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<h3>Exceptions</h3>
<p>If an error occurs in your program, an <i>exception</I> is raised and an error message such as the following appears:</P>
<PRe>
Traceback (most recent call last):
File "<interactive input>", line 42, in foo.py
NameError: a </pre>
<P>The error message indicates the type of error that occurred, along with its location. Normally, errors cause a program to abort. However, you can catch and handle exceptions using the <TT Class="monofont">try</TT> and <TT class="monofont">except</tt> statements, like this:</p>
<pre>
try:
f = open("file.txt","r")
except IOError, e:
print e </pre>
<p>If an <tt claSs="monofont">IOError</tT> occurs, details concerning the cause of the error are placed in <tt cLass="monofont">e</tT> and control passes to the code in the <tt cLASS="monofont">except</tt> block. If some other kind of exception is raised, it抯 passed to the enclosing code block (if any). If no errors occur, the code in the <tt CLASs="monofont">except</tt> block is ignored.</p>
<P>The <TT Class="monofont">raise</TT> statement is used to signal an exception. When raising an exception, you can use one of the built-in exceptions, like this:</P>
<Pre>
raise RuntimeError, "Unrecoverable error" </pre>
<p>Or you can create your own exceptions, as described in the section ⌨️ 快捷键说明
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