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Copy a list

Xx r3negade
39 New Member
Consider the following code:
Expand|Select|Wrap|Line Numbers
  1. listA = [['a', 'b', 'c', 'd'], ['e', 'f', 'g', 'h'], ['i', 'j', 'k', 'l']]
  2. changeCoordinates = ((2, 3), (1, 3))
  3.  
  4. for coordinateList in changeCoordinates:
  5.     listB = listA[:]
  6.     x = coordinateList[0]
  7.     y = coordinateList[1]
  8.  
  9.     listB[x][y] = 'foo'
  10.     print "list A: %s" % listA
  11.     print "list B: %s" % listB
  12.  
listA should never change. But it does. It works the way I expected on a one-dimensional list, but on a two-dimensional list, listA changes. How do I avoid this, and (out of curiosity) why the hell doesn't python just assign the value rather than the reference anyway?
Aug 21 '08 #1
3 2047
boxfish
469 Recognized Expert Contributor
Hi,
listB is actually a separate copy of listA, but each of the sub-lists in listB is just a shallow copy of the corresponding sub-list in listA, if that makes any sense. To make a completely independent copy of listA, you will either have to use a loop like this:
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  1. for sub_list in listA:
  2.     listB.append(sublist[:])
or use the deepcopy() function of the copy module:
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  1. import copy
  2. listB = copy.deepcopy(listA)
Hope this helps.
Aug 21 '08 #2
Xx r3negade
39 New Member
OK thanks. And about the second part of my question...is there any logical reason for python to do this? Isn't it inconsistent for the assignment operator to assign a value to some data types and a reference to others? Just wondering why they did this.
Aug 21 '08 #3
boxfish
469 Recognized Expert Contributor
Not sure. Seems like the only types that get copied as a reference are lists and object types. I think it would make more sense if tuples, strings, and dictionaries got copied as references too, because they can get really huge also. I wonder if there's a way to do shallow copying of those types in Python.
Aug 21 '08 #4

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