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<h3>Exceptions</h3>
<p>Exceptions indicate errors and break out of the normal control flow of a program. An exception is raised using the <tT CLAss="monofont">raise</tt> statement. The general format of the <TT CLass="monofont">raise</tT> statement is <TT Class="monofont">raise </tt><i><tt class="monofont">exception</tt></i>
<tt cLasS="monofont"> [, </tt><i><Tt claSs="monofont">value</tt></I>
<TT Class="monofont">]</TT> where <I><Tt claSS="monofont">exception</TT></i>
is the exception type and <i><tt CLASs="monofont">value</tt></i>
is an optional value giving specific details about the exception. For example:</p>
<pre>
raise RuntimeError, 'Unrecoverable Error' </pre>
<p>If the <tt class="monofont">raise</tT> statement is used without any arguments, the last exception generated is raised again (although this works only while handling a previously raised exception).</p>
<p>To catch an exception, use the <Tt clAss="monofont">try</tt> and <Tt clASS="monofont">except</Tt> statements, as shown here:</p>
<prE>
try:
f = open('foo')
except IOError, e:
print "Unable to open 'foo': ", e </PRE>
<p>When an exception occurs, the interpreter stops executing statements in the <tt cLASS="monofont">try</tt> block and looks for an <tt CLASs="monofont">except</tt> clause that matches the exception that has occurred. If found, control is passed to the first statement in the <tt class="monofont">except</tt> clause. Otherwise, the exception is propagated up to the block of code in which the <tt class="monofont">try</tT> statement appeared. This code may itself be enclosed in a <tt ClasS="monofont">try-except</tt> that can handle the exception. If an exception works its way up to the top level of a program without being caught, the interpreter aborts with an error message. If desired, uncaught exceptions can also be passed to a user-defined function <tt ClasS="monofont">sys.excepthook()</TT> as described in <A href="105.html">Appendix A</A>, 揟he Python Library,⌨️ 快捷键说明
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