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added notes to gzip example

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Mark Pilgrim
2009-08-16 13:55:20 -04:00
parent 234c41e9b6
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@@ -414,16 +414,22 @@ AttributeError: '_io.BufferedReader' object has no attribute 'encoding'</samp></
<samp class=p>you@localhost:~$ </samp><kbd>python3</kbd>
<samp class=p>>>> </samp><kbd class=pp>import gzip</kbd>
<samp class=p>>>> </samp><kbd class=pp>with gzip.open('out.log.gz', mode='wb') as z_file:</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>with gzip.open('out.log.gz', mode='wb') as z_file:</kbd> <span class=u>&#x2460;</span></a>
<samp class=p>... </samp><kbd class=pp> z_file.write('A nine mile walk is no joke, especially in the rain.'.encode('utf-8'))</kbd>
<samp class=p>... </samp>
<samp class=p>>>> </samp><kbd class=pp>exit()</kbd>
<samp class=p>you@localhost:~$ </samp><kbd>ls -l out.log.gz</kbd>
<a><samp class=p>you@localhost:~$ </samp><kbd>ls -l out.log.gz</kbd> <span class=u>&#x2461;</span></a>
<samp>-rw-r--r-- 1 mark mark 79 2009-07-19 14:29 out.log.gz</samp>
<samp class=p>you@localhost:~$ </samp><kbd>gunzip out.log.gz</kbd>
<samp class=p>you@localhost:~$ </samp><kbd>cat out.log</kbd>
<a><samp class=p>you@localhost:~$ </samp><kbd>gunzip out.log.gz</kbd> <span class=u>&#x2462;</span></a>
<a><samp class=p>you@localhost:~$ </samp><kbd>cat out.log</kbd> <span class=u>&#x2463;</span></a>
<samp>A nine mile walk is no joke, especially in the rain.</samp></pre>
<ol>
<li>You should always open gzipped files in binary mode. (Note the <code>'b'</code> character in the <code>mode</code> argument.)
<li>I constructed this example on Linux. If you&#8217;re not familiar with the command line, this command is showing the &#8220;long listing&#8221; of the gzip-compressed file you just created in the Python Shell. This listing shows that the file exists (good), and that it is 79 bytes long. That&#8217;s actually larger than the string you started with! The gzip file format includes a fixed-length header that contains some metadata about the file, so it&#8217;s inefficient for extremely small files.
<li>The <code>gunzip</code> command (pronounced &#8220;gee-unzip&#8221;) decompresses the file and stores the contents in a new file named the same as the compressed file but without the <code>.gz</code> file extension.
<li>The <code>cat</code> command displays the contents of a file. This file contains the string you originally wrote directly to the compressed file <code>out.log.gz</code> from within the Python Shell.
</ol>
<p class=a>&#x2042;