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<FONT><h3>Exception Handling</h3>
				<p>Exceptions are mostly used for error handling and event notification. They work by breaking the regular flow of a program and jumping to a special set of statements that handle the exception case. Python has many standard exceptions, which are exceptions already built into the language. Python also supports user-defined exceptions, which are exceptions created by users. The provided exceptions are almost no different from user-defined exceptions梩he only difference is that they are defined in one of the files in the standard library (<tT CLAss="monofont">exceptions.py</tt>).</P>

				<P>Any unexpected program behavior drives the interpreter to raise an exception. Many scenarios can help an exception to be raised, such as dividing a number by zero or reading from a nonexistent file. Note that the programmer can also manually <A Name="idx1073743079"></a><A NAMe="idx1073743080"></a>raise exceptions with the <a name="idx1073743081"></a><a name="idx1073743082"></a><tt clasS="monofont">raise</tt> statement.</P>

				<p>The default behavior of Python, when it encounters unhandled exceptions, is to terminate the program and to display a traceback message that describes the error condition. My goal in this chapter is to show you how to handle those exceptions.</p>

				<p>If you don't handle exceptions in your program, Python's interpreter returns a traceback message that shows the error message, the exception type, the function that contains the error, and the line of code that has caused the error. Hence, a complete history of what has caused the error is provided.<A name="idx1073743083"></A><a naME="idx1073743084"></A></P>

				<p>So that you can start learning how Python raises and handles exceptions, I will define the following example:</p>

				<prE>
					
&gt;&gt;&gt; a = { "a":1,"b":2}
&gt;&gt;&gt; def returnelement(element):
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