List Methods and Mutations

Adding elements: append, insert, extend

Mutation is changing a list in place, without making a new list. Methods such as append, insert, extend, pop and remove mutate the list they are called on.

Use .append(x) to add to the end, .insert(i, x) to insert at an index, and .extend(iterable) to merge another list.

Notice that tasks.append([1, 2]) would add a nested list as a single element, while .extend([1, 2]) unpacks and appends each item.

Removing elements: pop, remove, clear

Use .pop(index) to remove and return an item by index, or .remove(value) to delete the first occurrence of a value.

Calling .pop() without arguments removes and returns the last element in O(1) time (functioning as a stack).

In-place sort() versus sorted()

list.sort() sorts the list in place and returns None. The built-in sorted(list) returns a new sorted list, leaving the original unchanged.

Do not write nums = nums.sort(), as that rebinds nums to None because .sort() modifies in place.

Aliasing versus copying

Alias is a second name for the same list. Writing b = a does not copy the list, so a change made through b also shows when you read a.

Assigning a list to another name gives that name the same list. To get a separate list, copy it with .copy() or [:].

backup is another name for the same list, so it shows "bag". saved is a new list made before the change, so it does not. is checks whether two names point to the same object.