Create an empty list of specific size in Python

This post will discuss how to create an empty list with a given size in Python.

To assign any value to a list using the assignment operator at position i, a[i] = x, the list’s size should be at least i+1. Otherwise, it will raise an IndexError, as shown below:

if __name__ == '__main__':

a = []

a[0] = 1

print(a)

# raise IndexError: list assignment index out of range

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The solution is to create an empty list of None when list items are not known in advance. This can be easily done, as shown below:

if __name__ == '__main__':

a = [None] * 5

print(a)

# prints [None, None, None, None, None]

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The above code will create a list of size 5, where each position is initialized by None. None is frequently used in Python to represent the absence of a value.

Another alternative for creating empty lists with a given size is to use list comprehensions:

if __name__ == '__main__':

a = [None for x in range(5)]

print(a)

# prints [[], [], [], [], []]

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Which solution to use?

The first solution works well for non-reference types like numbers. But you might run into referencing errors in some cases. For example, [[]] * 5 will result in the list containing the same list object repeated 5 times.

if __name__ == '__main__':

a = [[]] * 5

a[0].append(1)

print(a)

# prints [[1], [1], [1], [1], [1]]

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The solution to this problem is using list comprehensions like this:

if __name__ == '__main__':

a = [[] for x in range(5)]

a[0].append(1)

print(a)

# prints [[1], [], [], [], []]

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That’s all about creating an empty list with the given size in Python.

Also See:

Initialize a list with the same values in Python

Create an empty list of specific size in Python

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