List Methods

1The question that sorts them all

On the Strings page, every method handed back a brand-new string and left the original alone — it had no choice, because a string cannot be changed. A list can. So a list method has a second option open to it: instead of building you a new list, it can reach into the one you already have and rearrange it.

Methods that do that are said to work in place. They change the list, and then they hand back None — Python's word for nothing at all. The change is the answer, so there is nothing left to give you.

2Adding: append(), insert(), extend()

Three ways to put something in, and they differ in where and how many.

marks = [50, 20, 40]

marks.append(30)          # one item, at the end
print(marks)

marks.insert(1, 99)       # one item, at position 1
print(marks)

marks.extend([70, 80])    # every item of another list
print(marks)

[50, 20, 40, 30]
[50, 99, 20, 40, 30]
[50, 99, 20, 40, 30, 70, 80]

a = [1, 2]
b = [1, 2]

a.append([3, 4])     # goes in as ONE item
b.extend([3, 4])     # goes in as TWO items

print(a)
print(b)
print(len(a))
print(len(b))

[1, 2, [3, 4]]
[1, 2, 3, 4]
3
4

a now has a list inside a list, and its length is 3, not 4 — because as far as a is concerned, [3, 4] is a single item that happens to be a list. append() adds the thing you gave it. extend() adds what is inside the thing you gave it.

3Removing: remove() and pop()

Two ways to take something out, and the difference is what you know about it. Use remove() when you know the value. Use pop() when you know the position — or when you want the item back.

marks = [50, 20, 40, 20]

marks.remove(20)      # by VALUE — the first 20 only
print(marks)

last = marks.pop()    # by POSITION — the last one, and it hands it back
print(last)
print(marks)

first = marks.pop(0)  # position 0
print(first)
print(marks)

[50, 40, 20]
20
[50, 40]
50
[40]

Two things are worth pausing on. remove(20) took out only the first 20 — the second one survived. And pop() is the odd one out of every method on this page: it changes the list and hands back the item it removed, which is why last holds 20.

marks = [50, 20, 40]

marks.remove(99)      # 99 was never in there

Traceback (most recent call last):
  File "removing_error.py", line 3, in <module>
    marks.remove(99)      # 99 was never in there
ValueError: list.remove(x): x not in list

4Reordering: sort(), reverse() — and sorted()

sort() puts the list in order, smallest first. reverse() flips it end to end — it does not sort, it just turns the list back to front. Both work in place.

marks = [50, 20, 40]

print(sorted(marks))       # a NEW sorted list
print(marks)               # ...and marks has not moved

marks.sort()               # now change marks itself
print(marks)

marks.reverse()            # flip it end to end
print(marks)

marks.sort(reverse=True)   # sort, biggest first
print(marks)

[20, 40, 50]
[50, 20, 40]
[20, 40, 50]
[50, 40, 20]
[50, 40, 20]

Read the first two lines together, because they are the whole point. sorted(marks) printed a sorted list — and the very next line shows marks still in its original order. Nothing was changed. Two lines later, marks.sort() changed it for good.

marks = [50, 20, 40]

marks = marks.sort()   # DON'T — sort() gives back None
print(marks)

None

Write marks.sort() on its own line and then use marks. Or, if you want to keep the original order too, use ordered = sorted(marks) — that is exactly what sorted() is for.

5Asking: count() and index()

You have met both of these already, on strings. They behave the same way on a list — and, like every question, they leave the list alone.

marks = [50, 20, 40, 20]

print(marks.count(20))   # how many 20s?
print(marks.index(20))   # where is the first 20?
print(marks.index(40))

2
1
2

count() answers how many. index() answers where — and only ever reports the first match, which is why two 20s still give the single answer 1. Ask for something that is not there and index() raises a ValueError, exactly as it did on a string.

6The functions: len(), min(), max(), sum()

These are functions, not methods — the list goes inside the parentheses, with no dot. None of them changes anything.

marks = [50, 20, 40, 20]

print(len(marks))
print(min(marks))
print(max(marks))
print(sum(marks))
print(sum(marks) / len(marks))   # the average

4
20
50
130
32.5

7list(): building a list out of something else

list() is one more function, and it does what int() and str() did in the casting lesson: it takes a value and gives you the list version of it. Hand it any sequence and it lays the items out one by one.

letters = list('hello')
numbers = list((1, 2, 3))
empty = list()

print(letters)
print(numbers)
print(empty)

['h', 'e', 'l', 'l', 'o']
[1, 2, 3]
[]

list('hello') is the useful one: it splits the word into its five separate characters, and now they sit in something you can change — which the string itself would never allow.

8Try it

Change the list, print it, break it. The methods need a name to work on, so this one is a real program rather than a single prompt line.

marks = [50, 20, 40, 20]

marks.append(30)
marks.insert(0, 10)
marks.remove(20)

print(marks)
print(sorted(marks))
print(marks.count(20), marks.index(40))
print(len(marks), min(marks), max(marks), sum(marks))

9Recap