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Mark Pilgrim
2009-09-26 00:05:54 -04:00
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@@ -216,7 +216,7 @@ ValueError: I/O operation on closed file.</samp>
<p>Here&#8217;s the kicker: no matter how or when you exit the <code>with</code> block, Python will close that file&hellip; even if you &#8220;exit&#8221; it via an unhandled exception. That&#8217;s right, even if your code raises an exception and your entire program comes to a screeching halt, that file will get closed. Guaranteed.
<blockquote class=note>
<p><span class=u>&#x261E;</span>In technical terms, the <code>with</code> statement creates a <dfn>runtime context</dfn>. In these examples, the stream object acts as a <dfn>context manager</dfn>. Python creates the stream object <var>a_file</var> and tells it that it is entering a runtime context. When the <code>with</code> code block is completed, Python tells the stream object that it is exiting the runtime context, and the stream object calls its own <code>close()</code> method. See <a href=special-method-names.html#context-managers>Appendix B, &#8220;Context Managers&#8221;</a> for details.
<p><span class=u>&#x261E;</span>In technical terms, the <code>with</code> statement creates a <dfn>runtime context</dfn>. In these examples, the stream object acts as a <dfn>context manager</dfn>. Python creates the stream object <var>a_file</var> and tells it that it is entering a runtime context. When the <code>with</code> code block is completed, Python tells the stream object that it is exiting the runtime context, and the stream object calls its own <code>close()</code> method. See <a href=special-method-names.html#context-managers>Appendix B, &#8220;Classes That Can Be Used in a <code>with</code> Block&#8221;</a> for details.
</blockquote>
<p>There&#8217;s nothing file-specific about the <code>with</code> statement; it&#8217;s just a generic framework for creating runtime contexts and telling objects that they&#8217;re entering and exiting a runtime context. If the object in question is a stream object, then it does useful file-like things (like closing the file automatically). But that behavior is defined in the stream object, not in the <code>with</code> statement. There are lots of other ways to use context managers that have nothing to do with files. You can even create your own, as you&#8217;ll see later in this chapter.