python - Item assignment in a nested list -

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i'm trying change item in nested list. thought straightforward. have following:

temp = [1, 2, 3, 4] my_list = [temp in xrange(4)] print "my_list = ", my_list  out: my_list =  [[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]

so normal list of lists. want access 1 item:

print my_list[0][1]  out: 2

as expected. problem comes when changing item. want change item in my_list[0][1], following:

my_list[0][1]= "a" print my_list  out: [[1, 'a', 3, 4], [1, 'a', 3, 4], [1, 'a', 3, 4], [1, 'a', 3, 4]]

why changing 4 positions, instead of one? how avoid it?

since lists mutable objects when repeat list inside 1 made list of same objects (all of them point 1 memory address),for getting rid of problem need copy nested lists in each iteration :

>>> temp = [1, 2, 3, 4] >>> my_list = [temp[:] in xrange(4)] >>> my_list[0][1]= "a" >>> print my_list [[1, 'a', 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]] >>>

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