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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.
Exception handling in Tcl

Exceptions are a wonderful fail safe way of trapping errors in most modern programming languages. Although the natural way for a newcomer to programming to handle unusual situations is to test for each possible situation, that becomes a guessing game trying to work out everything that may happen, and things can get overlooked. But with exceptions, you're saying "if this doesn't work out for any reason, please jump to the following recovery code."

You would think with a language such as Tcl that's approaching 30 years old, exceptions might be missing - but you're only half right in that thought. For Tcl uses a catch command to run another command (or block of commands) under a subsidiary control, returning a true value if something hasn't functioned in the normal way. You can specify a variable to grab the system message telling you what didn't work as well. So
   if {[catch {set many [expr $value * 100]} excuse]} {
will return a false value if the calculation works, and a true value (together with an explanation of the reason in excuse it it fails. Full example [here].

With an exception caught in Tcl in that way, you can continue the program running rather than have it exit with an error message - and that's a big benefit in Tcl, where syntax checking only happens as the code runs, so a lot of time may be lost by a program that crashes out towards the end of a long run.
(written 2013-11-14, updated 2013-11-16)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T214 - Tcl/Tk - Other Facilities in Tcl
  [239] What and why for the epoch - (2005-03-08)
  [364] pu daily and p hourly - (2005-06-30)
  [366] Error handling in Tcl through catch - (2005-07-02)
  [461] Shortened interactive commands - (2005-10-11)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4525] What does Tcl do if you try to run a command that is not defined? - (2015-10-10)
  [4762] Coverage map in Tcl - how many times has each proc been called? - (2017-09-28)


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Some other Articles
Handling JSON in Python (and a csv, marshall and pickle comparison)
If elif elif elif - multiway selection in Python
Lists in Tcl - fundamentals in a commented source code example
Tcl + Tk (Wish) - an introduction and revision example
Exception handling in Tcl
Writing the perfect program in Tcl?
Regular Expression Substitution - Tcl
With the Shpigs and Donkeys
A weekend away - Four in a Bed returns to Happy Donkey Hill
Excellent Railfuture conference, but some setup lessons
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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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