Python 101: `[[ ] for i in range(len(nums) + 1)]` and `[[] * (len(nums) + 1)]`

They are not the same. Let’s break down the differences between [[ ] for i in range(len(nums) + 1)] and [[] * (len(nums) + 1)] in detail:

Explanation of Each Expression

1. Expression 1: [[] for i in range(len(nums) + 1)]

   • What it does:
     This list comprehension creates a new list of empty lists for each iteration in the range of len(nums) + 1.

   • How it works:

     • range(len(nums) + 1) generates a sequence of numbers starting from 0 up to len(nums).
     • For each number in this range, a new empty list [] is created and added to the outer list.
     • The result is a list of separate empty lists.

   • Example:

     nums = [1, 2, 3]
     result = [[] for i in range(len(nums) + 1)]
     print(result)  # Output: [[], [], [], []]

     Here, since len(nums) is 3, the range is from 0 to 3 (inclusive), resulting in 4 separate empty lists.

2. Expression 2: [[] * (len(nums) + 1)]

   • What it does:
     This expression creates a single list by repeating the empty list [] (len(nums) + 1) times.

   • How it works:

     • [] is repeated (len(nums) + 1) times using the multiplication operator *.
     • However, multiplying a list by a number does not create separate copies of the elements inside the list. Instead, it creates references to the same list multiple times.
     • As a result, any change to one of the lists will be reflected in all the "copies" because they all point to the same memory location.

   • Example:

     nums = [1, 2, 3]
     result = [[] * (len(nums) + 1)]
     print(result)  # Output: [[]]

     This creates a single empty list inside the outer list. In Python, multiplying an empty list by any number does not change its size or content, so you end up with [[]] (one empty list).

Summary

• [[ ] for i in range(len(nums) + 1)] creates len(nums) + 1 independent empty lists.
• [[] * (len(nums) + 1)] results in a list containing a single empty list, as multiplying an empty list [] by any number does not create separate lists.

Comparison Table

Tip

If you want to create a list of independent empty lists, use list comprehension or a method that explicitly creates separate copies. Avoid using multiplication with lists as it can lead to unintended results when working with mutable objects like lists.

Warning:
If you modify one of the lists created using the * multiplication method, all lists will be modified due to shared references.