Python: Unexpected sublist changes with list of lists

If you are encountering unexpected changes in sublists while working with a list of lists in Python, there are a few possible explanations. One common issue is that mutable objects, such as lists, are passed by reference in Python, meaning that changes to a sublist can affect the original list.

To avoid this issue, you can use the copy() method to create a new copy of the sublist before making any changes. For example:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
sublist_copy = my_list[0].copy()
sublist_copy.append(4)
print(my_list) # output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

In this example, we create a copy of the first sublist using the copy() method. We then append a value to the copied sublist, without affecting the original list.

Another approach is to use list comprehension to create a new list of lists, with each sublist copied using the copy() method. For example:

my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
new_list = [sublist.copy() for sublist in my_list]
new_list[0].append(4)
print(my_list) # output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

In this example, we create a new list of lists using list comprehension, with each sublist copied using the copy() method. We then append a value to the first sublist of the new list, without affecting the original list.

By using these methods to create copies of sublists, you can avoid unexpected changes and ensure that your list of lists behaves as expected.