 9 specifying the level of compression: 1 produces theleast compression, and 9 (default) produces the most compression.

If mode is 'r', the input file may be the concatenation of multiplecompressed streams.

BZ2File provides all of the members specified by theio.BufferedIOBase, except for detach() and truncate().Iteration and the with statement are supported.

BZ2File also provides the following method:

• peek([n])
• Return buffered data without advancing the file position. At least onebyte of data will be returned (unless at EOF). The exact number of bytesreturned is unspecified.

注解

While calling peek() does not change the file position ofthe BZ2File, it may change the position of the underlying fileobject (e.g. if the BZ2File was constructed by passing a fileobject for filename).

3.3 新版功能.

3.0 版后已移除: The keyword argument buffering was deprecated and is now ignored.

在 3.1 版更改: 支持了 with 语句。

在 3.3 版更改: The fileno(), readable(), seekable(), writable(),read1() and readinto() methods were added.

在 3.3 版更改: Support was added for filename being a file object instead of anactual filename.

在 3.3 版更改: The 'a' (append) mode was added, along with support for readingmulti-stream files.

在 3.4 版更改: 添加了 'x' (仅创建) 模式。

在 3.5 版更改: The read() method now accepts an argument ofNone.

在 3.6 版更改: 接受一个 类路径对象。

增量压缩和解压

• class bz2.BZ2Compressor(compresslevel=9)
• Create a new compressor object. This object may be used to compress dataincrementally. For one-shot compression, use the compress() functioninstead.

compresslevel, if given, must be an integer between 1 and 9. Thedefault is 9.

• compress(data)
• Provide data to the compressor object. Returns a chunk of compressed dataif possible, or an empty byte string otherwise.

When you have finished providing data to the compressor, call theflush() method to finish the compression process.

• flush()
• 结束压缩进程，返回内部缓冲中剩余的压缩完成的数据。

The compressor object may not be used after this method has been called.

• class bz2.BZ2Decompressor
• Create a new decompressor object. This object may be used to decompress dataincrementally. For one-shot compression, use the decompress() functioninstead.

注解

This class does not transparently handle inputs containing multiplecompressed streams, unlike decompress() and BZ2File. Ifyou need to decompress a multi-stream input with BZ2Decompressor,you must use a new decompressor for each stream.

• decompress(data, max_length=-1)
• Decompress data (a bytes-like object), returninguncompressed data as bytes. Some of data may be bufferedinternally, for use in later calls to decompress(). Thereturned data should be concatenated with the output of anyprevious calls to decompress().

If max_length is nonnegative, returns at most max_length_bytes of decompressed data. If this limit is reached and furtheroutput can be produced, the needs_input attribute willbe set to False. In this case, the next call todecompress() may provide _data as b'' to obtainmore of the output.

If all of the input data was decompressed and returned (eitherbecause this was less than max_length bytes, or becausemax_length was negative), the needs_input attributewill be set to True.

Attempting to decompress data after the end of stream is reachedraises an EOFError. Any data found after the end of thestream is ignored and saved in the unused_data attribute.

在 3.5 版更改: Added the max_length parameter.

• eof
• 若达到了数据流末尾标识符则为 True。

3.3 新版功能.

• unused_data
• 压缩数据流的末尾还有数据。

If this attribute is accessed before the end of the stream has beenreached, its value will be b''.

• needs_input
• False if the decompress() method can provide moredecompressed data before requiring new uncompressed input.

3.5 新版功能.

一次性压缩或解压

• bz2.compress(data, compresslevel=9)
• Compress data, a bytes-like object.

compresslevel, if given, must be an integer between 1 and 9. Thedefault is 9.

For incremental compression, use a BZ2Compressor instead.

• bz2.decompress(data)
• Decompress data, a bytes-like object.

If data is the concatenation of multiple compressed streams, decompressall of the streams.

For incremental decompression, use a BZ2Decompressor instead.

在 3.3 版更改: 支持了多数据流的输入。

用法示例

Below are some examples of typical usage of the bz2 module.

Using compress() and decompress() to demonstrate round-trip compression:

>>> import bz2

>>> data = b"""\
... Donec rhoncus quis sapien sit amet molestie. Fusce scelerisque vel augue
... nec ullamcorper. Nam rutrum pretium placerat. Aliquam vel tristique lorem,
... sit amet cursus ante. In interdum laoreet mi, sit amet ultrices purus
... pulvinar a. Nam gravida euismod magna, non varius justo tincidunt feugiat.
... Aliquam pharetra lacus non risus vehicula rutrum. Maecenas aliquam leo
... felis. Pellentesque semper nunc sit amet nibh ullamcorper, ac elementum
... dolor luctus. Curabitur lacinia mi ornare consectetur vestibulum."""

>>> c = bz2.compress(data)
>>> len(data) / len(c)  # Data compression ratio
1.513595166163142

>>> d = bz2.decompress(c)
>>> data == d  # Check equality to original object after round-trip
True

Using BZ2Compressor for incremental compression:

>>> import bz2

>>> def gen_data(chunks=10, chunksize=1000):
...     """Yield incremental blocks of chunksize bytes."""
...     for _ in range(chunks):
...         yield b"z" * chunksize
...
>>> comp = bz2.BZ2Compressor()
>>> out = b""
>>> for chunk in gen_data():
...     # Provide data to the compressor object
...     out = out + comp.compress(chunk)
...
>>> # Finish the compression process.  Call this once you have
>>> # finished providing data to the compressor.
>>> out = out + comp.flush()

The example above uses a very "nonrandom" stream of data(a stream of b"z" chunks). Random data tends to compress poorly,while ordered, repetitive data usually yields a high compression ratio.

Writing and reading a bzip2-compressed file in binary mode:

>>> import bz2

>>> data = b"""\
... Donec rhoncus quis sapien sit amet molestie. Fusce scelerisque vel augue
... nec ullamcorper. Nam rutrum pretium placerat. Aliquam vel tristique lorem,
... sit amet cursus ante. In interdum laoreet mi, sit amet ultrices purus
... pulvinar a. Nam gravida euismod magna, non varius justo tincidunt feugiat.
... Aliquam pharetra lacus non risus vehicula rutrum. Maecenas aliquam leo
... felis. Pellentesque semper nunc sit amet nibh ullamcorper, ac elementum
... dolor luctus. Curabitur lacinia mi ornare consectetur vestibulum."""

>>> with bz2.open("myfile.bz2", "wb") as f:
...     # Write compressed data to file
...     unused = f.write(data)

>>> with bz2.open("myfile.bz2", "rb") as f:
...     # Decompress data from file
...     content = f.read()

>>> content == data  # Check equality to original object after round-trip
True