Rather than having to check ahead of time for every possible error, Python provides an exception handling capability too. Simply try to perform you action, and define what's to be done in an except block if the action you want can't be completed.

Here's an example in which we (try to) read an integer value from the user. If our user types a value that's not an integer, the exception is caught and the prompt repeated

def gnum(prompt):

   while 1:

      try:

         rv = int(input(prompt+": "))

         break

      except:

         print "eh?"

   return rv



first = gnum("First number")

second = gnum("Second number")

print "Sum is "+str(first+second)

By the way - note the use of a named block of code (a function) to avoid the need to repeat code. Good programming practise!
(written 2005-07-17, updated 2006-06-05)

 Y109 - Python - Exceptions
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  [2018] UnboundLocalError - Python Message - (2009-01-31)
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