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This week, we're updating our course layouts and descriptions. Presentation and materials always gently change over time, but just occasionally there's a need to make a step change to clear out some of the old and roll in the new. That's now happening - but over a long and complex site it's not instant and you'll see sections of the site changing up to and including 19th September.

See also [here] for status update
 
Robust user input (exception handling) example in Python

One of the questions in the "exceptions" section of the Python Course asks my delegates to "Graham Proof" a piece of code:

first = int(input("First number: "))
second = int(input("Second number: "))
print "Sum is "+str(first+second)


The idea is that I come round the room and put really awkward inputs into customer's programs, as would a hacker or a non-thinking user. Not only is there a need to trap exceptions, but input should be changed to raw_input, and the input / error handling delegated to a function so that an error on the second input doesn't push the user back to remaking the first input, nor does it result in code duplication. Here is what I came up with:

def getval(pr):
 while 1:
  try:
    result = raw_input(pr+" ")
    val = float(result)
    return val
  except EOFError:
    print "Ran out of Data"
    exit()
  except:
    print "nah"
 
first = getval("first number")
second = getval("second number")
 
print "Sum is "+str(first+second)


A further comment on the EOFError. On some systems, you can continue to read even if you've received an EOF - RedHat Fedora Linux being one of them. On others, such as my Mac with OS X, reading past the end of file results in the code returning a further EOF without checking / waiting so see if there's any more data. My code above deals with the case on both types of operating system ...
(written 2009-09-17, updated 2009-09-20)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y109 - Python - Exceptions
  [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)
  [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)
  [381] Exceptions in Python - (2005-07-17)


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Autumn Fruits and Bristol Old Station
Further North - long summer days and lovely countryside
Removal of technical resources from this site
TypeError: super() argument 1 must be type, not classobj (Python)
Robust user input (exception handling) example in Python
Testing code in Python - doctest, unittest and others
Pound Sign in Python Program
But I am reading from a file - no need to prompt (Perl)
Low Sun - Autumn is here
Hotel Booking Scam / Cost of calls to 070 numbers
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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