Python Lists, Tuples, Sets, and Dictionaries

Python provides four core built-in collection types, and choosing the right one is one of the most practical skills in the language. Lists and dictionaries are mutable and extremely common for everyday data manipulation; tuples are immutable and useful for fixed groupings of values; sets are unordered collections optimized for membership testing and eliminating duplicates.

Understanding exactly which of these types can be changed after creation — and which cannot — prevents an entire category of bugs, especially when a collection is passed around and modified in multiple places in a program.

Why it matters

Almost all data in a Python program sits in one of these four structures, and data from files, databases and APIs arrives as lists and dictionaries. Choosing between them is the first design decision in most functions, and each choice has consequences for what the code can do and how fast it runs.

Lists — Ordered and Mutable

A list is an ordered, mutable collection written with square brackets, e.g. [1, 2, 3]. You can add, remove, or change elements after creation using methods like append(), remove(), insert(), and pop(), or by assigning directly to an index. Lists can hold mixed types and can be nested inside one another.

Tuples — Ordered and Immutable

A tuple is an ordered, immutable collection written with parentheses, e.g. (1, 2, 3). Once created, a tuple's contents cannot be changed, added to, or removed — attempting to do so raises a TypeError. Tuples are commonly used for fixed groupings of related values (like coordinates) and as dictionary keys, since they are hashable as long as their contents are hashable, unlike lists.

Sets — Unordered and Unique

A set is an unordered collection of unique, hashable elements, written with curly braces, e.g. {1, 2, 3}, or created from another iterable with set(). Sets automatically discard duplicates and support fast membership testing (x in my_set) and mathematical set operations like union (|), intersection (&), and difference (-). Sets are mutable, but their elements must themselves be immutable/hashable.

Dictionaries — Key-Value Pairs

A dictionary maps unique keys to values, written with curly braces containing key: value pairs, e.g. {"name": "Alice", "age": 30}. Dictionaries are mutable — you can add, update, or delete key-value pairs after creation — and since Python 3.7, they preserve insertion order. Values are accessed by key rather than by numeric position: my_dict["name"] rather than my_dict[0].

Indexing and Slicing

Lists and tuples (both ordered sequences) support indexing with square brackets, where index 0 is the first element and -1 is the last. Slicing with the syntax [start:stop:step] extracts a sub-sequence without modifying the original — start is inclusive, stop is exclusive, and step controls the interval (a negative step reverses direction). Sets and dictionaries, being unordered by concept, do not support indexing or slicing.

Examples

fruits = ["apple", "banana", "cherry"]
fruits.append("date")
fruits[0] = "avocado"
print(fruits)

coordinates = (10, 20)
try:
    coordinates[0] = 99
except TypeError as e:
    print("Error:", e)

['avocado', 'banana', 'cherry', 'date']
Error: 'tuple' object does not support item assignment

numbers = [1, 2, 2, 3, 3, 3, 4]
unique_numbers = set(numbers)
print(unique_numbers)

person = {"name": "Sam", "age": 25}
person["city"] = "Austin"
print(person)

letters = ["a", "b", "c", "d", "e"]
print(letters[1:4])
print(letters[::-1])

{1, 2, 3, 4}
{'name': 'Sam', 'age': 25, 'city': 'Austin'}
['b', 'c', 'd']
['e', 'd', 'c', 'b', 'a']

Exercise

From the list of purchases, print the distinct items in alphabetical order, and then a dictionary that counts how many times each item was bought.

Expected output: ['bag', 'book', 'pen'] then {'pen': 3, 'book': 2, 'bag': 1}

purchases = ["pen", "book", "pen", "bag", "book", "pen"]

# TODO: print the distinct items, sorted

# TODO: build a dictionary of item -> count, and print it

purchases = ["pen", "book", "pen", "bag", "book", "pen"]

print(sorted(set(purchases)))

counts = {}
for item in purchases:
    counts[item] = counts.get(item, 0) + 1
print(counts)

Quick quiz

Interview questions