"zlib" --- 与 **gzip** 兼容的压缩
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======================================================================

For applications that require data compression, the functions in this
module allow compression and decompression, using the zlib library.
The zlib library has its own home page at https://www.zlib.net.
There are known incompatibilities between the Python module and
versions of the zlib library earlier than 1.1.3; 1.1.3 has a security
vulnerability, so we recommend using 1.1.4 or later.

zlib 的函数有很多选项，一般需要按特定顺序使用。本文档没有覆盖全部的用
法。更多详细信息请于 http://www.zlib.net/manual.html 参阅官方手册。

要读写 ".gz" 格式的文件，请参考 "gzip" 模块。

此模块中可用的异常和函数如下：

exception zlib.error

   在压缩或解压缩过程中发生错误时的异常。

zlib.adler32(data[, value])

   计算 *data* 的 Adler-32 校验值。(Adler-32 校验的可靠性与 CRC32 基本
   相当，但比计算 CRC32 更高效。) 计算的结果是一个 32 位的整数。参数
   *value* 是校验时的起始值，其默认值为 1。借助参数 *value* 可为分段的
   输入计算校验值。此算法没有加密强度，不应用于身份验证和数字签名。此
   算法的目的仅为验证数据的正确性，不适合作为通用散列算法。

   在 3.0 版更改: 返回值永远是无符号数。要在所有的 Python 版本和平台上
   获得相同的值，请使用 "adler32(data) & 0xffffffff"。

zlib.compress(data, /, level=-1)

   压缩 *data* 中的字节，返回含有已压缩内容的 bytes 对象。参数 *level*
   为整数，可取值为 "0" 到 "9" 或 "-1"，用于指定压缩等级。"1"
   (Z_BEST_SPEED) 表示最快速度和最低压缩率，"9" (Z_BEST_COMPRESSION)
   表示最慢速度和最高压缩率。"0" (Z_NO_COMPRESSION) 表示不压缩。参数默
   认值为 "-1" (Z_DEFAULT_COMPRESSION)。Z_DEFAULT_COMPRESSION 是速度和
   压缩率之间的平衡 (一般相当于设压缩等级为 6)。函数发生错误时抛出
   "error" 异常。

   在 3.6 版更改: 现在，*level* 可作为关键字参数。

zlib.compressobj(level=-1, method=DEFLATED, wbits=MAX_WBITS, memLevel=DEF_MEM_LEVEL, strategy=Z_DEFAULT_STRATEGY[, zdict])

   返回一个 压缩对象，用来压缩内存中难以容下的数据流。

   参数 *level* 为压缩等级，是整数，可取值为 "0" 到 "9" 或 "-1"。"1"
   (Z_BEST_SPEED) 表示最快速度和最低压缩率，"9" (Z_BEST_COMPRESSION)
   表示最慢速度和最高压缩率。"0" (Z_NO_COMPRESSION) 表示不压缩。参数默
   认值为 "-1" (Z_DEFAULT_COMPRESSION)。Z_DEFAULT_COMPRESSION 是速度和
   压缩率之间的平衡 (一般相当于设压缩等级为 6)。

   *method* 表示压缩算法。现在只支持 "DEFLATED" 这个算法。

   参数 *wbits* 指定压缩数据时所使用的历史缓冲区的大小 (窗口大小)，并
   指定压缩输出是否包含头部或尾部。参数的默认值是 "15" (MAX_WBITS)。参
   数的值分为几个范围：

   * +9 到 +15：窗口大小以 2 为底的对数。即这些值对应着 512 到 32768
     的窗口大小。更大的值会提供更好的压缩，同时内存开销也会更大。压缩
     输出会包含 zlib 特定格式的头部和尾部。

   * −9 到 −15：绝对值为窗口大小以 2 为底的对数。压缩输出仅包含压缩数
     据，没有头部和尾部。

   * +25 到 +31 = 16 + (9 到 15)：后 4 个比特位为窗口大小以 2 为底的对
     数。压缩输出包含一个基本的 **gzip** 头部，并以校验和为尾部。

   参数 *memLevel* 指定内部压缩操作时所占用内存大小。参数取 "1" 到 "9"
   。更大的值占用更多的内存，同时速度也更快输出也更小。

   参数 *strategy* 用于调节压缩算法。可取值为  "Z_DEFAULT_STRATEGY"、
   "Z_FILTERED"、"Z_HUFFMAN_ONLY"、"Z_RLE" (zlib 1.2.0.1) 或 "Z_FIXED"
   (zlib 1.2.2.2)。

   参数 *zdict* 指定预定义的压缩字典。它是一个字节序列 (如 "bytes" 对
   象)，其中包含用户认为要压缩的数据中可能频繁出现的子序列。频率高的子
   序列应当放在字典的尾部。

   在 3.3 版更改: 添加关键字参数 *zdict*。

zlib.crc32(data[, value])

   计算 *data* 的 CRC (循环冗余校验) 值。计算的结果是一个 32 位的整数
   。参数 *value* 是校验时的起始值，其默认值为 0。借助参数 *value* 可
   为分段的输入计算校验值。此算法没有加密强度，不应用于身份验证和数字
   签名。此算法的目的仅为验证数据的正确性，不适合作为通用散列算法。

   在 3.0 版更改: 返回值永远是无符号数。要在所有的 Python 版本和平台上
   获得相同的值，请使用 "crc32(data) & 0xffffffff"。

zlib.decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)

   解压 *data* 中的字节，返回含有已解压内容的 bytes 对象。参数 *wbits*
   取决于 *data* 的格式，具体参见下边的说明。*bufsize* 为输出缓冲区的
   起始大小。函数发生错误时抛出 "error" 异常。

   The *wbits* parameter controls the size of the history buffer (or
   "window size"), and what header and trailer format is expected. It
   is similar to the parameter for "compressobj()", but accepts more
   ranges of values:

   * +8 to +15: The base-two logarithm of the window size.  The input
     must include a zlib header and trailer.

   * 0: Automatically determine the window size from the zlib header.
     Only supported since zlib 1.2.3.5.

   * −8 to −15: Uses the absolute value of *wbits* as the window size
     logarithm.  The input must be a raw stream with no header or
     trailer.

   * +24 to +31 = 16 + (8 to 15): Uses the low 4 bits of the value as
     the window size logarithm.  The input must include a gzip header
     and trailer.

   * +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value as
     the window size logarithm, and automatically accepts either the
     zlib or gzip format.

   When decompressing a stream, the window size must not be smaller
   than the size originally used to compress the stream; using a too-
   small value may result in an "error" exception. The default *wbits*
   value corresponds to the largest window size and requires a zlib
   header and trailer to be included.

   *bufsize* is the initial size of the buffer used to hold
   decompressed data.  If more space is required, the buffer size will
   be increased as needed, so you don't have to get this value exactly
   right; tuning it will only save a few calls to "malloc()".

   在 3.6 版更改: *wbits* and *bufsize* can be used as keyword
   arguments.

zlib.decompressobj(wbits=MAX_WBITS[, zdict])

   Returns a decompression object, to be used for decompressing data
   streams that won't fit into memory at once.

   The *wbits* parameter controls the size of the history buffer (or
   the "window size"), and what header and trailer format is expected.
   It has the same meaning as described for decompress().

   The *zdict* parameter specifies a predefined compression
   dictionary. If provided, this must be the same dictionary as was
   used by the compressor that produced the data that is to be
   decompressed.

   注解:

     If *zdict* is a mutable object (such as a "bytearray"), you must
     not modify its contents between the call to "decompressobj()" and
     the first call to the decompressor's "decompress()" method.

   在 3.3 版更改: Added the *zdict* parameter.

压缩对象支持以下方法：

Compress.compress(data)

   压缩 *data* 并返回 bytes 对象，这个对象含有 *data* 的部分或全部内容
   的已压缩数据。所得的对象必须拼接在上一次调用 "compress()" 方法所得
   数据的后面。缓冲区中可能留存部分输入以供下一次调用。

Compress.flush([mode])

   压缩所有缓冲区的数据并返回已压缩的数据。参数 *mode* 可以传入的常量
   为："Z_NO_FLUSH"、"Z_PARTIAL_FLUSH"、"Z_SYNC_FLUSH"、"Z_FULL_FLUSH"
   、"Z_BLOCK" (zlib 1.2.3.4) 或 "Z_FINISH"。默认值为 "Z_FINISH"。
   "Z_FINISH" 关闭已压缩数据流并不允许再压缩其他数据，"Z_FINISH"  以外
   的值皆允许这个对象继续压缩数据。调用 "flush()" 方法并将 *mode* 设为
   "Z_FINISH" 后会无法再次调用 "compress()"，此时只能删除这个对象。

Compress.copy()

   返回此压缩对象的一个拷贝。它可以用来高效压缩一系列拥有相同前缀的数
   据。

在 3.8 版更改: Added "copy.copy()" and "copy.deepcopy()" support to
compression objects.

解压缩对象支持以下方法：

Decompress.unused_data

   A bytes object which contains any bytes past the end of the
   compressed data. That is, this remains "b""" until the last byte
   that contains compression data is available.  If the whole
   bytestring turned out to contain compressed data, this is "b""", an
   empty bytes object.

Decompress.unconsumed_tail

   A bytes object that contains any data that was not consumed by the
   last "decompress()" call because it exceeded the limit for the
   uncompressed data buffer.  This data has not yet been seen by the
   zlib machinery, so you must feed it (possibly with further data
   concatenated to it) back to a subsequent "decompress()" method call
   in order to get correct output.

Decompress.eof

   A boolean indicating whether the end of the compressed data stream
   has been reached.

   This makes it possible to distinguish between a properly-formed
   compressed stream, and an incomplete or truncated one.

   3.3 新版功能.

Decompress.decompress(data, max_length=0)

   Decompress *data*, returning a bytes object containing the
   uncompressed data corresponding to at least part of the data in
   *string*.  This data should be concatenated to the output produced
   by any preceding calls to the "decompress()" method.  Some of the
   input data may be preserved in internal buffers for later
   processing.

   If the optional parameter *max_length* is non-zero then the return
   value will be no longer than *max_length*. This may mean that not
   all of the compressed input can be processed; and unconsumed data
   will be stored in the attribute "unconsumed_tail". This bytestring
   must be passed to a subsequent call to "decompress()" if
   decompression is to continue.  If *max_length* is zero then the
   whole input is decompressed, and "unconsumed_tail" is empty.

   在 3.6 版更改: *max_length* can be used as a keyword argument.

Decompress.flush([length])

   All pending input is processed, and a bytes object containing the
   remaining uncompressed output is returned.  After calling
   "flush()", the "decompress()" method cannot be called again; the
   only realistic action is to delete the object.

   The optional parameter *length* sets the initial size of the output
   buffer.

Decompress.copy()

   Returns a copy of the decompression object.  This can be used to
   save the state of the decompressor midway through the data stream
   in order to speed up random seeks into the stream at a future
   point.

在 3.8 版更改: Added "copy.copy()" and "copy.deepcopy()" support to
decompression objects.

通过下列常量可获取模块所使用的 zlib 库的版本信息：

zlib.ZLIB_VERSION

   构建此模块时所用的 zlib 库的版本字符串。它的值可能与运行时所加载的
   zlib 不同。运行时加载的 zlib 库的版本字符串为
   "ZLIB_RUNTIME_VERSION"。

zlib.ZLIB_RUNTIME_VERSION

   解释器所加载的 zlib 库的版本字符串。

   3.3 新版功能.

参见:

  模块 "gzip"
     读写 **gzip** 格式的文件。

  http://www.zlib.net
     zlib 库项目主页。

  http://www.zlib.net/manual.html
     zlib 库用户手册。提供了库的许多功能的解释和用法。
