Changing a List

A list is not fixed once it is made. Items can be added, removed, replaced and reordered, which is exactly what you need when a program collects results as it goes. Most of the tools that change a list work on the list itself and give back nothing, and that detail is behind one of the classic beginner bugs, met at the end of this page.

Think of a to-do list on a whiteboard. You add items at the bottom, rub some out, and rewrite others, all on the same board. Copying the list neatly onto a second board is a different act, and Python keeps the two just as separate.

Adding

append adds one item to the end of a list. insert puts an item at a position you choose, moving everything after it along by one. Both change the list you call them on.

>>> todo = ["email", "shopping"]
>>> todo.append("laundry")
>>> todo
['email', 'shopping', 'laundry']
>>> todo.insert(0, "coffee")
>>> todo
['coffee', 'email', 'shopping', 'laundry']

Removing

remove deletes the first item equal to the value you give it. pop removes an item by position, the last one if you give no position, and hands the removed item back to you, which is useful when you want to do something with it. Removing a value that is not in the list is a ValueError, and the message says the item is not in the list.

>>> todo.remove("email")
>>> todo
['coffee', 'shopping', 'laundry']
>>> done = todo.pop()
>>> done
'laundry'
>>> todo
['coffee', 'shopping']
>>> todo.remove("dishes")
Traceback (most recent call last):
  File "<python-input-10>", line 1, in <module>
    todo.remove("dishes")
    ~~~~~~~~~~~^^^^^^^^^^
ValueError: list.remove(x): x not in list

Changing One Item

To replace an item, assign to its position, exactly as you would assign to a name. Afterwards the list has the same length and a new value in that place.

>>> todo[1] = "groceries"
>>> todo
['coffee', 'groceries']

Sorting: in Place or a Copy

There are two ways to sort, and the difference matters. The sort method reorders the list itself and gives back None. The sorted function leaves the original alone and gives back a new, sorted list.

>>> scores = [70, 95, 58, 81]
>>> sorted(scores)
[58, 70, 81, 95]
>>> scores
[70, 95, 58, 81]
>>> scores.sort()
>>> scores
[58, 70, 81, 95]

Now the bug. Because sort gives back None, writing scores = scores.sort() sorts the list and then points the name at the None it returned. The sorted list is lost, and printing the name shows only None. The same trap applies to append and the other methods that change a list in place; pop is the exception, since it hands back the item it removed.

scores = [70, 95, 58, 81]
scores = scores.sort()
print(scores)

None