24. List methods

You have built lists by hand and walked them with for and enumerate. Python lists also come with built-in methods — functions that belong to the list and do the common jobs for you: sorting, reversing, copying, searching, counting. Learn these and you rarely need to write the loop yourself.

This chapter uses the lists from Chapter 22.

Sorting a list: .sort() and sorted()

.sort() reorders the list in place — it changes the original.

scores = [30, 12, 7, 24]
scores.sort()
print(scores)   # [7, 12, 24, 30]

It sorts text alphabetically too:

names = ["Cara", "Ada", "Ben"]
names.sort()
print(names)   # ['Ada', 'Ben', 'Cara']

sorted(lst) does the same but returns a new list, leaving the original unchanged:

scores = [30, 12, 7, 24]
ascending = sorted(scores)
print(ascending)   # [7, 12, 24, 30]
print(scores)      # [30, 12, 7, 24]  (unchanged)

Sorting your own way

Pass a key function to sort by a computed value, or reverse=True for descending order:

scores = [30, 12, 7, 24]
scores.sort(reverse=True)
print(scores)   # [30, 24, 12, 7]

words = ["banana", "fig", "apple", "kiwi"]
words.sort(key=len)           # sort by string length
print(words)   # ['fig', 'kiwi', 'apple', 'banana']

Reversing a list: .reverse()

.reverse() flips the list in place:

items = ["a", "b", "c"]
items.reverse()
print(items)   # ['c', 'b', 'a']

To get a reversed copy without changing the original, use the slice items[::-1] or list(reversed(items)):

items = ["a", "b", "c"]
flipped = items[::-1]
print(flipped)   # ['c', 'b', 'a']
print(items)     # ['a', 'b', 'c']  (unchanged)

Joining a list into text

", ".join(lst) glues every item into one string, the separator between each. Items must be strings.

fruits = ["apple", "banana", "cherry"]
print(", ".join(fruits))    # apple, banana, cherry
print(" - ".join(fruits))   # apple - banana - cherry
print("".join(fruits))      # applebananacherry

For a list of numbers, convert them first:

scores = [7, 12, 24, 30]
print(" ".join(str(n) for n in scores))   # 7 12 24 30

Copying a list: .copy()

.copy() returns a shallow copy — a new list with the same items:

original = [1, 2, 3]
copy = original.copy()
copy[0] = 99
print(original)   # [1, 2, 3]  (unchanged)
print(copy)       # [99, 2, 3]

Without .copy(), assigning a list to another variable gives you a second reference to the same list (covered in Chapter 22).

Searching: .index() and .count()

.index(value) returns the position of the first occurrence:

fruits = ["apple", "banana", "cherry", "banana"]
print(fruits.index("banana"))   # 1

It raises a ValueError if the value is not in the list. Check with in first if you are not sure:

if "grape" in fruits:
    print(fruits.index("grape"))
else:
    print("not found")

.count(value) returns how many times a value appears:

print(fruits.count("banana"))   # 2
print(fruits.count("grape"))    # 0

List comprehensions

A list comprehension builds a new list from an expression applied to each item. The syntax is [expression for item in iterable]:

nums = [1, 2, 3, 4, 5]
doubled = [x * 2 for x in nums]
print(doubled)   # [2, 4, 6, 8, 10]

You can add a condition to filter items:

evens = [x for x in nums if x % 2 == 0]
print(evens)   # [2, 4]

This is a compact way to express what would otherwise be a loop with .append. You do not need to use comprehensions immediately; the loop form is equally correct. Use whichever you find clearer.

Homework

Homework files are in exercises/24/homework/.

Stuck or finished? Open the homework solutions page.