python標准庫介紹——29 zlib 模塊詳解

==zlib 模塊==


(可選) ``zlib`` 模塊為 "zlib" 壓縮提供支持. (這種壓縮方法是 "deflate".) 

[Example 2-43 #eg-2-43] 展示了如何使用 ``compress`` 和 ``decompress`` 函數接受字符串參數.

====Example 2-43. 使用 zlib 模塊壓縮字符串====[eg-2-43]

```
File: zlib-example-1.py

import zlib

MESSAGE = "life of brian"

compressed_message = zlib.compress(MESSAGE)
decompressed_message = zlib.decompress(compressed_message)

print "original:", repr(MESSAGE)
print "compressed message:", repr(compressed_message)
print "decompressed message:", repr(decompressed_message)

*B*original: 'life of brian'
compressed message: 'x\234\313\311LKU\310OSH*\312L\314\003\000!\010\004\302'
decompressed message: 'life of brian'*b*
```

文件的內容決定了壓縮比率, [Example 2-44 #eg-2-44] 說明了這點.

====Example 2-44. 使用 zlib 模塊壓縮多個不同類型文件====[eg-2-44]

```
File: zlib-example-2.py

import zlib
import glob

for file in glob.glob("samples/*"):

    indata = open(file, "rb").read()
    outdata = zlib.compress(indata, zlib.Z_BEST_COMPRESSION)

    print file, len(indata), "=>", len(outdata),
    print "%d%%" % (len(outdata) * 100 / len(indata))

*B*samples\sample.au 1676 => 1109 66%
samples\sample.gz 42 => 51 121%
samples\sample.htm 186 => 135 72%
samples\sample.ini 246 => 190 77%
samples\sample.jpg 4762 => 4632 97%
samples\sample.msg 450 => 275 61%
samples\sample.sgm 430 => 321 74%
samples\sample.tar 10240 => 125 1%
samples\sample.tgz 155 => 159 102%
samples\sample.txt 302 => 220 72%
samples\sample.wav 13260 => 10992 82%*b*
```

你也可以實時地壓縮或解壓縮數據, 如 [Example 2-45 #eg-2-45] 所示.

====Example 2-45. 使用 zlib 模塊解壓縮流====[eg-2-45]

```
File: zlib-example-3.py

import zlib

encoder = zlib.compressobj()

data = encoder.compress("life")
data = data + encoder.compress(" of ")
data = data + encoder.compress("brian")
data = data + encoder.flush()

print repr(data)
print repr(zlib.decompress(data))

*B*'x\234\313\311LKU\310OSH*\312L\314\003\000!\010\004\302'
'life of brian'*b*
```

[Example 2-46 #eg-2-46] 把解碼對象封裝到了一個類似文件對象的類中, 
實現了一些文件對象的方法, 這樣使得讀取壓縮文件更方便.

====Example 2-46. 壓縮流的仿文件訪問方式====[eg-2-46]

```
File: zlib-example-4.py

import zlib
import string, StringIO

class ZipInputStream:

    def _ _init_ _(self, file):
        self.file = file
        self._ _rewind()

    def _ _rewind(self):
        self.zip = zlib.decompressobj()
        self.pos = 0 # position in zipped stream
        self.offset = 0 # position in unzipped stream
        self.data = ""

    def _ _fill(self, bytes):
        if self.zip:
            # read until we have enough bytes in the buffer
            while not bytes or len(self.data) < bytes:
                self.file.seek(self.pos)
                data = self.file.read(16384)
                if not data:
                    self.data = self.data + self.zip.flush()
                    self.zip = None # no more data
                    break
                self.pos = self.pos + len(data)
                self.data = self.data + self.zip.decompress(data)

    def seek(self, offset, whence=0):
        if whence == 0:
            position = offset
        elif whence == 1:
            position = self.offset + offset
        else:
            raise IOError, "Illegal argument"
        if position < self.offset:
            raise IOError, "Cannot seek backwards"

        # skip forward, in 16k blocks
        while position > self.offset:
            if not self.read(min(position - self.offset, 16384)):
                break

    def tell(self):
        return self.offset

    def read(self, bytes = 0):
        self._ _fill(bytes)
        if bytes:
            data = self.data[:bytes]
            self.data = self.data[bytes:]
        else:
            data = self.data
            self.data = ""
        self.offset = self.offset + len(data)
        return data

    def readline(self):
        # make sure we have an entire line
        while self.zip and "\n" not in self.data:
            self._ _fill(len(self.data) + 512)
        i = string.find(self.data, "\n") + 1
        if i <= 0:
            return self.read()
        return self.read(i)

    def readlines(self):
        lines = []
        while 1:
            s = self.readline()
            if not s:
                break
            lines.append(s)
        return lines

#
# try it out

data = open("samples/sample.txt").read()
data = zlib.compress(data)

file = ZipInputStream(StringIO.StringIO(data))
for line in file.readlines():
    print line[:-1]

*B*We will perhaps eventually be writing only small
modules which are identified by name as they are
used to build larger ones, so that devices like
indentation, rather than delimiters, might become
feasible for expressing local structure in the
source language.
    -- Donald E. Knuth, December 1974*b*
```