How to insert elements in a nested List in Python?

Question

I do not have much experience in Python. All I want to do is to insert elements in nested lists.I have two lists which seems to be similar but their behaviour is completely different.

list1 = [['a','b']] * 3 list2 = [['a','b'],['a','b'],['a','b']]

When I output print these two lists both give same output: [['a', 'b'], ['a', 'b'], ['a', 'b']]

But when I try to insert elements in nested lists both do that in a different way. Below is the code for inserting elements in nested list.

list1 = [['a','b']] * 3
for item in list1:
  item.append("Hello")
print (list1)

This outputs

[['a', 'b', 'Hello', 'Hello', 'Hello'], ['a', 'b', 'Hello', 'Hello', 'Hello'], ['a', 'b', 'Hello', 'Hello', 'Hello']]

While when I define list in the following way it does exactly what I want.

list2 = [['a','b'],['a','b'],['a','b']]
for item in list2:
  item.append("Hello")
print (list2)

This gives following output: [['a', 'b', 'Hello'], ['a', 'b', 'Hello'], ['a', 'b', 'Hello']].

Why are these two behaving differently? list1 = [['a','b']] * 3 list2 = [['a','b'],['a','b'],['a','b']] Screenshot of Program output

pythomatic · Answer 1 · 2018-09-27T16:18:52.263

2

When you use the * operator here, you are saying "I want 3 of these".

So you're getting 3 references to the same ['a', 'b']. In your case, you're adding 'Hello' to that same ['a', 'b'] reference.

List of lists changes reflected across sublists unexpectedly

If you want to make 3 separate references, try using list comprehension:

>>> x = [['a', 'b'] for i in range(0, 3)]
>>> x
[['a', 'b'], ['a', 'b'], ['a', 'b']]
>>> x[0].append('Hello')
>>> x
[['a', 'b', 'Hello'], ['a', 'b'], ['a', 'b']]

edited Sep 27 '18 at 16:18

answered Sep 27 '18 at 15:37

pythomatic

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4
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Thanks, So how Do I create a nested list with variable length . Is it possible to do so? – Varun Bhandari Sep 27 '18 at 15:51
I would use a list comprehension: `x = [['a', 'b'] for i in range(0, 3)]`. The difference here is that this will create a new `['a', 'b']` each time. – pythomatic Sep 27 '18 at 16:17
Thanks, I got the answer in the question asked earlier. Thanks for the answer. – Varun Bhandari Sep 27 '18 at 16:20

score 2 · Answer 2 · answered Sep 27 '18 at 15:38

list1 = [['a', 'b']] * 3

This creates a list of lists, as you know. However, the nested lists are actually all references to the same list object.

So when you iterate over list1 with

for item in list1:

item refers to the same list object on each iteration. So you repeated append to the same list.

On the other hand, list2 in your example code is explicitly assigned a list with three different lists. Those lists happen to have the same elements, but they are distinct lists.

How to insert elements in a nested List in Python?

2 Answers2