Array — Efficient arrays of numeric values in Python

Jennifer Nicholas

Answered on 16th Apr, 2019

Array is a very popular data structure in C/C++, Java etc. In these languages array is defined as a collection of more than one elements of similar data type. Python doesn't have any built-in equivalent of array. It's List as well as Tuple is a collection of elements but they may of different types.

Python's array module emulates C type array. The module defines 'array' class. Following constructor creates an array object:

array(typecode, initializer)

The typecode argument determines the type of array. Initializer should be a sequence with all elements of matching type.

Following statement creates an integer array object:

>>> import array
>>> arr = array.array('i', range(5))
>>> arr
array('i', [0, 1, 2, 3, 4])
>>> type(arr)
<class 'array.array'>
>>> array.typecodes
'bBuhHiIlLqQfd'

The array module defines typecodes attribute which returns a string. Each character in the string represents a type code indicating C type and equivalent Python type:

The initializer argument can be a byte like object. Following example builds an array from byte representation of string.

>>> arr1 = array.array('b', b'Hello')
>>> arr1
array('b', [72, 101, 108, 108, 111])

The array class defines following methods:

array.buffer_info()

This method returns a tuple (address, length) giving the current memory address and the length in elements of the buffer used to hold array’s contents.

>>> arr = array.array('i', [0, 1, 2, 3, 4])
>>> arr.buffer_info()
(2201141755144, 5)

count()

This method returns the number of occurrences of certain element in the array.

>>> arr = array.array('i', [0, 1, 2, 3, 4])
>>> arr.count(2)
1

extend()

This method appends items from iterable to the end of the array or iterable which must have exactly the same type code; if not, TypeError will be raised.

>>> arr = array.array('i', [0, 1, 2, 3, 4])
>>> arr1 = array.array('i',[10,20,30])
>>> arr.extend(arr1)
>>> arr
array('i', [0, 1, 2, 3, 4, 10, 20, 30])

fromfile()

This method reads n items (as machine values) from the file object and appends to an array.

In following example, we first open a file in binary write mode.

>>> file = open('test.txt','wb')
>>> file.write(b'Hello Python')
12
>>> file.close()

We now use this file to append its data to array object.

>>> a = array.array('i')
>>> file = open('test.txt','rb')
>>> a.fromfile(file,file.tell())
>>> a
array('i', [1819043144, 2035294319, 1852794996])

append()

This method appends a new item to the end of the array

fromlist()

This method appends items from the list to array. This is equivalent to for x in list: a.append(x)

>>> a = array.array('i')
>>> a.append(10)
>>> a
array('i', [10])
>>> num = [20,30,40,50]
>>> a.fromlist(num)
>>> a
array('i', [10, 20, 30, 40, 50])

insert()

Insert a new item in the array before specified position

>>> a = array.array('i', [10, 20, 30, 40, 50])
>>> a.insert(2,25)
>>> a
array('i', [10, 20, 25, 30, 40, 50])

pop()

This method returns item at given index after removing it from the array.

>>> a = array.array('i', [10, 20, 30, 40, 50])
>>> x = a.pop(2)
>>> x
30
>>> a
array('i', [10, 20, 40, 50])

remove()

This method removes first occurrence of given item from the array.

>>> a = array.array('i', [10, 20, 30, 40, 50])
>>> a.remove(30)
>>> a
array('i', [10, 20, 40, 50])

tofile()

This method write all items to the file object having write permission enabled.

>>> a = array.array('i', [10, 20, 30, 40, 50])
>>> file = open("test.txt","wb")
>>> a.tofile(file)
>>> file.close()
>>> file = open("test.txt","rb")
>>> file.read()
b'\n\x00\x00\x00\x14\x00\x00\x00\x1e\x00\x00\x00(\x00\x00\x002\x00\x00\x00'