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open file as late as possible (in __iter__, not __init__)

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This commit is contained in:
Mark Pilgrim
2009-07-14 00:28:46 -04:00
parent d86eb45f24
commit 34bbe9e9d6
2 changed files with 8 additions and 11 deletions
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examples/plural6.py
Regular → Executable
+1 -3
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@@ -17,11 +17,9 @@ def build_match_and_apply_functions(pattern, search, replace):
class LazyRules:
rules_filename = 'plural6-rules.txt'
def __init__(self):
def __iter__(self):
self.pattern_file = open(self.rules_filename)
self.cache = []
def __iter__(self):
self.cache_index = 0
return self
iterators.html
+7 -8
View File
@@ -217,11 +217,9 @@ All three of these class methods, <code>__init__</code>, <code>__iter__</code>,
<pre><code class=pp>class LazyRules:
rules_filename = 'plural6-rules.txt'
def __init__(self):
def __iter__(self):
self.pattern_file = open(self.rules_filename)
self.cache = []
def __iter__(self):
self.cache_index = 0
return self
@@ -253,16 +251,17 @@ rules = LazyRules()</code></pre>
<pre><code class=pp>class LazyRules:
rules_filename = 'plural6-rules.txt'
<a> def __init__(self): <span class=u>&#x2460;</span></a>
<a> def __iter__(self): <span class=u>&#x2460;</span></a>
<a> self.pattern_file = open(self.rules_filename) <span class=u>&#x2462;</span></a>
<a> self.cache = [] <span class=u>&#x2461;</span></a></code></pre>
<a> self.cache = [] <span class=u>&#x2461;</span></a>
self.cache_index = 0</code></pre>
<ol>
<li>The <code>__init__()</code> method is only going to be called once, when you instantiate the class and assign it to <var>rules</var>.
<li>Since this is only going to get called once, it&#8217;s the perfect place to open the pattern file. You&#8217;ll read it later; no point doing more than you absolutely have to until absolutely necessary!
<li>The <code>__iter__()</code> method is only going to be called once, after you instantiate the class, assign it to <var>rules</var>, and call <code>iter(rules)</code> to create the iterator.
<li>Since this is only going to get called once, it&#8217;s the perfect place to open the pattern file. No point doing more than you absolutely have to until absolutely necessary!
<li>Also, this is a good place to initialize the cache, which you&#8217;ll use later as you read the patterns from the pattern file.
</ol>
<p>Before we continue, let&#8217;s take a closer look at <var>rules_filename</var>. It&#8217;s not defined within the <code>__init__()</code> method. In fact, it&#8217;s not defined within <em>any</em> method. It&#8217;s defined at the class level. It&#8217;s a <i>class variable</i>, and although you can access it just like an instance variable (<var>self.rules_filename</var>), it is shared across all instances of the <code>LazyRules</code> class.
<p>Before we continue, let&#8217;s take a closer look at <var>rules_filename</var>. It&#8217;s not defined within the <code>__iter__()</code> method. In fact, it&#8217;s not defined within <em>any</em> method. It&#8217;s defined at the class level. It&#8217;s a <i>class variable</i>, and although you can access it just like an instance variable (<var>self.rules_filename</var>), it is shared across all instances of the <code>LazyRules</code> class.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>import plural6</kbd>