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<H1><A NAME="SECTION009000000000000000000">
7. Class, instance, method and URL</A>
</H1>
So far, we've seen that the URL <a class="url" href="http://localhost:8000">http://localhost:8000</a> triggers a call to the <var>index</var> method of the <var>Root</var>
class. The URL <a class="url" href="http://localhost:8000/viewResult">http://localhost:8000/viewResult</a> triggers a call to the <var>viewResult</var> method of the <var>Root</var>
class.
<UL>
<LI>How does this magic work ?
</LI>
<LI>I know it cannot call the method of a class. It has to call the method of <b>an instance</b> of a class. How do you explain that ?
</LI>
<LI>What if I type the URL <a class="url" href="http://localhost:8000/dir1/dir2/dir3/page">http://localhost:8000/dir1/dir2/dir3/page</a>
</LI>
</UL>
<P>
Let's answer these questions:
<P>
First of all, when you declare a CherryClass in the source file:
<div class="verbatim"><pre>
CherryClass ClassName:
</pre></div>
<P>
CherryPy (or, to be more precise: the executable generated by CherryPy) will automatically create an instance of
this class, called <var>className</var>. This is the name of the class with a lower case first letter. (This is the reason
why CherryClass names should always start with an upper case letter)
<P>
This instance is a global variable and can be acces from anywhere in the program.
<P>
Based on the URL, how does CherryPy know which method of which class instance to call ?
<P>
It uses a simple mechanism:
For the URL <var>host/dir1/dir2/dir3/page</var>, it will call the <var>dir1_dir2_dir3.page()</var> method. So it will expect
your program to have a CherryClass called <var>Dir1_dir2_dir3</var>, which should have a method called <var>page</var>.
<P>
There are 2 special cases:
<UL>
<LI>if there is no first directory (in other words, the URL is in the form: <var>host/page</var>, then it will call the
<var>root.page()</var> method. This means that both URL <var>host/page</var> and <var>host/root/page</var> are perfectly equivalent.
</LI>
<LI>if there is no first directory and no page (in other words, the URL is just the host name), then it will call the
<var>root.index()</var> method.
</LI>
</UL>
<b>Important</b>: In CherryPy-0.9, a new feature was added: if the page <a class="url" href="http://localhost:8000/dir1/dir2/dir3">http://localhost:8000/dir1/dir2/dir3</a> is
requested, CherryPy will convert it first to <var>dir1_dir2.dir3()</var>, so it will expect a <var>dir3</var> method in the
<var>Dir1_dir2</var> CherryClass. But if no such method exists, then it will look for an <var>index</var> method in the
<var>Dir1_dir2_dir3</var> CherryClass. (this would also correspond to <a class="url" href="http://localhost:8000/dir1/dir2/dir3/index">http://localhost:8000/dir1/dir2/dir3/index</a>).
<P>
In the next chapter, we'll see how to use inheritance when we have similar modules inside a website...
<P>
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<span class="release-info">Release 0.10, documentation updated on 19 March 2004.</span>
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