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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
G504 - Well House Consultants - Writing Notes
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  [2812] What is Perl? - (2010-06-15)
  [2414] Hello World - a good traditional start to a Java course - (2009-09-22)
  [2192] Copy writing - allowing for the cut - (2009-05-21)
  [2010] How long should a training module be? - (2009-01-27)
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Y105 - Python - Functions, Modules and Packages
  [4212] Python functions - an introduction to how they work - (2013-11-16)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [3945] vargs in Python - how to call a method with unknown number of parameters - (2012-12-06)
  [3931] Optional positional and named parameters in Python - (2012-11-23)
  [3885] Default local - a good choice by the author of Python - (2012-10-08)
  [3852] Static variables in Python? - (2012-08-29)
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  [3695] Functions are first class variables in Lua and Python - (2012-04-13)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3474] Python Packages - groupings of modules. An introduction - (2011-10-11)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3459] Catching the fishes first? - (2011-09-27)
  [3280] Passing parameters to Python functions - the options you have - (2011-05-07)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
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  [2766] Optional and named parameters to Python functions/methods - (2010-05-15)
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  [340] Code and code maintainance efficiency - (2005-06-08)
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  [303] Lambdas in Python - (2005-05-06)
  [294] Python generator functions, lambdas, and iterators - (2005-04-28)
  [105] Distance Learning - (2004-10-31)
  [96] Variable Scope - (2004-10-22)


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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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