Data Types and Structures in Python

Python's built-in collections cover many everyday data tasks. Choose a structure for its semantics, not just its syntax.

Lists: ordered and mutable

Lists preserve order and support indexed access, slicing, and in-place changes.

fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
fruits.insert(1, "kiwi")
fruits.remove("banana")
last_fruit = fruits.pop()
fruits[0] = "mango"

print(fruits)      # ['mango', 'kiwi', 'cherry']
print(last_fruit)  # orange

remove(value) raises ValueError when the value is absent; pop(index) removes and returns an item and raises IndexError for an invalid index.

Tuples: ordered and immutable

Tuples cannot have their items reassigned. They are useful for fixed records and unpacking. Parentheses improve readability, but the comma creates a tuple; a one-item tuple therefore needs a trailing comma.

point = (3, 5)
x, y = point
single = ("only",)

print(x, y)         # 3 5
print(len(single))  # 1

Immutability is shallow: a tuple itself cannot be changed, but it may contain a mutable object such as a list.

Dictionaries: key-value mappings

Dictionaries map unique, hashable keys to values and preserve insertion order. Assigning an existing key replaces its value.

record = {"name": "Alice", "age": 30}
record["city"] = "Berlin"
record["age"] = 31
city = record.pop("city")

print(record)  # {'name': 'Alice', 'age': 31}
print(city)    # Berlin

record[key] raises KeyError for a missing key. Use record.get(key, default) when absence is expected and a default is meaningful.

Sets: unique, unordered values

Sets support membership tests and mathematical set operations. Their iteration and printed order should not be treated as stable. Use set(), not {}, for an empty set.

observed = {"red", "blue", "red"}
allowed = {"blue", "green"}

print(sorted(observed))            # ['blue', 'red']
print(sorted(observed & allowed))  # ['blue']
print(sorted(observed - allowed))  # ['red']

Set elements and dictionary keys must be hashable. Immutable values such as strings and suitable tuples are common choices; lists are not hashable.

Choosing a collection

Need	Typical choice
Ordered sequence that changes	list
Fixed ordered record or unpacking	tuple
Lookup by unique key	dict
Unique values or set algebra	set

DataFrames are provided by pandas rather than Python itself and are covered in the dedicated pandas fundamentals section.