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finished files chapter
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Mark Pilgrim
@@ -44,6 +44,7 @@ Classname Legend
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.pp = "pretty print" = apply syntax highlighting to this code block
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.pf = "padded frame" = black border with internal padding
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.fr = "framed" = black border, no padding
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.ss = "screenshot" = image, floated right, with margin
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.note = "note/caution/important" = indented block for tips/gotchas/language comparisons
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.baa = "best available ampersand" = wrapper block for ampersands
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@@ -160,6 +161,10 @@ form div, #level {
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line-height:1;
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margin:0.7em 0;
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}
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.ss {
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float: right;
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margin: 0 0 1.75em 1.75em;
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}
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/* links */
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+18
-18
@@ -16,7 +16,7 @@ body{counter-reset:h1 12}
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<p id=level>Difficulty level: <span class=u title=intermediate>♦♦♦♢♢</span>
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<h1>Files</h1>
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<blockquote class=q>
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<p><span class=u>❝</span> FIXME <span class=u>❞</span><br>— FIXME
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<p><span class=u>❝</span> A nine mile walk is no joke, especially in the rain. <span class=u>❞</span><br>— Harry Kemelman, <cite>The Nine Mile Walk</cite>
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</blockquote>
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<p id=toc>
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<h2 id=divingin>Diving In</h2>
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@@ -296,16 +296,14 @@ ValueError: I/O operation on closed file.</samp>
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<p>Did you notice the <code>encoding</code> parameter that got passed in to the <code>open()</code> function while you were <a href=#writing>opening a file for writing</a>? It’s important; don’t ever leave it out! As you saw in the beginning of this chapter, files don’t contain <i>strings</i>, they contain <i>bytes</i>. Reading a “string” from a text file only works because you told Python what encoding to use to read a stream of bytes and convert it to a string. Writing text to a file presents the same problem in reverse. You can’t write characters to a file; <a href=strings.html#byte-arrays>characters are an abstraction</a>. In order to write to the file, Python needs to know how to convert your string into a sequence of bytes. The only way to be sure it’s performing the correct conversion is to specify the <code>encoding</code> parameter when you open the file for writing.
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<h3 id=write>Write A Little, Write A Lot</h3>
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<p>FIXME write(), writelines(), .writeable
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<h2 id=binary>Binary Files</h2>
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<p>FIXME
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<p class=ss><img src=examples/beauregard.jpg alt='my dog Beauregard' width=100 height=100>
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<p>Not every file contains text. Some of them contain pictures of my dog.
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<pre class=screen>
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<a><samp class=p>>>> </samp><kbd class=pp>an_image = open('examples/beauregard-100x100.jpg', mode='rb')</kbd> <span class=u>①</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>an_image = open('examples/beauregard.jpg', mode='rb')</kbd> <span class=u>①</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>an_image.mode</kbd> <span class=u>②</span></a>
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<samp class=pp>'rb'</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>an_image.name</kbd> <span class=u>③</span></a>
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@@ -315,22 +313,24 @@ ValueError: I/O operation on closed file.</samp>
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File "<stdin>", line 1, in <module>
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AttributeError: '_io.BufferedReader' object has no attribute 'encoding'</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>Opening a file in binary mode is simple but subtle. The only difference from opening it in text mode is that the <code>mode</code> parameter contains a <code>'b'</code>.
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<li>The file object you get from opening a file in binary mode has many of the same attributes, including <code>mode</code>, which reflects the <code>mode</code> parameter you passed into the <code>open()</code> function.
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<li>File objects for binary files also have a <code>name</code> attribute, just like file objects for text files.
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<li>Here’s one difference, though: the file object for a binary file has no <code>encoding</code> attribute. That makes sense, right? You&#re reading (or writing) bytes, not strings, so there’s no conversion for Python to do. What you get out of a binary file is exactly what you put into it, no conversion necessary.
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</ol>
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<p>Did I mention you’re reading bytes? Oh yes you are.
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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>an_image.tell()</kbd> <span class=u>①</span></a>
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<samp class=p>>>> </samp><kbd class=pp>an_image.tell()</kbd>
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<samp class=pp>0</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>data = image.read(3)</kbd> <span class=u>②</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>data = image.read(3)</kbd> <span class=u>①</span></a>
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<samp class=p>>>> </samp><kbd class=pp>data</kbd>
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<samp class=pp>b'\xff\xd8\xff'</samp>
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<a><samp class=p>>>> </samp><kbd class=pp>type(data)</kbd> <span class=u>③</span></a>
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<a><samp class=p>>>> </samp><kbd class=pp>type(data)</kbd> <span class=u>②</span></a>
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<samp class=pp><class 'bytes'></samp>
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<samp class=p>>>> </samp><kbd class=pp>an_image.tell()</kbd>
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<a><samp class=p>>>> </samp><kbd class=pp>an_image.tell()</kbd> <span class=u>③</span></a>
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<samp class=pp>3</samp>
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<samp class=p>>>> </samp><kbd class=pp>an_image.seek(0)</kbd>
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<samp class=pp>0</samp>
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@@ -338,9 +338,9 @@ AttributeError: '_io.BufferedReader' object has no attribute 'encoding'</samp></
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<samp class=p>>>> </samp><kbd class=pp>len(data)</kbd>
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<samp class=pp>3150</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>Like text files, you can read binary files a little bit at a time. But there’s a crucial difference…
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<li>…you’re reading bytes, not strings. Since you opened the file in binary mode, the <code>read()</code> method takes <em>the number of bytes to read</em>, not the number of characters.
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<li>That means that there’s never <a href=#read>an unexpected mismatch</a> between the number you passed into the <code>read()</code> method and the position index you get out of the <code>tell()</code> method. The <code>read()</code> method reads bytes, and the <code>seek()</code> and <code>tell()</code> methods track the number of bytes read. For binary files, they’ll always agree.
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</ol>
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<h2 id=file-like-objects>File-like Objects</h2>
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@@ -9,7 +9,6 @@ body{counter-reset:h1 0}
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.i{list-style:none;margin:0;padding:0}
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#which{padding-top:1.75em}
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h2,.i>li{clear:both}
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.i li .ss{float:right;margin:0 0 1.75em 1.75em}
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</style>
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<link rel=stylesheet type=text/css media='only screen and (max-device-width: 480px)' href=mobile.css>
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<link rel=stylesheet media=print href=print.css>
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@@ -199,10 +198,6 @@ Type "help", "copyright", "credits" or "license" for more information.
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<p class='ss nm'><img src=i/mac-interactive-shell.png width=522 height=538 alt='[Mac Python Shell, a graphical interactive shell for Python]'>
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<p>The Python Shell is where you will spend most of your time exploring Python. Examples throughout this book will assume that you can find your way into the Python Shell.
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<!--
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<li>
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<p class='ss nm'><img src=i/mac-pythonlauncher.png width=432 height=536 alt='[Python Launcher preferences window]'>
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-->
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</ol>
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<p>[Skip to <a href=#idle>using the Python Shell</a>]
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