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finished #close section
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Mark Pilgrim
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@@ -28,13 +28,14 @@ body{counter-reset:h1 12}
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<pre class=nd><code class=pp>a_file = open('examples/chinese.txt', encoding='utf-8')</code></pre>
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<p>Python has a built-in <code>open()</code> function, which takes a filename as an argument. Here the filename is <code class=pp>'examples/chinese.txt'</code>. There are four interesting things about this filename:
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<p>Python has a built-in <code>open()</code> function, which takes a filename as an argument. Here the filename is <code class=pp>'examples/chinese.txt'</code>. There are five interesting things about this filename:
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<ol>
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<li>It’s not just the name of a file; it’s a combination of a directory path and a filename. A hypothetical file-opening function could have taken two arguments — a directory path and a filename — but the <code>open()</code> function only takes one. In Python, whenever you need a “filename,” you can include some or all of a directory path as well.
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<li>The directory path uses a forward slash, but I didn’t say what operating system I was using. Windows uses backward slashes to denote subdirectories, while Mac OS X and Linux use forward slashes. But in Python, forward slashes always Just Work, even on Windows.
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<li>The directory path does not begin with a slash or a drive letter, so it is called a <i>relative path</i>. Relative to what, you might ask? Patience, grasshopper.
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<li>It’s a string. All modern operating systems (even Windows!) use Unicode to store the names of files and directories. Python 3 fully supports non-<abbr>ASCII</abbr> pathnames.
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<li>It doesn’t need to be on your local disk. You might have a network drive mounted. That “file” might be a figment of <a href=http://en.wikipedia.org/wiki/Filesystem_in_Userspace>an entirely virtual filesystem</a>. If your computer considers it a file and can access it as a file, Python can open it.
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</ol>
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<p>But that call to the <code>open()</code> function didn’t stop at the filename. There’s another argument, called <code>encoding</code>. Oh dear, <a href=strings.html#boring-stuff>that sounds dreadfully familiar</a>.
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@@ -92,47 +93,62 @@ UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 28: chara
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<pre class=screen>
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<samp class=p>>>> </samp><kbd class=pp>a_file = open('examples/chinese.txt', encoding='utf-8')</kbd>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read()</kbd>
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<samp class=pp>'Dive Into Python 是为有经验的程序员编写的一本 Python 书。\n'</samp></pre>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read()</kbd> <span class=u>①</span></a>
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<samp class=pp>'Dive Into Python 是为有经验的程序员编写的一本 Python 书。\n'</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read()</kbd> <span class=u>②</span></a>
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<samp class=pp>''</samp></pre>
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<ol>
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<li>FIXME
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<li>Once you open a file (with the correct encoding), reading from it is just a matter of calling the file object’s <code>read()</code> method. The result is a string.
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<li>Perhaps somewhat surprisingly, reading the file again does not raise an exception. Python does not consider reading past end-of-file to be an error; it simply returns an empty string.
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</ol>
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<p>FIXME
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<p>What if you want to re-read a file?
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<pre class=screen>
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<samp class=p>>>> </samp><kbd class=pp>a_file.seek(0)</kbd>
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# continued from the previous example
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read()</kbd> <span class=u>①</span></a>
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<samp class=pp>''</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.seek(0)</kbd> <span class=u>②</span></a>
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<samp class=pp>0</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read(16)</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read(16)</kbd> <span class=u>③</span></a>
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<samp class=pp>'Dive Into Python'</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd> <span class=u>④</span></a>
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<samp class=pp>' '</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd> <span class=u>⑤</span></a>
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<samp class=pp>'是'</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.tell()</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.tell()</kbd> <span class=u>⑥</span></a>
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<samp class=pp>20</samp></pre>
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<ol>
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<li>FIXME
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<li>
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<li>
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<li>
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<li>
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<li>
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</ol>
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<p>FIXME
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<pre class=screen>
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<samp class=p>>>> </samp><kbd class=pp>a_file.seek(17)</kbd>
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# continued from the previous example
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.seek(17)</kbd> <span class=u>①</span></a>
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<samp class=pp>17</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd> <span class=u>②</span></a>
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<samp class=pp>'是'</samp>
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<samp class=p>>>> </samp><kbd class=pp>a_file.tell()</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.tell()</kbd> <span class=u>③</span></a>
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<samp class=pp>20</samp></pre>
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<ol>
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<li>FIXME
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<li>
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<li>
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</ol>
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<p>FIXME
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<pre class=screen>
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<samp class=p>>>> </samp><kbd class=pp>a_file.seek(18)</kbd>
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<samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.seek(18)</kbd> <span class=u>①</span></a>
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<samp class=pp>18</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read(1)</kbd> <span class=u>②</span></a>
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<samp class=traceback>Traceback (most recent call last):
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File "<pyshell#12>", line 1, in <module>
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a_file.read(1)
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@@ -141,45 +157,48 @@ UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 28: chara
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UnicodeDecodeError: 'utf8' codec can't decode byte 0x98 in position 0: unexpected code byte</samp></pre>
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<ol>
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<li>FIXME
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<li>
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</ol>
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<h3>6.2.2. Closing Files</h3>
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<h3 id=close>Closing Files</h3>
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<p>Open files consume system resources, and depending on the file mode, other programs may not be able to access them. It’s important to close files as soon as you’re finished with them.
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<!--
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<div class=example><h3>Example 6.5. Closing a File</h3><pre class=screen>
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<samp class=p>>>> </samp><kbd>f</kbd>
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<open file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
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<samp class=p>>>> </samp><kbd>f.closed</kbd> <span>①</span>
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False
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<samp class=p>>>> </samp><kbd>f.close()</kbd> <span>②</span>
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<samp class=p>>>> </samp><kbd>f</kbd>
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<closed file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
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<samp class=p>>>> </samp><kbd>f.closed</kbd> <span>③</span>
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True
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<samp class=p>>>> </samp><kbd>f.seek(0)</kbd> <span>④</span>
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<samp class=traceback>Traceback (innermost last):
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File "<interactive input>", line 1, in ?
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ValueError: I/O operation on closed file</samp>
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<samp class=p>>>> </samp><kbd>f.tell()</kbd>
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<samp class=traceback>Traceback (innermost last):
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File "<interactive input>", line 1, in ?
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ValueError: I/O operation on closed file</samp>
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<samp class=p>>>> </samp><kbd>f.read()</kbd>
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<samp class=traceback>Traceback (innermost last):
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File "<interactive input>", line 1, in ?
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ValueError: I/O operation on closed file</samp>
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<samp class=p>>>> </samp><kbd>f.close()</kbd> <span>⑤</span></pre>
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<ol>
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<li>The <var>closed</var> attribute of a file object indicates whether the object has a file open or not. In this case, the file is still open (<var>closed</var> is <code>False</code>).
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<li>To close a file, call the <code>close</code> method of the file object. This frees the lock (if any) that you were holding on the file, flushes buffered writes (if any) that the system hadn’t gotten around to actually writing yet, and releases the system resources.
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<li>The <var>closed</var> attribute confirms that the file is closed.
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<li>Just because a file is closed doesn’t mean that the file object ceases to exist. The variable <var>f</var> will continue to exist until it <a href="#fileinfo.scope" title="Example 5.8. Trying to Implement a Memory Leak">goes out of scope</a> or gets manually deleted. However, none of the methods that manipulate an open file will work once the file has been closed; they all raise an exception.
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<li>Calling <code>close</code> on a file object whose file is already closed does <em>not</em> raise an exception; it fails silently.
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-->
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<pre class='nd screen'>
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# continued from the previous example
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<samp class=p>>>> </samp><kbd class=pp>a_file.close()</kbd></pre>
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<p>FIXME checking if a file is closed
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<p>Well <em>that</em> was anticlimactic.
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<p>The file object <var>a_file</var> still exists; calling its <code>close()</code> method doesn’t destroy the object itself. But it’s not terribly useful.
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<pre class=screen>
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# continued from the previous example
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.read()</kbd> <span class=u>①</span></a>
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<samp class=traceback>Traceback (most recent call last):
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File "<pyshell#24>", line 1, in <module>
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a_file.read()
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ValueError: I/O operation on closed file.</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.seek(0)</kbd> <span class=u>②</span></a>
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<samp class=traceback>Traceback (most recent call last):
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File "<pyshell#25>", line 1, in <module>
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a_file.seek(0)
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ValueError: I/O operation on closed file.</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.tell()</kbd> <span class=u>③</span></a>
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<samp class=traceback>Traceback (most recent call last):
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File "<pyshell#26>", line 1, in <module>
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a_file.tell()
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ValueError: I/O operation on closed file.</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.close()</kbd> <span class=u>④</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>a_file.closed</kbd> <span class=u>⑤</span></a>
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<samp class=pp>True</samp></pre>
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<ol>
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<li>You can’t read from a closed file; that raises an <code>IOError</code> exception.
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<li>You can’t seek in a closed file either.
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<li>There’s no current position in a closed file, so the <code>tell()</code> method also fails.
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<li>Perhaps surprisingly, calling the <code>close()</code> method on a file object whose file has been closed does <em>not</em> raise an exception. It’s just a no-op.
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<li>Closed file objects do have one useful attribute: the <code>closed</code> attribute will confirm that the file is closed.
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</ol>
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<h3 id=with>Using The <code>with</code> Statement</h3>
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