Python Basics — Creating & Accessing Lists

Creating & Accessing Lists

A list is an ordered collection of items enclosed in square brackets []. Lists can hold any data type, and items can be different types.

Creating Lists

# Empty list
empty = []
also_empty = list()

# List of numbers
numbers = [1, 2, 3, 4, 5]

# List of strings
fruits = ["apple", "banana", "cherry"]

# Mixed types
mixed = [1, "hello", 3.14, True, None]

# From range
nums = list(range(1, 6))   # [1, 2, 3, 4, 5]

# From a string
chars = list("Python")     # ['P', 'y', 't', 'h', 'o', 'n']

Indexing

Lists use zero-based indexing, just like strings:

fruits = ["apple", "banana", "cherry", "date"]

# Positive indexing
print(fruits[0])   # apple
print(fruits[1])   # banana
print(fruits[3])   # date

# Negative indexing
print(fruits[-1])  # date
print(fruits[-2])  # cherry

Index:    0         1         2        3
       ["apple", "banana", "cherry", "date"]
Neg:    -4        -3        -2       -1

Modifying Elements

Unlike strings, lists are mutable:

fruits = ["apple", "banana", "cherry"]

fruits[1] = "blueberry"
print(fruits)  # ['apple', 'blueberry', 'cherry']

Built-in Functions with Lists

numbers = [4, 2, 8, 1, 5]

print(len(numbers))   # 5
print(min(numbers))   # 1
print(max(numbers))   # 8
print(sum(numbers))   # 20
print(sorted(numbers))  # [1, 2, 4, 5, 8]  (returns new list)

Checking Membership

fruits = ["apple", "banana", "cherry"]

print("banana" in fruits)      # True
print("grape" in fruits)       # False
print("grape" not in fruits)   # True

Empty Lists and Truthiness

empty = []
full = [1, 2, 3]

print(bool(empty))   # False (empty list is falsy)
print(bool(full))    # True

# Pythonic way to check if a list has items
if fruits:
    print("List has items")
else:
    print("List is empty")

List Repetition and Concatenation

# Concatenation
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print(c)  # [1, 2, 3, 4, 5, 6]

# Repetition
zeros = [0] * 5
print(zeros)  # [0, 0, 0, 0, 0]

# Caution with mutable elements!
matrix = [[0]] * 3   # [[0], [0], [0]] — all point to the SAME inner list!
matrix[0][0] = 1
print(matrix)         # [[1], [1], [1]] — all changed!

# Safe way
matrix = [[0] for _ in range(3)]
matrix[0][0] = 1
print(matrix)         # [[1], [0], [0]] — only first changed

Iterating Over Lists

fruits = ["apple", "banana", "cherry"]

# Simple iteration
for fruit in fruits:
    print(fruit)

# With index
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# Using range (less Pythonic)
for i in range(len(fruits)):
    print(fruits[i])

Summary

• Create lists with [], list(), or list(range(...))
• Access elements with positive [0] or negative [-1] indexing
• Lists are mutable — elements can be changed in place
• Use len(), min(), max(), sum() for common operations
• Use in to check membership
• Empty lists are falsy; non-empty lists are truthy
• Be careful with [[]] * n — use comprehensions for independent inner lists

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