Manage Sequences of Values with Lists

Problem to Solve

I want to build, modify, retrieve elements from, and iterate over a sequence of values.

A list is basically just like a tuple, except that it can be modified in all kinds of ways after it has been created: Lists are mutable. You can add elements, remove elements, change the value of existing elements, and so on.

You can do almost anything with a list that you can do with a tuple, so I won't repeat those recipes here.

  • (Exception: The fact that tuples are immutable makes them hashable, which means that they can be used as keys in dictionaries and as elements of sets. That is occasionally useful.)

Again, I'll point you to some online introductions:

A Few Recipes

change the value of an existing list element

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1[2] = 99  # change the value at index 2
print(list1)  # yields [1, 2, 99, 4, 5]

append an element to a list

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.append(6)  # add 6 to the end of the list
print(list1)  # yields [1, 2, 3, 4, 5, 6]

append multiple elements to a list

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.extend([6, 7, 8])  # add 6, 7, and 8 to the end of the list
print(list1)  # yields [1, 2, 3, 4, 5, 6, 7, 8]

Note that append() can only add one element at a time; to add multiple elements at once, use extend().

insert an element at a specific index

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.insert(2, 99)  # insert 99 at index 2
print(list1)  # yields [1, 2, 99, 3, 4, 5]

remove a specific value from a list

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.remove(3)  # remove the first occurrence of 3
print(list1)  # yields [1, 2, 4, 5]

remove an element at a specific index

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.pop(2)  # remove the element at index 2
print(list1)  # yields [1, 2, 4, 5]

The pop() method returns the value of the element its removing, so you can do something like this:

list1 = [1, 2, 3, 4, 5]  # a list of integers
i = 2
print(f"Removed {list1.pop(i)} from position {i} position in the list")
print(list1)  # yields [1, 2, 4, 5]

remove the last element from a list

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.pop()  # pop() with no argument means "the last element"
print(list1)  # yields [1, 2, 3, 4]

remove all elements from a list

list1 = [1, 2, 3, 4, 5]  # a list of integers
list1.clear()  # remove all elements
print(list1)  # yields []

sort a list

list1 = [5, 2, 4, 3, 1]  # a list of integers
list1.sort()  # sort the list in place
print(list1)  # yields [1, 2, 3, 4, 5]

sort a list in reverse order

list1 = [5, 2, 4, 3, 1]  # a list of integers
list1.sort(reverse=True)  # sort the list in place, in reverse order
print(list1)  # yields [5, 4, 3, 2, 1]

build a list iteratively

This is a very common idiom in computational physics:

list1 = []  # an empty list
for i in range(5):
    list1.append(i**2)  # add the square of i to the list
print(list1)  # yields [0, 1, 4, 9, 16]

create a list from a string

list1 = list("abc")  # a list of characters
print(list1)  # yields ['a', 'b', 'c']

create a list from a range object

list1 = list(range(5))  # a list of integers
print(list1)  # yields [0, 1, 2, 3, 4]