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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.
Sample code with errors in it on our web site

Feedback is lifeblood ... ignore it at your peril, and remember that for each person who lets you know you have a problem, a further ten will have noticed and not said anything. From my mailbox:

> Your webpage
>
> http://www.wellho.net/resources/ex.php4?item=y105/locvar.py
>
> contains the following code ...
>
> [snip]
>
> which is erroneous.

I totally agree. It does generate an error in the last line. Why?

The immediate answer is that variables used within defined functions / methods in Python are local to those functions, and unless returned or declared global are NOT available outside in the calling code. When you think about it for a minute of two, this is an excellent default behavior as it means that you don't clutter up your main variable name space with internal names from other piece of code you have packaged into self-contained units. If your language uses "default global" which means that variables are shared unless you declare otherwise, you can end up with the most enormous problems when you come to build an application using code blocks (functions / methods / objects) from several sources and they have a variable name conflict. Perl's use strict pragma is an example of a language that is "default global" but has an addition to let you circumvent the problems it causes.

There is a second very different answer too if you are asking why we have erroneous code on our site. Many of the examples on the site come from our training courses, and were initially uploaded as a service to our delegates - so that they could try out the code after the course without the need to retype it. Some of the examples intentionally show the sort of things that generate error messages, and the notes for the course go through the "why" and "wherefore" of that - indeed, on of the most important things you learn on a programming course is how to 'read' the error messages - interpreting what they mean - so of necessity there will be failing examples in the notes, thus on our site.

Jxx's feedback is doubly useful - it alerted me to the fact that a page put up for a very good reason has now become much more visible, and that the explanation that was offered further on in the page:
>>> Remember that some of our examples show you how not to do things - check in your notes.
was far too well buried, and generic.

Rather than change an example which is there for good cause, and forms part of our Python Programming and learning to program in Python courses, I have added some comments to say that it shows how an error is generated, and why. Always good to comment your code ;-) and hopefully much clearer now. It will, though, need to be revised further in the near future to take accocunt of Python 3 ;-)

Jxx continued:

> I'm not trying to be a smart-ass since python is certainly not
> my best language but you really shouldn't have erroneous code
> advertising your expertise.

Totally agree (with the conclusion - he probably knows a lot more Python that he lets on!) - and "Thank you" letter sent. The page, if you would care to click here still illustrates the error, but now clearly explains why the code doesn't work. Which is what you want to learn on a training course.
(written 2009-10-29, updated 2009-10-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y105 - Python - Functions, Modules and Packages
  [96] Variable Scope - (2004-10-22)
  [105] Distance Learning - (2004-10-31)
  [294] Python generator functions, lambdas, and iterators - (2005-04-28)
  [303] Lambdas in Python - (2005-05-06)
  [308] Call by name v call by value - (2005-05-11)
  [340] Code and code maintainance efficiency - (2005-06-08)
  [386] What is a callback? - (2005-07-22)
  [418] Difference between import and from in Python - (2005-08-18)
  [561] Python's Generator functions - (2006-01-11)
  [668] Python - block insets help with documentation - (2006-04-04)
  [745] Python modules. The distribution, The Cheese Shop and the Vaults of Parnassus. - (2006-06-05)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [775] Do not duplicate your code - (2006-06-23)
  [821] Dynamic functions and names - Python - (2006-08-03)
  [900] Python - function v method - (2006-10-20)
  [912] Recursion in Python - (2006-11-02)
  [913] Python - A list of methods - (2006-11-03)
  [949] Sludge off the mountain, and Python and PHP - (2006-11-27)
  [959] It's the 1st, not the 1nd 1rd or 1th. - (2006-12-01)
  [1134] Function / method parameters with * and ** in Python - (2007-04-04)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)
  [1464] Python Script - easy examples of lots of basics - (2007-12-08)
  [1784] Global - Tcl, PHP, Python - (2008-09-03)
  [1790] Sharing variables with functions, but keeping them local too - Python - (2008-09-09)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1871] Optional and named parameters in Python - (2008-11-05)
  [1879] Dynamic code - Python - (2008-11-11)
  [2011] Conversion of OSI grid references to Eastings and Northings - (2009-01-28)
  [2439] Multiple returns from a function in Python - (2009-10-06)
  [2440] Optional parameters to Python functions - (2009-10-07)
  [2506] Good example of recursion in Python - analyse an RSS feed - (2009-11-18)
  [2520] Global and Enable - two misused words! - (2009-11-30)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [2766] Optional and named parameters to Python functions/methods - (2010-05-15)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [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)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3280] Passing parameters to Python functions - the options you have - (2011-05-07)
  [3459] Catching the fishes first? - (2011-09-27)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3474] Python Packages - groupings of modules. An introduction - (2011-10-11)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3695] Functions are first class variables in Lua and Python - (2012-04-13)
  [3766] Python timing - when to use a list, and when to use a generator - (2012-06-16)
  [3852] Static variables in Python? - (2012-08-29)
  [3885] Default local - a good choice by the author of Python - (2012-10-08)
  [3931] Optional positional and named parameters in Python - (2012-11-23)
  [3945] vargs in Python - how to call a method with unknown number of parameters - (2012-12-06)
  [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)
  [4212] Python functions - an introduction to how they work - (2013-11-16)
  [4361] Multiple yields and no loops in a Python generator? - (2014-12-22)
  [4407] Python - even named code blocks are objects - (2015-01-28)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4441] Reading command line parameters in Python - (2015-02-23)
  [4448] What is the difference between a function and a method? - (2015-03-04)
  [4645] What are callbacks? Why use them? An example in Python - (2016-02-11)
  [4662] Recursion in Python - the classic example - (2016-03-07)
  [4719] Nesting decorators - (2016-11-02)
  [4722] Embedding more complex code into a named block - (2016-11-04)
  [4724] From and Import in Python - where is the module loaded from? - (2016-11-06)

G504 - Well House Consultants - Writing Notes
  [4] Seeing the wood for the trees. - (2004-08-06)
  [7] Writing on a Sunday - (2004-08-08)
  [9] Study room - the Oxford train - (2004-08-10)
  [394] A year on - should we offer certified PHP courses - (2005-07-28)
  [398] Training course material - why we write our own - (2005-07-30)
  [407] Theft of training material - (2005-08-09)
  [797] Writing up new C / C++ notes. - (2006-07-09)
  [979] Empty seats, Nodding Donkeys and buses - (2006-12-11)
  [982] Notes from the white board - (2006-12-14)
  [1950] Copyright of Training Notes and Web Site - (2008-12-18)
  [2010] How long should a training module be? - (2009-01-27)
  [2192] Copy writing - allowing for the cut - (2009-05-21)
  [2414] Hello World - a good traditional start to a Java course - (2009-09-22)
  [2812] What is Perl? - (2010-06-15)
  [2828] Sharing our programs - easy. Sharing our data - harder. - (2010-06-26)
  [3152] Jargon busting - (2011-01-30)
  [4009] Clear, concise examples - Ruby classes and objects. - (2013-02-17)
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)


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Some other Articles
How do I set up a constant in Python?
Finding text and what surrounds it - contextual grep
Clustering on Tomcat
Load balancing with sticky sessions (httpd / Tomcat)
Sample code with errors in it on our web site
Pantomimes around Melksham - 2009/2010 season
Accidentally typed ci rather than vi?
How did I do THAT?
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Tcl - uplevel to run code at calling level
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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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