Appending to 2D lists in Python

Question

I ve encountered what I think is a strange behavior in Python  and I d like somebody to explain it if possible   I ve created an empty 2D list  listy        3  print listy                 The following works as I d expect   listy 1     1 2  yields       1 2        listy 1  append 3  yields       1 2 3        However  when I append to one of the empty lists  python appends to ALL of the sublists  as follows   listy 2  append 1  yields   1    1 2 3    1     Can anyone explain to me why this behavior occurs

User · Answer

Came here to see how to append an item to a 2D array, but the title of the thread is a bit misleading because it is exploring an issue with the appending.

The easiest way I found to append to a 2D list is like this:

list=[[]]

list.append((var_1,var_2))

This will result in an entry with the 2 variables var_1, var_2. Hope this helps!

User · Answer

You haven t created three different empty lists  You ve created one empty list  and then created a new list with three references to that same empty list  To fix the problem use this code instead   listy       for i in range 3     Running your example code now gives the result you probably expected    gt  gt  gt  listy       for i in range 3    gt  gt  gt  listy 1     1 2   gt  gt  gt  listy       1  2        gt  gt  gt  listy 1  append 3   gt  gt  gt  listy       1  2  3        gt  gt  gt  listy 2  append 1   gt  gt  gt  listy       1  2  3    1

User · Answer

3 is not the same as                It s as if you d said  a      listy    a  a  a    In other words  all three list references refer to the same list instance

[python] Appending to 2D lists in Python

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