Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Python and Tcl - public course schedule [here]
Private courses on your site - see [here]
Please ask about maintenance training for Perl, PHP, Lua, etc
 
Callbacks - a more complex code sandwich

When you write a piece of code, you're normally putting the filling into the sandwich; there's a built-in program in your computer that controls the loading and running of the code thst you've written, and there is a whole library of standard pieces of code that you call to perform the low level operations. So thus you provide the sandwich filling.

A program as simple as
print "Hello World"
(and that will work in Perl, Python or PHP amongst others) illustrates this - the operating systen runs it, and the built in function within the language chosen deals with the detail of output.

But there are a few occasions where the mechanism gets a bit more complex - where one of the functions itself calls BACK to your code. Let's say you're sorting, but you don't want to write your own sort algorithm. How will that work? Well - you can call the sort function but then how do you tell it whether record a comes before or after record b? The easiest answer is to pass the name of a function that you've written to answer just such a question into the language's sort routine and have it callback to your code.

During last week's course, I was describing this mechanism and in order to make it clearer, I wrote my own piece of code to do the sorting ... but then took that out to use Python's own sort routine. I've posted the example up to our longer Python examples - you can see both the version that uses the callback, and my example of what really happens inside.

Callbacks are used in most programming languages, but not usually all that often. Big uses are sorting, and for even handling on GUIs (Graphic User Interfaces).
(written 2007-08-19, updated 2007-08-21)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P307 - Perl/Tk
  [3009] Expect in Perl - a short explanation and a practical example - (2010-10-22)
  [1340] Tk locks up - 100% c.p.u. on a simple program (Tcl, Perl, Python) - (2007-09-09)
  [738] (Perl) Callbacks - what are they? - (2006-05-30)
  [599] Perl/Tk real time display - (2006-02-10)
  [596] The magic of -textvariable - (2006-02-08)
  [595] Add a friendly front end with Tk - (2006-02-08)

Y111 - Python - More on Collections and Sequences
  [4442] Mutable v Immuatble objects in Python, and the implication - (2015-02-24)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)
  [3797] zip in Python - (2012-07-05)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3150] Python dictionaries - mutable and immutable keys and values - (2011-01-29)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2894] Sorting people by their names - (2010-07-29)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [1873] List Comprehensions in Python - (2008-11-06)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [899] Python - extend v append on a list - (2006-10-20)
  [633] Copying a reference, or cloning - (2006-03-05)
  [386] What is a callback? - (2005-07-22)
  [61] Python is a fabulous language - (2004-09-24)


Back to
Dates for Easter - 2008 to 2015
Previous and next
or
Horse's mouth home
Forward to
What do people look for on a hotel web site?
Some other Articles
Business travel by train in the USA
Tratum Technologies
Some one line Perl tips and techniques
What do people look for on a hotel web site?
Callbacks - a more complex code sandwich
Dates for Easter - 2008 to 2015
Good to be home
Troy, up state New York
Python class rattling around
Regular expressions made easy - building from components
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


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.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2019: 404 The Spa • Melksham, Wiltshire • United Kingdom • SN12 6QL
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/1310_Cal ... dwich.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat