Robust user input (exception handling) example in Python

Archive - Originally posted on "The Horse's Mouth" - 2009-09-17 20:02:52 - Graham Ellis
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 ...

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Robust user input (exception handling) example in Python