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<h3>Defining New Exceptions</h3>
<p>All the built-in exceptions are defined in terms of classes. To create a new exception, create a new class definition that inherits from <tT CLAss="monofont">exceptions. Exception</tt>  such as the following:</P>

<PRE>

import exceptions 
# Exception class 
class NetworkError(exceptions.Exception): 
     def _ _init_ _(self,args=None): 
         self.args = args </pre>

<p>The name <TT CLass="monofont">args</tt>  should be used as shown. This allows the value used in the <tt class="monofont">raise</tt> statement to be properly printed in tracebacks and other diagnostics. In other words,</p>

<pre>

raise NetworkError, "Cannot find host." </prE>

<p>creates an instance of <tT claSs="monofont">NetworkError</tt>  using the call</p>

<Pre>

NetworkError("Cannot find host.") </pRE>

<P>The object that is created will print itself as <Tt claSS="monofont">NetworkError: Cannot find host</TT>. If you use a name other than <tt clASS="monofont">self.args</Tt>  or you don抰 store the argument, this feature won抰 work correctly.</p>

<p>When an exception is raised, the optional value supplied in the <tT CLAss="monofont">raise</tt>  statement is used as the argument to the exception抯 class constructor. If the constructor for an exception requires more than one argument, it can be raised in two ways:</p>

<pre>

import exceptions 
# Exception class 
class NetworkError(exceptions.Exception): 
     def _ _init_ _(self,errno,msg): 
     self.args = (errno, msg) 
     self.errno = errno 
     self.errmsg = msg 

# Raises an exception (multiple arguments) 
def error2(): 
     raise NetworkError(1, 'Host not found') 

# Raises an exception (multiple arguments) 
def error3(): 
     raise NetworkError, (1, 'Host not found') </pre>

<p>Class-based exceptions enable you to create hierarchies of exceptions. For instance, the <tt class="monofont">NetworkError</tT>  exception defined earlier could serve as a base class for a variety of more specific errors. For example:</p>

<pRe>

class HostnameError(NetworkError): 
    pass 

class TimeoutError(NetworkError): 
    pass 

def error3(): 
    raise HostnameError 

def error4(): 
    raise TimeoutError 

try: 
    error3() 
except NetworkError: 
    import sys 
    print sys.exc_type    # Prints exception type </prE>

<p>In this case, the <tt cLass="monofont">exceptNetworkError</TT> statement catches any exception derived from <TT clasS="monofont">NetworkError</TT>. To find the specific type of error that was raised, examine the variable <Tt claSS="monofont">sys.exc_type</TT>. Similarly, the <tt clASS="monofont">sys.exc_value</Tt>  variable contains the value of the last exception. Alternatively, the <tt class="monofont">sys.exc_info()</tt> function can be used to retrieve exception information in a manner that doesn抰 rely on global variables and is thread-safe.</p>
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<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, &copy;&nbsp;2002 New Riders Publishing</font></TD></TR></TABLE></P>
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