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Python Lists

Introduction

A list in Python is an ordered, mutable collection of objects. It is the closest equivalent to an "array" in other languages, but with significantly more built-in flexibility.

Creating Lists

You can create lists using square brackets [] or the list() constructor.

# Empty list
empty1 = []
empty2 = list()

# Initialized list
nums = [1, 2, 3]

# Mixed types (valid, but rare in DSA problems)
mixed = [1, "two", 3.0, [4, 5]]

Pre-allocating Lists

Often in interviews (especially for Dynamic Programming), you need a list of a specific size initialized with a default value.

# Create a list of 10 zeros
dp = [0] * 10
print(dp) # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
This is an \(O(N)\) operation and is highly optimized.

Mutability

Unlike strings, lists are mutable. You can modify them in-place without creating a new object.

nums = [1, 2, 3]
nums[0] = 99
print(nums) # [99, 2, 3]

The Shared Reference Trap

Because lists are mutable, assigning one list to another variable does not create a copy. Both variables reference the same list in memory.

a = [1, 2, 3]
b = a
b.append(4)

print(a) # [1, 2, 3, 4]
If you need an independent copy, use a.copy() or a[:].

Lists vs Arrays

In languages like C or Java, an "array" is a contiguous block of memory holding values of the exact same type. The size is fixed at creation.

Python lists are different: 1. Dynamic Sizing: They grow and shrink automatically. 2. Heterogeneous: They can store elements of different types. 3. Array of Pointers: Under the hood, a Python list is an array of pointers (references) to the actual objects, not an array of the objects themselves.

If an interviewer asks you to use an "array", they almost always mean a Python list.

(Note: Python does have an array module for C-style arrays of primitives, but it is rarely used in standard algorithmic interviews).

Summary

  • Lists are ordered and mutable.
  • [0] * N is the idiomatic way to initialize a list of size \(N\).
  • Assigning b = a copies the reference, not the list.
  • Python lists are arrays of pointers, meaning they can store mixed data types.