Unexpected extra zero in python list when assigned explicitly

Question

I want the to directly populate particular sections of a list from a string that is returned by a function. I want to do some binary arithmetic on these elements later. When I access the list using subscript, the elements are not getting stored as I expect. A simplified version of my code is:

   reg_bin_list = ['0']*2
   reg_bin_list[0:1] = "10"

   print reg_bin_list

This puts an extra zero at the end: ['1', '0', '0']

Instead of that if do :

   reg_bin_list = "10"

Then the list is printed correctly as I expect i.e. ['1', '0']

Can anyone help me understand when I access using the index operator, what is going on. Why am I seeing an extra zero at the end?

possible duplicate of [How assignment works with python list slice](http://stackoverflow.com/questions/10623302/how-assignment-works-with-python-list-slice) — TigerhawkT3, Jul 23 '15 at 18:54

score 4 · Accepted Answer · edited May 23 '17 at 12:29

4

The problem is with your slicing. [0:1] starts from index 0 and ends before index 1. Hence change the slice to [0:2]

>>> reg_bin_list = ['0']*2
>>> reg_bin_list[0:2] = "10"  # Note the change
>>> reg_bin_list
['1', '0']

Why am I seeing an extra zero at the end?

It is the additional 0 left behind in the original list.

Do have a look at Explain Python's slice notation, for a nice explanation as to why.

edited May 23 '17 at 12:29

Community

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answered Jul 23 '15 at 18:38

Bhargav Rao

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A convenient way to remember this is that a slice `x[a:b]` has length `b-a`. – amalloy Jul 23 '15 at 18:41

score 1 · Answer 2 · answered Jul 23 '15 at 18:39

When slicing a list using - list[<start>:<stop>] - the stop index is exclusive, that means it does not get included in the slice.

Hence, when you do -

reg_bin_list[0:1] = "10"

This is only taking the reg_bin_list[0] , and hence it is insert '1' and '0' starting at 0th position, leading to the 3 length list.

You should use -

reg_bin_list[0:2] = "10"

Example -

>>> reg_bin_list[0:2] = "10"
>>> reg_bin_list
['1', '0']

Unexpected extra zero in python list when assigned explicitly

2 Answers2