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
  [4444] Elements of an exception in Python - try, except, else, finally - (2015-02-28)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [3930] Reporting the full stack trace when you catch a Python exception - (2012-11-22)
  [3913] How many times ... has this loco headed west through Tenby? - Python exceptions - (2012-11-05)
  [3664] Error checking in a Python program - making your program robust via exceptions - (2012-03-22)
  [3441] Pressing ^C in a Python program. Also Progress Bar. - (2011-09-15)
  [3433] Exceptions - a fail-safe way of trapping things that may go wrong - (2011-09-11)
  [3177] Insurance against any errors - Volcanoes and Python - (2011-02-19)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2622] Handling unusual and error conditions - exceptions - (2010-02-03)
  [2408] Robust user input (exception handling) example in Python - (2009-09-17)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2281] Python - using exceptions to set a fallback - (2009-07-12)
  [2018] UnboundLocalError - Python Message - (2009-01-31)
  [1236] Trying things in Python - (2007-06-18)
  [1042] Nested exceptions in Python - (2007-01-18)

Back to Bridging to the customer requirement

Previous and next or Horse's mouth home

Forward to Central London Courses - Perl, PHP, Python, Tcl, MySQL

Feast or famine
Managing conflict and disappointment
Overloading of operators on standard objects in Python
Central London Courses - Perl, PHP, Python, Tcl, MySQL
Exceptions in Python
Bridging to the customer requirement
Getting the lighting right
A word of admiration for the London cabbie
A Strengthened City
What brings people to my web site?