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)

 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)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1310] Callbacks - a more complex code sandwich - (2007-08-19)
  [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)

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