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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.
Exceptions in Python

Rather than having to check ahead of time for every possible error, Python provides an exception handling capability too. Simply try to perform you action, and define what's to be done in an except block if the action you want can't be completed.

Here's an example in which we (try to) read an integer value from the user. If our user types a value that's not an integer, the exception is caught and the prompt repeated


def gnum(prompt):
while 1:
try:
rv = int(input(prompt+": "))
break
except:
print "eh?"
return rv

first = gnum("First number")
second = gnum("Second number")
print "Sum is "+str(first+second)


By the way - note the use of a named block of code (a function) to avoid the need to repeat code. Good programming practise!
(written 2005-07-17, updated 2006-06-05)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y109 - Python - Exceptions
  [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)
  [3433] Exceptions - a fail-safe way of trapping things that may go wrong - (2011-09-11)
  [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)


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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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