As part of a previous post, I was looking at the "Internal Server Errors" logged on our web server over the past 3 months ... and I found one coming from a Python / CGI demonstration which I wrote and uploaded for a delegate a couple of months back.

Internal server error 500 (by default on Apache httpd) gives little away to the web site visitor - it just says "this page is broken" really. And that's sensible - by displaying more data, it could give the recipient of the error message clues as to the structure, and thus possible weaknesses, or the web site. However, the error message will have been logged in the server's error log file so that the developer / web site admin / maintainer can go back and find out a little more.

It turned out that the server error was being reported when script was called up directly rather than via the form that was intended to go with it, and so it tried to match a regular expression in a variable that wasn't a string. The failure was in

  strikes = words.findall(fill["stuff"])

The first (and obvious) thought is "let's add a test to make sure that doesn't happen again". But to do so would fix the immediate problem without giving consideration to any other problems that might occur. In other words - adding a test is not fail safe. What I have elected to do instead is to catch an exception. Catching an excpetion lets me deal with all the things that might go wrong in the regular expression match, including those I haven't even though of. So it IS a fail safe approach. Here is the code with the exception handler added:

  try:
    strikes = words.findall(fill["stuff"])
  except:
    strikes = ["'no input'"]

The form is [here] and the (now modified) complete script is [here].

On last week's Ruby Programming Course, we also looked at exceptions. There's a nice, several stage example in the course notes that shows ...
• An example which will crash is the file does not exist - [here]
• The same example, but with any errors which are encountered being rescued - [here]
• A final example of exceptions, showing how you can segment different exception types into different handlers, yet still have the safety net of all exceptions being caught. Source code [here].
(written 2011-09-11)

 Y109 - Python - Exceptions
  [381] Exceptions in Python - (2005-07-17)
  [1042] Nested exceptions in Python - (2007-01-18)
  [1236] Trying things in Python - (2007-06-18)
  [2018] UnboundLocalError - Python Message - (2009-01-31)
  [2281] Python - using exceptions to set a fallback - (2009-07-12)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2408] Robust user input (exception handling) example in Python - (2009-09-17)
  [2622] Handling unusual and error conditions - exceptions - (2010-02-03)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [3177] Insurance against any errors - Volcanoes and Python - (2011-02-19)
  [3441] Pressing ^C in a Python program. Also Progress Bar. - (2011-09-15)
  [3664] Error checking in a Python program - making your program robust via exceptions - (2012-03-22)
  [3913] How many times ... has this loco headed west through Tenby? - Python exceptions - (2012-11-05)
  [3930] Reporting the full stack trace when you catch a Python exception - (2012-11-22)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4444] Elements of an exception in Python - try, except, else, finally - (2015-02-28)

R111 - Ruby - Exceptions.
  [1875] What are exceptions - Python based answer - (2008-11-08)
  [2615] String to number conversion with error trapping in Ruby - (2010-02-01)
  [2620] Direct access to object variable (attributes) in Ruby - (2010-02-02)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [3260] Ruby - a training example that puts many language elements together to demonstrate the whole - (2011-04-23)
  [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12)
  [4008] Reading and checking user inputs - first lessons - Ruby - (2013-02-17)
  [4675] Exceptions in Ruby - throwing, catching and using - (2016-05-17)

Back to 3 digit HTTP status codes - what are they, which are most common, which should be a concern?

Previous and next or Horse's mouth home

Forward to Sundays - and over eating

Light bulbs
Moving from scripting to Object Orientation in Python
Sundays - and over eating
Exceptions - a fail-safe way of trapping things that may go wrong
3 digit HTTP status codes - what are they, which are most common, which should be a concern?
Ruby at both extremes of your website
Sigils - the characters on the start of variable names in Perl, Ruby and Fortran
Searching through all the files in or below a directory - Ruby, Tcl, Perl
How many days to Christmas?