Skip to content

Unpacking Iterables

Introduction

Python allows you to assign elements of a list or tuple to multiple variables in a single, readable line. This is called unpacking.

It is widely used in interviews for returning multiple values from a function, swapping variables, or extracting coordinates from a matrix.

Basic Unpacking

If the number of variables on the left matches the number of elements on the right, Python unpacks them sequentially.

point = [10, 20]

# Unpacking the list
x, y = point
print(x) # 10
print(y) # 20

Common Interview Mistake: ValueError

If the number of variables does not exactly match the length of the iterable, Python raises a ValueError.

# ValueError: too many values to unpack (expected 2)
x, y = [10, 20, 30]

The Swapping Idiom

Because Python evaluates the entire right side before performing any assignments, you can unpack to easily swap variables without a temporary variable.

a = 1
b = 2

a, b = b, a
This is the standard, expected way to swap elements in Python.

Advanced Unpacking (The * Operator)

If you only care about the first or last few elements of a list, you can use the * operator to gather the remaining items into a new list.

nums = [1, 2, 3, 4, 5]

# Get the first element, put the rest in a list
first, *rest = nums
print(first) # 1
print(rest)  # [2, 3, 4, 5]

You can put the * anywhere.

# First, last, and middle
first, *middle, last = nums
print(first)  # 1
print(middle) # [2, 3, 4]
print(last)   # 5

Using * in Function Arguments

You can unpack a list directly into function arguments using *.

def add(a, b, c):
    return a + b + c

nums = [10, 20, 30]
print(add(*nums)) # 60

Time and Space Complexity

  • Time Complexity: \(O(N)\) where \(N\) is the number of elements being unpacked.
  • Space Complexity: \(O(1)\) for strict unpacking (assigning references), but \(O(K)\) when using *rest because a new list of \(K\) elements is constructed.

Summary

  • Use a, b = b, a to swap variables.
  • Use a, *rest = list to capture arbitrary numbers of elements.
  • Unpacking improves code readability and eliminates the need for manual indexing x = nums[0]; y = nums[1].