Lists

Last updated on 2026-10-21 | Edit this page

Estimated time: 30 minutes

Overview

Questions

  • How can I store multiple values in one object.

Objectives

  • Explain what a Python list is.
  • Explain why programs need collections of values.
  • Write programs that create lists, index them, slice them, and modify them.

Lists stores values of any type

A list stores multiple values in one object, in a specific order. Values in a list can be called “elements” or “items”.

A list is represented by comma-separated items enclosed in [].

PYTHON 

first_list = ["my", "first", "list"]
print(first_list)

OUTPUT 

['my', 'first', 'list']

Items in a list do not have to be of the same type.

PYTHON 

lucky_numbers = ["15", 31, 12.0]
print(lucky_numbers)

OUTPUT 

['15', 31, 12.0]

Use len to find out how many values are in a list.

PYTHON 

print(len(first_list))

OUTPUT 

3

Lists are indexed like strings

Just like strings, lists are indexed from 0 to n-1, where n is the length of the list. Use [] to select indexed values.

PYTHON 

alphabet = "abcde"
words = ["apple", "boy", "camel", "dog"]

print(alphabet[1])
print(words[1])

OUTPUT 

b
boy

Make sure you check the length of a list before indexing into it. As with strings, indexing into a value that doesn’t exist (because the list is too short) causes an IndexError.

PYTHON 

words[5] # This will fail

OUTPUT 

IndexError                                Traceback (most recent call last)
Cell In[80], line 1
----> 1 words[5]

IndexError: list index out of range

You can also use the same syntax [start:stop] you used to express slices for strings with lists.

PYTHON 

print(words[:2])

OUTPUT 

['apple', 'boy']
Challenge

List Indexing, String Indexing

How would I get a slice of a specific character in a specific item in words? Let’s say I want to get a substring containing the letter y in boy.

PYTHON 

words = ["apple", "boy", "camel", "dog"]
print(words_____) # Add indexing here

You can accomplish this by indexing into the second item in words [1] and indexing into the third character of that [2].

Nested indexing is read from left to right, starting from the outermost collection.

PYTHON 

words[1][2]

OUTPUT 

'y'

Values in lists can be reassigned

Use indexing on the left side of an assignment statement to replace a value inside a list.

Lists are mutable: they can be changed after they are instantiated.

PYTHON 

pressures = [0.273, 0.275, 0.277, 0.275, 0.276]
pressures[0] = 0.26
print('Pressures is now:', pressures)

OUTPUT 

pressures is now: [0.265, 0.275, 0.277, 0.275, 0.276]

The append method adds items to the end of a list

Use the .append method to add items to the end of a list.

PYTHON 

primes = [2, 3, 5]
print('primes is initially:', primes)
primes.append(7)
print('primes has become:', primes)

OUTPUT 

primes is initially: [2, 3, 5]
primes has become: [2, 3, 5, 7]

The .append() method returns None and modifies the list in place. That is, it directly modifies the list that calls the method rather than returning a copy.

This is an intentional design choice.

PYTHON 

returned_by_append = primes.append(11) # Append 11 but return None
print(returned_by_append)
print(primes)

OUTPUT 

None
[2, 3, 5, 7, 11]

Extending a list concatenates two lists together

Use the .extend() method to add a list passed in as an argument to the end of that list.

PYTHON 

housemates = ["dave", "rose"]
guests = ["bob", "yao", "sam"]
guests.extend(housemates) # Also modifies in place
print("The following people will be at my party", guests)
print("There are", len(guests), "guests")

OUTPUT 

The following people will be at my party ['bob', 'yao', 'sam', 'dave', 'rose']
There are 5 guests

Appending a list to a list creates a nested list: a list inside of a list.

PYTHON 

ml = [1, "dog", "antelope"]
ml2 = [2, 2, 3]
ml.append(ml2)
print(ml)

OUTPUT 

[1, 'dog', 'antelope', [2, 2, 3]]

The empty list contains no values.

We don’t have to create lists with items already in them. The empty list [] is a list with length 0.

PYTHON 

x = []
len(x)

OUTPUT 

0

You can think of it as “the zero of lists.”

The empty list is useful because we can add items to it using methods like .append() and .extend().

PYTHON 

x.append(4)
x.append(5)
print(x)

OUTPUT 

[4, 5]
Challenge

Append or Extend?

Can you make z into [1, 2, 3, 4, 5] using .append()? How about with .extend()?

PYTHON 

z = [1, 2, 3]

We can append 4, then 5.

PYTHON 

z = [1, 2, 3]
z.append(4)
z.append(5)
print(z)

OUTPUT 

[1, 2, 3, 4, 5]

Or we can extend the list with [4, 5].

PYTHON 

z = [1, 2, 3] # Reset z
z.extend([4,5])
print(z)

OUTPUT 

[1, 2, 3, 4, 5]

Inserting into list with .insert()

So far, we’ve looked at methods that modify the end of a list.

The method .insert(i, x) inserts item x before the item at position i.

PYTHON 

alpha = ["apple", "berry", "durian"]
alpha.insert(2, "cherry") # Before the item at index 2 ("durian") 
print(alpha)

OUTPUT 

['apple', 'berry', 'cherry', 'durian']

Removing items from lists: del, .remove(), and .pop()

We use del list_name[index] to remove an element from a list. - del is not a function or a method, but a statement in the language.

PYTHON 

primes = [2, 3, 5, 7, 9]
print('primes before removing', primes)
del primes[4]
print('primes after removing', primes)

OUTPUT 

primes before removing: [2, 3, 5, 7, 9]
primes after removing: [2, 3, 5, 7]

However, it is more common to see items removed from lists using the .remove() and .pop() methods. .remove(x) will remove the first instance of value(x) from a list. Like .append(), the .remove() function returns None.

PYTHON 

housemates = ["rose", "sam"] 
housemates.remove("rose") 
housemates

OUTPUT 

['sam']

It will return a ValueError if there are no instances of x in the list. Now that “rose” has already moved out, .remove() will fail because it cannot find a “rose” in the list to remove.

PYTHON 

housemates.remove("rose") # This will fail

OUTPUT 

----> 1 housemates.remove("rose")

ValueError: list.remove(x): x not in list

Let’s say two new housemates move in: “josh” and “joe”.

PYTHON 

housemates.append("josh")
housemates.append("joe")
print("There are now", len(housemates), "people in the house.")

The landlord imposes a new rule. Only two people are allowed in the house. The person who moved in most recently (the end of the list) must move out.

We can use the .pop() method to remove (and return) the last item in a list.

PYTHON 

evicted = housemates.pop() # Removes and returns the last item in a list
print(evicted, "was evicted.")
print(housemates, "live in the house")

outputjoe was evicted. ['sam', 'josh'] live in the house

You cannot .pop() values from an empty list.

PYTHON 

empty_house = []
empty_house.pop() # This will fails

OUTPUT 

IndexError                                Traceback (most recent call last)
Cell In[23], line 2
      1 empty_house = []
----> 2 empty_house.pop()

IndexError: pop from empty list

Character strings are immutable.

We cannot change the characters in a string after it has been created because Python considers the string to be a single primitive value.

Primitives (strings, ints, floats, booleans) are immutable. This means that when we want to change the value of a string or number variable, we can only replace the old value with a completely new value.

PYTHON 

element = "Helium"
element[0] = 'C'

ERROR 

TypeError: 'str' object does not support item assignment

Lists and character strings are both collections.

Mutable and immutable objects: when a copy isn’t a copy

Be careful when modifying data in-place. If two variables refer to the same list, and you modify the list value, it will change for both variables!

This has to do with the mechanics of assignment and mutability in Python.

PYTHON 

x = [1, 3]
y = x # y points to x
x.append(5)
print(x)
print("y", y, "is another name for x")

OUTPUT 

[1, 3, 5]
y [1, 3, 5] is another name for x

If you wish to make two copies of the same list, use the .copy() method instead. This method returns a new list with the same contents. This principle applies for all mutable variables. (We’ll learn about another mutable data type—the DataFrame—later in this workshop.)

PYTHON 

x = [1, 3]
y = x.copy() # Y points to a different, mutable list
x.append(5)
print("x", x)
print("y", y, "hasn't changed.")

OUTPUT 

x [1, 3, 5]
y [1, 3] hasn't changed.

For additional reading on this topic, see this question from the official Python FAQ.

Two ways of sorting lists

There are two common ways to sort lists by their values: the sorted() function which returns a sorted collection as a copy and the .sort() method which sorts a list in place.

PYTHON 

numbers = [1, 5, 3, 4, 2]
snumbers = sorted(numbers) # Returns a copy
print(numbers) # Unchanged
print(snumbers)

OUTPUT 

[1, 5, 3, 4, 2]
[1, 2, 3, 4, 5]

Notice that the .sort() method returns None and permanently changes numbers.

PYTHON 

print("Numbers before .sort()", numbers)
numbers.sort() # Permanently changes numbers
print("Numbers after", numbers)

OUTPUT 

Numbers before .sort() [1, 5, 3, 4, 2]
Numbers after [1, 2, 3, 4, 5]

However, sorting a list only works when all its values are of the same data type. Remember those rules about comparing items of different types with min() and max()?

PYTHON 

valid_list = ["a", 3, [1]] # This will work

PYTHON 

valid_list.sort()

OUTPUT 

TypeError                                 Traceback (most recent call last)
Cell In[27], line 1
----> 1 valid_list.sort()

TypeError: '<' not supported between instances of 'int' and 'str'
Challenge

Fill in the Blanks

Fill in the blanks so that the program below produces the output shown.

PYTHON 

values = ____
values.____(1)
values.____(3)
values.____(5)
print('first time:', values)
values = values[____]
print('second time:', values)

OUTPUT 

first time: [1, 3, 5]
second time: [3, 5]

PYTHON 

values = []
values.append(1)
values.append(3)
values.append(5)
print('first time:', values)
values = values[1:]
print('second time:', values)
Challenge

How Large is a Slice?

If start and stop are both non-negative integers, how long is the list values[start:stop]?

The list values[start:stop] has up to stop - start elements. For example, values[1:4] has the 3 elements values[1], values[2], and values[3]. Why ‘up to’? As we saw in episode 2, if stop is greater than the total length of the list values, we will still get a list back but it will be shorter than expected.

Challenge

From Strings to Lists and Back

Given this:

PYTHON 

print('string to list:', list('tin'))
print('list to string:', ''.join(['g', 'o', 'l', 'd']))

OUTPUT 

string to list: ['t', 'i', 'n']
list to string: gold
  1. What does list('some string') do?
  2. What does '-'.join(['x', 'y', 'z']) generate?
  1. list('some string') converts a string into a list containing all of its characters.
  2. join returns a string that is the concatenation of each string element in the list and adds the separator between each element in the list. This results in x-y-z. The separator between the elements is the string that provides this method.
Challenge

Working With the End

What does the following program print?

PYTHON 

element = 'helium'
print(element[-1])
  1. How does Python interpret a negative index?
  2. If a list or string has N elements, what is the most negative index that can safely be used with it, and what location does that index represent?
  3. If values is a list, what does del values[-1] do?
  4. How can you display all elements but the last one without changing values? (Hint: you will need to combine slicing and negative indexing.)

The program prints m.

  1. Python interprets a negative index as starting from the end (as opposed to starting from the beginning). The last element is -1.
  2. The last index that can safely be used with a list of N elements is element -N, which represents the first element.
  3. del values[-1] removes the last element from the list.
  4. values[:-1]
Challenge

Slice Bounds

What does the following program print?

PYTHON 

element = 'lithium'
print(element[0:20])
print(element[-1:3])

OUTPUT 

lithium

The first statement prints the whole string, since the slice goes beyond the total length of the string. The second statement returns an empty string, because the slice goes “out of bounds” of the string.

Challenge

Sort and Sorted

What do these two programs print? In simple terms, explain the difference between sorted(letters) and letters.sort().

PYTHON 

# Program A
letters = list('gold')
result = sorted(letters)
print('letters is', letters, 'and result is', result)

PYTHON 

# Program B
letters = list('gold')
result = letters.sort()
print('letters is', letters, 'and result is', result)

Program A prints

OUTPUT 

letters is ['g', 'o', 'l', 'd'] and result is ['d', 'g', 'l', 'o']

Program B prints

OUTPUT 

letters is ['d', 'g', 'l', 'o'] and result is None

sorted(letters) returns a sorted copy of the list letters (the original list letters remains unchanged), while letters.sort() sorts the list letters in-place and does not return anything.

Key Points
  • A list stores many values in a single structure.
  • Use an item’s index to fetch it from a list.
  • Lists’ values can be replaced by assigning to them.
  • Appending items to a list lengthens it.
  • Use del to remove items from a list entirely.
  • The empty list contains no values.
  • Lists may contain values of different types.
  • Character strings can be indexed like lists.
  • Character strings are immutable.
  • Indexing beyond the end of the collection is an error.