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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
List Comprehensions in Python

How do you perform an operation on every member of a list, producing a new list? A List Comprehension in Python is a structure in which a for loop is written within a list's square brackets. Its purpose is to allow the programmer to write short code that's used to transform each element of a source list, generating a new list without the need to append new items one by one to that new list. Here are some examples:

squares = [x*x for x in range(10)]
print squares
# result - [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]


and

breakfast = ["Orange Juice", "Cereal", "Actimel"]
size2 = [len(x) for x in breakfast]
print size2
# result - [12, 6, 7]


One of the more conventional alternatives to a list comprehension is a map function call (but that's more limited). Here is that example we have just see re-coded in the form of a map for comparison:

size1 = map(len,breakfast)
print size1
# result - [12, 6, 7]


But here are some examples that wouldn't be so easy with a map - if (for example) you wanted to run a method on each object in a list, rather than a function on each member:

shout = [x.upper() for x in breakfast]
print shout
# result - ['ORANGE JUICE', 'CEREAL', 'ACTIMEL']


And here's another feature - you can add an if clause onto a list comprehension which allows you to select (filter out) certain records if you wish:

shout = [x.upper() for x in breakfast if x != "Cereal"]
print shout
# result - ['ORANGE JUICE', 'ACTIMEL']


Full source code of this example - here
(written 2008-11-06, updated 2008-11-07)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y111 - Python - More on Collections and Sequences
  [61] Python is a fabulous language - (2004-09-24)
  [386] What is a callback? - (2005-07-22)
  [633] Copying a reference, or cloning - (2006-03-05)
  [899] Python - extend v append on a list - (2006-10-20)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [1310] Callbacks - a more complex code sandwich - (2007-08-19)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [2894] Sorting people by their names - (2010-07-29)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3150] Python dictionaries - mutable and immutable keys and values - (2011-01-29)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3797] zip in Python - (2012-07-05)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)
  [4442] Mutable v Immuatble objects in Python, and the implication - (2015-02-24)


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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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