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<FONT><h3>Code Examples</h3>
<p>This first example returns the square root of a given input value. If the input value is negative or if it is a character, two traceback messages are displayed.</p>
<H5>
Listing 4.1 Square root (File squareroot.py)</H5>
<PRe claSS="monofont">
1: ###
2: # Program: Square root
3: # Author: Andre S Lessa
4: ###
5:
6: ### import modules
7:
8: import sys, traceback, math
9:
10: try:
11: n = float(raw_input("Please, enter a number: "))
12: print "The sqrt of %f is %f" % (n, math.sqrt(n))
13:
14: except (ValueError, TypeError, OverflowError):
15: print "-----------------------------------------"
16: print "This is the standard traceback message:"
17: print ""
18: traceback.print_exc()
19:
20: print "-----------------------------------------"
21: print "This is the customized traceback message:"
22: print ""
23: info = sys.exc_info()
24: exc_type = info[0]
25: exc_value = info[1]
26: exc_traceback = info[2]
27:
28: trace = traceback.extract_tb(sys.exc_traceback)
29: print "Exception Type: ", exc_type
30: print "Error Message: ", exc_value
31: print "File name: ", trace[0][0]
32: print "Error message: ", trace[0][1]
33: print "Line: ", trace[0][2]
34: print "Function: ", trace[0][3]
35: else:
36: print "Everything went just fine."
</PRe>
<p>The <tt CLASs="monofont">except</tt> clause in line 14 covers <tt class="monofont">ValueError, OverflowError,</tt> and <tt class="monofont">TypeError</tT> exceptions.</p>
<p>The <Tt clAss="monofont">else</tt> clause in line 35 is only executed when no exception is raised.</P>
<p>The next lines show the two traceback messages that are displayed by this program: Python standard traceback message and a customized version.</p>
<pRE>
C:\python> s:\python\squareroot.py
Please, enter a number: i
-----------------------------------------
This is the standard traceback message:
Traceback (innermost last):
File "s:\python\squareroot.py", line 11, in ?
n = float(raw_input("Please, enter a number: "))
ValueError: invalid literal for float(): i
-----------------------------------------
This is the customized traceback message:
Exception Type: exceptions.ValueError
Error Message: invalid literal for float(): i
File name: s:\python\squareroot.py
Error message: 11
Line: ?
Function: n = float(raw_input("Please, enter a number: "))
</PRe>
<p>This example uses multiple <tt CLASs="monofont">except</tt> clauses (lines 17 and 20). It also takes advantage of the <tT CLAss="monofont">assert</tt> command to raise a debug exception (line 15).</P>
<H5>
Listing 4.2 Internet country codes (File countrycode.py)</H5>
<Pre class="monofont">
1: ###
2: # Program: Country code
3: # Author: Andre S Lessa
4: ###
5:
6: ### import modules
7:
8: import sys, string
9:
10: matrix = { "brazil":"br","france":"fr","argentina":"ar","usa":"us"}
11:
12: def getcode(country):
13: try:
14: data = matrix[string.lower(country)]
15: assert data != "br", "You cannot select this country " + }
"for this action!"
16: return data
17: except KeyError:
18: print sys.exc_type, ":", "%s is not in the list." % }
sys.exc_value
19: print
20: except AssertionError, b:
21: print b
22: print
23:
24: while 1:
25: country = raw_input("Enter the country name or press x to exit: ")
26: if country == "x":
27: break
28: code = getcode(country)
29: if code != None:
30: print "%s's country code is %s" % (country, code)
31: print
</pre>
<p>The following <i>screen dump</i> shows the execution of this program. Note that the program doesn't end after an exception has been raised.</p>
<pre>
C:\>python s:\python\ countrycode.py
Enter the country name or press x to exit: Mexico
exceptions.KeyError : mexico is not in the list.
Enter the country name or press x to exit: USA
USA's country code is us
Enter the country name or press x to exit: Brazil
You cannot select this country for this action!
Enter the country name or press x to exit: Argentina
Argentina's country code is ar
Enter the country name or press x to exit: x
C:\Python>
</pre>
<P>See more <i>exception handling</i> cases in the final section of the next chapter.</P>
</fonT>
<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 1/30/2002<br>Python Developer's Handbook, © 2002 Sams Publishing</font></TD></TR></TABLE></P>
<TABLE border=0 width="100%" cellspacing=0 cellpadding=0><TR><td align=left width="15%" class="headingsubbarbg"><a href="100.html" title="Summary"><font size="1">< BACK</font></a></td><td align=center width="70%" class="headingsubbarbg"><font size="1"><a href="popanote.asp?pubui=oreilly&bookname=0672319942&snode=101" target="_blank" title="Make a public or private annnotation">Make Note</a> | <a href="101.html" title="Use a Safari bookmark to remember this section">Bookmark</a></font></td><td align=right width="15%" class="headingsubbarbg"><a href="103.html" title="5. Object-Oriented Programming"><font size="1">CONTINUE ></font></a></td></TR></TABLE>
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