Python : how to append new elements in a list of list? -

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here simple program:

 = [[]]*3   print str(a)   a[0].append(1)  a[1].append(2)  a[2].append(3)   print str(a[0])  print str(a[1])  print str(a[2])

here output expecting:

 [[], [], []]  [1]  [2]  [3]

but instead :

 [[], [], []]  [1, 2, 3]  [1, 2, 3]  [1, 2, 3]

there not here !

you must do

a = [[] in xrange(3)]

not

a = [[]]*3

now works:

$ cat /tmp/3.py = [[] in xrange(3)]  print str(a)  a[0].append(1) a[1].append(2) a[2].append(3)  print str(a[0]) print str(a[1]) print str(a[2])  $ python /tmp/3.py [[], [], []] [1] [2] [3]

when a = [[]]*3 same list [] 3 times in list. the same means when change 1 of them change of them (because there 1 list referenced 3 times).

you need create 3 independent lists circumvent problem. , list comprehension. construct here list consists of independent empty lists []. new empty list created each iteration on xrange(3) (the difference between range , xrange not important in case; xrange little bit better because not produce full list of numbers , returns iterator object instead).


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