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<H1><A name=SECTION001800000000000000000>Dealing with the imperfect (or how to 
handle errors)</A> </H1>
<P>So you now have the perfect program, it runs flawlessly, except for one 
detail, it will crash on invalid user input. Have no fear, for Python has a 
special control structure for you. It's called <CODE>try</CODE> and it tries to 
do something. Here is an example of a program with a problem: <PRE>print "Type Control C or -1 to exit"
number = 1
while number != -1:
    number = int(raw_input("Enter a number: "))
    print "You entered: ",number
</PRE>
<P>Notice how when you enter <CODE>@#&amp;</CODE> it outputs something like: <PRE>Traceback (innermost last):
  File "try_less.py", line 4, in ?
    number = int(raw_input("Enter a number: "))
ValueError: invalid literal for int(): @#&amp;
</PRE>
<P>As you can see the <CODE>int</CODE> function is unhappy with the number 
<CODE>@#&amp;</CODE> (as well it should be). The last line shows what the 
problem is; Python found a <CODE>ValueError</CODE>. How can our program deal 
with this? What we do is first: put the place where the errors occurs in a 
<CODE>try</CODE> block, and second: tell Python how we want 
<CODE>ValueError</CODE>s handled. The following program does this: <PRE>print "Type Control C or -1 to exit"
number = 1
while number != -1:
    try:
        number = int(raw_input("Enter a number: "))
        print "You entered: ",number
    except ValueError:
        print "That was not a number."
</PRE>
<P>Now when we run the new program and give it <CODE>@#&amp;</CODE> it tells us 
``That was not a number.'' and continues with what it was doing before. 
<P>When your program keeps having some error that you know how to handle, put 
code in a <CODE>try</CODE> block, and put the way to handle the error in the 
<CODE>except</CODE> block. 
<P>
<H1><A name=SECTION001810000000000000000>Exercises</A> </H1>
<P>Update at least the phone numbers program so it doesn't crash if a user 
doesn't enter any data at the menu. 
<P>
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<ADDRESS>Josh Cogliati <A 
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