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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
What are exceptions - Python based answer

How do you check for run time errors in your program? You'll include tests with if statements in almost every program you write, to ensure that user entered data is reasonable / as you would expect ... but you will also need to do more that that. You'll need to check whether a system function has run correctly - "Did that file open" type questions. One very commonly used approach to error checking of this sort is to call functions that return a special value (a cardinal value) in the event of an error - something that can never be a correct answer - and check for that value after every call. You'll see that approach used very widely indeed, especially in older languages.

But there's an alternative - the exception. With exceptions, you write code in a block that says "try" to perform a particular task, and if something goes wrong an exception is thrown and the code jumps to an exception / error recovery block. What are the advantages of exceptions:

a) There's no need to use cardinal values that may turn out to be not so improssible in the normal run of events anyway

b) You don't have to program for every possible error individually - you can lay out a safety net to catch any errors that occur within a certain area of the code irrespective of what they are

c) You have a need mechanism for correcting and bringing the code back together rather than having to code with lower level if tests.

Here's an example in Python. For clarity and completeness, in this program we raise the exception ourselves in the howlarge function rather than allowing the built in function called list to do so:

def howlarge(mylist):
   if isinstance(mylist,list):
      size = len(mylist)
   else:
      raise TypeError,"Only works on lists"
   return size


The main code for this short example is a single try block, in which we call the howlarge function multiple times. It will fail on the fourth of five calls, since the function traps any calls in which the object passed to it is not a llist - so you get three sets of results followed by an error message. You'll notice that if an exception is thrown, the code jumps to an appropriate handler and the rest of the try block is skipped.

try:
   print (howlarge([5,6,5,6,7]))
   print (howlarge([5,6,7]))
   print (howlarge([]))
   print (howlarge(14))
   print (howlarge([5,6,7]))
except StandardError,e:
   print "Oh - never mind ...."
   print e
print "Program Completed"


Once the except block has been completed (or the try block completed if no exception was thrown), the code runs from the end of the structure - in this case simply stating "Program Completed"

Dorothy:slee_liv grahamellis$ python oy
5
3
0
Oh - never mind ....
Only works on lists
Program Completed
Dorothy:slee_liv grahamellis$


Full source here. You'll find more on exceptions in Python, exceptions in C++, exceptions in Ruby and exceptions in Java here.
(written 2008-11-08)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
R111 - Ruby - Exceptions.
  [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)
  [2622] Handling unusual and error conditions - exceptions - (2010-02-03)
  [3177] Insurance against any errors - Volcanoes and Python - (2011-02-19)
  [3260] Ruby - a training example that puts many language elements together to demonstrate the whole - (2011-04-23)
  [3433] Exceptions - a fail-safe way of trapping things that may go wrong - (2011-09-11)
  [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)

R109 - Ruby - Strings and Regular Expressions
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)
  [986] puts - opposite of chomp in Ruby - (2006-12-15)
  [987] Ruby v Perl - interpollating variables - (2006-12-15)
  [1195] Regular Express Primer - (2007-05-20)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1588] String interpretation in Ruby - (2008-03-21)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1891] Ruby to access web services - (2008-11-16)
  [2293] Regular Expressions in Ruby - (2009-07-16)
  [2295] The dog is not in trouble - (2009-07-17)
  [2608] Search and replace in Ruby - Ruby Regular Expressions - (2010-01-31)
  [2614] Neatly formatting results into a table - (2010-02-01)
  [2623] Object Oriented Ruby - new examples - (2010-02-03)
  [2980] Ruby - examples of regular expressions, inheritance and polymorphism - (2010-10-02)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [3621] Matching regular expressions, and substitutions, in Ruby - (2012-02-23)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [3758] Ruby - standard operators are overloaded. Perl - they are not - (2012-06-09)
  [4388] Global Regular Expression matching in Ruby (using scan) - (2015-01-08)
  [4505] Regular Expressions for the petrified - in Ruby - (2015-06-03)
  [4549] Clarrissa-Marybelle - too long to really fit? - (2015-10-23)

J712 - Java - Exceptions
  [1066] Final, Finally and Finalize - three special words in Java - (2007-02-05)
  [2420] Exceptions in Java - why and how - (2009-09-24)
  [2862] Fail Safe Error Handling in Java via Exceptions - (2010-07-09)
  [3045] After Course Resources - do we publish sample answers. Example from Java Exceptions module. - (2010-11-13)
  [3048] String handling - from first steps to practical examples - (2010-11-13)
  [4350] Keyboard reader for Java programming newcomers - (2014-12-12)
  [4396] Java Utility class - flexible replacement for array. Also cacheing in objects and multiple catch clauses example. - (2015-01-16)

C236 - C and C based languages - Exceptions
  [799] The case for exceptions - (2006-07-11)
  [3068] Throwing your own exception in C++, and catching it - (2010-11-24)
  [3509] Operator Overloading, Exceptions, Pointers, References and Templates in C++ - new examples from our courses - (2011-11-06)


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Dynamic code - Python
Pascals Triangle in Python and Java
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Python Regular Expressions
What are exceptions - Python based answer
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List Comprehensions in Python
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Optional and named parameters in Python
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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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