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Declare an array in Python

David Y.

The Problem

How do I declare an array in Python?

The Solution

The default array type in Python is known as a list. To create a new empty list and assign it to a variable, we can use either of the following equivalent lines:

my_list = []
my_list = list()

We don’t have to instantiate our list as empty but can add any number of items of any type to it.

my_list = [1, "hello", True]

To create a list with many copies of the same value, we can use the multiplication operator:

my_list = [0] * 10  # will create a list containing ten 0s.

Note, using this syntax for more complex types can cause issues as the list is created as a reference copy.

basic = [1]
my_list = [basic]*10
print(my_list)    # [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]
basic[0] = 2
print(my_list)    # [[2], [2], [2], [2], [2], [2], [2], [2], [2], [2]]

Lists in Python can contain elements of different data types and do not have a predefined length. Python provides an array module that can be used to create lists that can only contain one type of element, though they can also be of arbitrary and varying lengths.

from array import array

my_int_array = array('i', [1, 2, 3, 4, 5])  # create an array of signed integers
my_int_array.append(6)  # will add 6 to the end of the array
my_int_array.append('a')  # will throw a TypeError: an integer is required (got type str)

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