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move some explanatory stuff to the files chapter

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Mark Pilgrim
2009-07-16 12:30:25 -04:00
parent e9b883b771
commit 6c585aeb60
1 changed files with 6 additions and 6 deletions
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generators.html
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@@ -274,15 +274,15 @@ $ $ s</code></pre>
return [matches_rule, apply_rule]
rules = []
<a>with open('plural4-rules.txt') as pattern_file: <span class=u>&#x2461;</span></a>
<a> for line in pattern_file: <span class=u>&#x2462;</span></a>
<a> pattern, search, replace = line.split(None, 3) <span class=u>&#x2463;</span></a>
<a> rules.append(build_match_and_apply_functions( <span class=u>&#x2464;</span></a>
<a>with open('plural4-rules.txt', encoding='utf-8') as pattern_file: <span class=u>&#x2461;</span></a>
<a> for line in pattern_file: <span class=u>&#x2462;</span></a>
<a> pattern, search, replace = line.split(None, 3) <span class=u>&#x2463;</span></a>
<a> rules.append(build_match_and_apply_functions( <span class=u>&#x2464;</span></a>
pattern, search, replace))</code></pre>
<ol>
<li>The <code>build_match_and_apply_functions()</code> function has not changed. You&#8217;re still using closures to build two functions dynamically that use variables defined in the outer function.
<li>The global <code>open()</code> function opens a file and returns a file object. In this case, the file we&#8217;re opening contains the pattern strings for pluralizing nouns. The <code>with</code> statement creates what&#8217;s called a <i>context</i>: when the <code>with</code> block ends, Python will automatically close the file, even if an exception is raised inside the <code>with</code> block. You&#8217;ll learn more about <code>with</code> blocks in the <a href=files.html>Files</a> chapter.
<li>The <code>for line in &lt;fileobject></code> idiom reads data from the open file, one line at a time, and assigns the text to the <var>line</var> variable. A &#8220;line&#8221; of a text file is just what you think it is&nbsp;&mdash;&nbsp;a sequence of characters delimited by a carriage return. Of course, it can&#8217;t really be that simple, can it? Text files can use several different characters to mark the end of a line. Some use a carriage return character, others use a line feed character, and some use both characters at the end of every line. Python handles all of these cases automatically, so you can say, &#8220;Hey, I want to read this text file one line at a time&#8221; and it will Just Work. You&#8217;ll learn more about reading from and writing to in the <a href=files.html>Files</a> chapter.
<li>The global <code>open()</code> function opens a file and returns a file object. In this case, the file we&#8217;re opening contains the pattern strings for pluralizing nouns. The <code>with</code> statement creates what&#8217;s called a <i>context</i>: when the <code>with</code> block ends, Python will automatically close the file, even if an exception is raised inside the <code>with</code> block. You&#8217;ll learn more about <code>with</code> blocks and file objects in the <a href=files.html>Files</a> chapter.
<li>The <code>for line in &lt;fileobject></code> idiom reads data from the open file, one line at a time, and assigns the text to the <var>line</var> variable. You&#8217;ll learn more about reading from and writing to in the <a href=files.html>Files</a> chapter.
<li>Each line in the file really has three values, but they&#8217;re separated by whitespace (tabs or spaces, it makes no difference). To split it out, use the <code>split()</code> string method. The first argument to the <code>split()</code> method is <code>None</code>, which means &#8220;split on any whitespace (tabs or spaces, it makes no difference).&#8221; The second argument is <code>3</code>, which means &#8220;split on whitespace 3 times, then discard the rest of the line.&#8221; A line like <code>[sxz]$ $ es</code> will be broken up into the list <code>['[sxz]$', '$', 'es']</code>, which means that <var>pattern</var> will get <code>'[sxz]$'</code>, <var>search</var> will get <code>'$'</code>, and <var>replace</var> will get <code>'es'</code>. That&#8217;s a lot of power in one little line of code.
</ol>