List Basics

A list is a sequence you can edit after you build it. By the end of this lesson you can change a list in place, tell which methods change it and which hand back a new value, and keep two names from silently sharing one list.

In Python Fundamentals you got lists from split() and walked them with for, enumerate and zip, and you saw that a string cannot be edited in place. A list can, and that single difference has consequences.

Editing in place

Assigning to an index or a slice changes the list itself:

The methods that grow and shrink a list:

append and extend differ on a list argument: after s = [1], s.append([7, 8]) gives [1, [7, 8]] (one new item, itself a list), while s.extend([7, 8]) gives [1, 7, 8].

Mutators return None

Every method that changes a list in place returns None, not the list:

Built-ins that only look at a list leave it alone and return something: len(s), sum(s), min(s), max(s), s.index(x) (position of the first match), s.count(x), x in s, and sorted(s), which returns a new sorted list. reversed(s) returns a lazy iterator, so wrap it: list(reversed([3, 1, 2])) is [2, 1, 3].

One list, two names

b = a copies the name, not the list:

Variables are labels attached to objects. b = a puts a second label on the same list, so a change through either label is visible through both. list(a), a[:] and a.copy() build a new list holding the same items.

That copy is shallow: the new outer list is separate, but any inner lists are still shared.

The same sharing explains [[0] * 3] * 3: it repeats ONE inner list three times, so grid[0][0] = 9 changes all three rows.

Finally, += and + differ on lists. y += [3] extends the list in place, so every name for it sees the change. y = y + [3] builds a new list and rebinds y, leaving the other names on the old one.

Common mistakes

• items = items.sort(). sort() returns None, so the next " ".join(items) raises TypeError and len(items) fails. Call items.sort() on its own line, or use sorted() when you want a value back.
• Saving a "backup" with =. backup = items followed by items.append("z") also changes backup, so restoring it restores nothing. Save list(items).
• remove on a missing value. [1, 2].remove(5) raises ValueError. Check 5 in s first.
• Expecting insert to fail past the end. [1, 2].insert(99, 7) gives [1, 2, 7]; it never raises.

Your exercise

Write a small command interpreter over one list of words, where every edit can be undone. The starter reads the commands and prints for show; the most likely mistake is saving history in a way that later edits quietly rewrite.