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<h2 class="title"><a name="soap.wsdl"></a>12.5. Introducing <span class="acronym">WSDL</span></h2>
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<div class="abstract">
<p>The <tt class="classname">SOAPProxy</tt> class proxies local method calls and transparently turns then into invocations of remote <span class="acronym">SOAP</span> methods. As you've seen, this is a lot of work, and <tt class="classname">SOAPProxy</tt> does it quickly and transparently. What it doesn't do is provide any means of method introspection.
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<p>Consider this: the previous two sections showed an example of calling a simple remote <span class="acronym">SOAP</span> method with one argument and one return value, both of simple data types. This required knowing, and keeping track of, the
service <span class="acronym">URL</span>, the service namespace, the function name, the number of arguments, and the datatype of each argument. If any of these is
missing or wrong, the whole thing falls apart.
</p>
<p>That shouldn't come as a big surprise. If I wanted to call a local function, I would need to know what package or module
it was in (the equivalent of service <span class="acronym">URL</span> and namespace). I would need to know the correct function name and the correct number of arguments. <span class="application">Python</span> deftly handles datatyping without explicit types, but I would still need to know how many argument to pass, and how many
return values to expect.
</p>
<p>The big difference is introspection. As you saw in <a href="../power_of_introspection/index.html">Chapter 4</a>, <span class="application">Python</span> excels at letting you discover things about modules and functions at runtime. You can list the available functions within
a module, and with a little work, drill down to individual function declarations and arguments.
</p>
<p><span class="acronym">WSDL</span> lets you do that with <span class="acronym">SOAP</span> web services. <span class="acronym">WSDL</span> stands for “<span class="quote">Web Services Description Language</span>”. Although designed to be flexible enough to describe many types of web services, it is most often used to describe <span class="acronym">SOAP</span> web services.
</p>
<p>A <span class="acronym">WSDL</span> file is just that: a file. More specifically, it's an XML file. It usually lives on the same server you use to access the
<span class="acronym">SOAP</span> web services it describes, although there's nothing special about it. Later in this chapter, we'll download the <span class="acronym">WSDL</span> file for the Google API and use it locally. That doesn't mean we're calling Google locally; the <span class="acronym">WSDL</span> file still describes the remote functions sitting on Google's server.
</p>
<p>A <span class="acronym">WSDL</span> file contains a description of everything involved in calling a <span class="acronym">SOAP</span> web service:
</p>
<div class="itemizedlist">
<ul>
<li>The service <span class="acronym">URL</span> and namespace
</li>
<li>The type of web service (probably function calls using <span class="acronym">SOAP</span>, although as I mentioned, <span class="acronym">WSDL</span> is flexible enough to describe a wide variety of web services)
</li>
<li>The list of available functions</li>
<li>The arguments for each function</li>
<li>The datatype of each argument</li>
<li>The return values of each function, and the datatype of each return value</li>
</ul>
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<p>In other words, a <span class="acronym">WSDL</span> file tells you everything you need to know to be able to call a <span class="acronym">SOAP</span> web service.
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