13.1. zlibgzip 互換の圧縮


このモジュールは、データ圧縮を必要とするアプリケーションが zlib ライブラリを使って圧縮および展開を行えるようにします。zlib ライブラリ自身の Webページは http://www.zlib.net です。Python モジュールと zlib ライブラリの 1.1.3 より前のバージョンには互換性のない部分があることが知られています。1.1.3 にはセキュリティホールが存在するため、1.1.4 以降のバージョンを利用することを推奨します。

zlib の関数にはたくさんのオプションがあり、場合によっては特定の順番で使わなければなりません。このドキュメントではそれら順番についてすべてを説明しようとはしていません。詳細は公式サイト http://www.zlib.net/manual.html にある zlib のマニュアルを参照してください。

.gz ファイルの読み書きのためには、 gzip モジュールを参照してください。

このモジュールで利用可能な例外と関数を以下に示します:

exception zlib.error

圧縮および展開時のエラーによって送出される例外です。

zlib.adler32(data[, value])

Computes an Adler-32 checksum of data. (An Adler-32 checksum is almost as reliable as a CRC32 but can be computed much more quickly.) The result is an unsigned 32-bit integer. If value is present, it is used as the starting value of the checksum; otherwise, a default value of 1 is used. Passing in value allows computing a running checksum over the concatenation of several inputs. The algorithm is not cryptographically strong, and should not be used for authentication or digital signatures. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm.

バージョン 3.0 で変更: Always returns an unsigned value. To generate the same numeric value across all Python versions and platforms, use adler32(data) & 0xffffffff.

zlib.compress(data, level=-1)

Compresses the bytes in data, returning a bytes object containing compressed data. level is an integer from 0 to 9 or -1 controlling the level of compression; 1 is fastest and produces the least compression, 9 is slowest and produces the most. 0 is no compression. The default value is -1 (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a default compromise between speed and compression (currently equivalent to level 6). Raises the error exception if any error occurs.

バージョン 3.6 で変更: level can now be used as a keyword parameter.

zlib.compressobj(level=-1, method=DEFLATED, wbits=15, memLevel=8, strategy=Z_DEFAULT_STRATEGY[, zdict])

Returns a compression object, to be used for compressing data streams that won’t fit into memory at once.

level is the compression level – an integer from 0 to 9 or -1. A value of 1 is fastest and produces the least compression, while a value of 9 is slowest and produces the most. 0 is no compression. The default value is -1 (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a default compromise between speed and compression (currently equivalent to level 6).

method は圧縮アルゴリズムです。現在、 DEFLATED のみサポートされています。

The wbits argument controls the size of the history buffer (or the 「window size」) used when compressing data, and whether a header and trailer is included in the output. It can take several ranges of values:

  • +9 to +15: The base-two logarithm of the window size, which therefore ranges between 512 and 32768. Larger values produce better compression at the expense of greater memory usage. The resulting output will include a zlib-specific header and trailer.
  • −9 to −15: Uses the absolute value of wbits as the window size logarithm, while producing a raw output stream with no header or trailing checksum.
  • +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value as the window size logarithm, while including a basic gzip header and trailing checksum in the output.

The memLevel argument controls the amount of memory used for the internal compression state. Valid values range from 1 to 9. Higher values use more memory, but are faster and produce smaller output.

strategy is used to tune the compression algorithm. Possible values are Z_DEFAULT_STRATEGY, Z_FILTERED, and Z_HUFFMAN_ONLY.

zdict is a predefined compression dictionary. This is a sequence of bytes (such as a bytes object) containing subsequences that are expected to occur frequently in the data that is to be compressed. Those subsequences that are expected to be most common should come at the end of the dictionary.

バージョン 3.3 で変更: Added the zdict parameter and keyword argument support.

zlib.crc32(data[, value])

Computes a CRC (Cyclic Redundancy Check) checksum of data. The result is an unsigned 32-bit integer. If value is present, it is used as the starting value of the checksum; otherwise, a default value of 0 is used. Passing in value allows computing a running checksum over the concatenation of several inputs. The algorithm is not cryptographically strong, and should not be used for authentication or digital signatures. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm.

バージョン 3.0 で変更: Always returns an unsigned value. To generate the same numeric value across all Python versions and platforms, use crc32(data) & 0xffffffff.

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

Decompresses the bytes in data, returning a bytes object containing the uncompressed data. The wbits parameter depends on the format of data, and is discussed further below. If bufsize is given, it is used as the initial size of the output buffer. Raises the error exception if any error occurs.

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().

バージョン 3.6 で変更: wbits and bufsize can be used as keyword arguments.

zlib.decompressobj(wbits=15[, zdict])

一度にメモリ上に置くことができないようなデータストリームを展開するための展開オブジェクトを返します。

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.

注釈

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.

バージョン 3.3 で変更: Added the zdict parameter.

圧縮オブジェクトは以下のメソッドをサポートしています:

Compress.compress(data)

Compress data, returning a bytes object containing compressed data for at least part of the data in data. This data should be concatenated to the output produced by any preceding calls to the compress() method. Some input may be kept in internal buffers for later processing.

Compress.flush([mode])

All pending input is processed, and a bytes object containing the remaining compressed output is returned. mode can be selected from the constants Z_SYNC_FLUSH, Z_FULL_FLUSH, or Z_FINISH, defaulting to Z_FINISH. Z_SYNC_FLUSH and Z_FULL_FLUSH allow compressing further bytestrings of data, while Z_FINISH finishes the compressed stream and prevents compressing any more data. After calling flush() with mode set to Z_FINISH, the compress() method cannot be called again; the only realistic action is to delete the object.

Compress.copy()

圧縮オブジェクトのコピーを返します。これを使うと先頭部分が共通している複数のデータを効率的に圧縮することができます。

Decompression objects support the following methods and attributes:

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.

バージョン 3.3 で追加.

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.

バージョン 3.6 で変更: 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.

オプション引数 length には出力バッファの初期サイズを指定します。

Decompress.copy()

展開オブジェクトのコピーを返します。これを使うとデータストリームの途中にある展開オブジェクトの状態を保存でき、未来のある時点で行なわれるストリームのランダムなシークをスピードアップするのに利用できます。

Information about the version of the zlib library in use is available through the following constants:

zlib.ZLIB_VERSION

The version string of the zlib library that was used for building the module. This may be different from the zlib library actually used at runtime, which is available as ZLIB_RUNTIME_VERSION.

zlib.ZLIB_RUNTIME_VERSION

The version string of the zlib library actually loaded by the interpreter.

バージョン 3.3 で追加.

参考

gzip モジュール
gzip 形式ファイルへの読み書きを行うモジュール。
http://www.zlib.net
zlib ライブラリホームページ。
http://www.zlib.net/manual.html
zlib ライブラリの多くの関数の意味と使い方を解説したマニュアル。