python - Cannot modify item of list within for loop -

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i've found out while executing for loop on list of mutable items cannot modify item, able modify element of item if element mutable. why?

# create alist contain mutable alist = [[1, 2, 3],]  #1 loop x in alist:     x = x + [9,] print alist # let's replace alist[0] list contain 1 , try modify alist[0] = [[[1,2],3]]  print2 alist # [[[[1, 2], 3]]] #2 loop x in alist:     x[0] = x[0] + [9,] # list modified ... print alist # [[[[1, 2], 3, 9]]], modified !

i aware modifying list iterating on isn't practice (it's better iterate on copy of it), please don't point me moment.

the reason of such behaviour while performing for loop #1 new list produced applying + operator list( can tracked getting id of x).

however in for loop #2 access element of item index x[0] object in memory being modified. can found calling id on x while iterating on alist

# create alist contain mutable alist = [[1, 2, 3],]  #1 loop x in alist:     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0])      # id(x): 4425690360 id(alist[0]) 4425690360 same?: true     x = x + [9,]     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0])      # id(x): 4425770696 id(alist[0]) 4425690360 same?: false # mutable item not modified, guess x copy of item print alist # [[1, 2, 3]] # let's replace alist[0] list contain 1 , try modify alist[0] = [[[1,2],3]]  print alist #2 loop x in alist:     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0])      # id(x): 4425811008 id(alist[0]) 4425811008 same?: true      x[0] = x[0] + [9,]     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0])     # id(x): 4425811008 id(alist[0]) 4425811008 same?: true # list modified ... print alist # [[[[1, 2], 3, 9]]]

to solve problem use array.append modify item within loop, see sample below:

alist = [[1, 2, 3],] x in alist:     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0])     x.append(9)     print id(x), id(alist[0]), 'same?:', id(x) == id(alist[0]) # list modified ... print alist # [[1, 2, 3, 9],]

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