Python 3 Lists

Python 3 Lists

Sequences are the most basic data structure in Python. Each element in a sequence is assigned a number—its position, or index. The first index is 0, the second is 1, and so on.

Python has six built-in sequence types, but the most common are lists and tuples.

Operations on sequences include indexing, slicing, addition, multiplication, and checking membership.

In addition, Python has built-in methods for determining the length of a sequence and the largest and smallest elements.

A list is the most commonly used Python data type and appears as a comma-separated list of values enclosed in square brackets.

List items do not need to be of the same type

To create a list, simply enclose the comma-separated items in square brackets. As shown below:

list1 = ['Google', 'Geekdoc', 1997, 2000];
list2 = [1, 2, 3, 4, 5 ];
list3 = ["a", "b", "c", "d"];

Like string indices, list indices start at 0. Lists can be truncated, combined, and so on.

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Accessing List Values

Use subscript indexing to access list values. You can also use square brackets to delimit characters, as shown below:

Example (Python 3.0+)

#!/usr/bin/python3

list1 = ['Google', 'Geekdoc', 1997, 2000];
list2 = [1, 2, 3, 4, 5, 6, 7 ];

print ("list1[0]: ", list1[0])
print ("list2[1:5]: ", list2[1:5])

Run Example »

The above example outputs:

list1[0]: Google
list2[1:5]: [2, 3, 4, 5]

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Update List

You can modify or update the data items in a list, or you can use the append() method to add list items, as shown below:

Example (Python 3.0+)

#!/usr/bin/python3

list = ['Google', 'Geekdoc', 1997, 2000]

print ("The third element is: ", list[2])
list[2] = 2001
print ("The updated third element is: ", list[2])

Note: We will discuss the use of the append() method in the next chapter.

The output of the above example is:

The third element is: 1997
The updated third element is: 2001

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Delete list elements

You can use del Statement to delete elements of a list, as shown in the following example:

Example (Python 3.0+)

#!/usr/bin/python3

list = ['Google', 'Geekdoc', 1997, 2000]

print ("Original list: ", list)
del list[2]
print ("Delete the third element: ", list)

The above example outputs:

Original list: ['Google', 'Geekdoc', 1997, 2000]
Delete the third element: ['Google', 'Geekdoc', 2000]

Note: We will discuss remove() in the next section. Method Usage

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Python List Script Operators

List operators (+ and *) are similar to string operators. The + operator is used to combine lists, and the * operator is used to repeat a list.

As shown below:

Python expression	Result	Description
len([1, 2, 3])	3	Length
[1, 2, 3] + [4, 5, 6]	[1, 2, 3, 4, 5, 6]	Combination
[‘Hi!’] * 4	[‘Hi!’, ‘Hi!’, ‘Hi!’, ‘Hi!’]	Repeat
3 in [1, 2, 3]	True	Is the element in the list?
for x in [1, 2, 3]: print(x, end=” “)	1 2 3	Iteration

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Python list truncation and concatenation

Python list truncation and string operation types are as follows:

L=['Google', 'Geekdoc', 'Taobao']

Operation:

Python expression	Result	Description
L[2]	‘Taobao’	Read the third element
L[-2]	‘Geekdoc’	Read the second to last element from the right: count from the right
L[1:]	[‘Geekdoc’, ‘Taobao’]	Output all elements starting from the second element

>>>L=['Google', 'Geekdoc', 'Taobao']
>>> L[2]
'Taobao'
>>> L[-2]
'Geekdoc'
>>> L[1:]
['Geekdoc', 'Taobao']
>>>

Lists also support concatenation operations:

>>>squares = [1, 4, 9, 16, 25]
>>> squares += [36, 49, 64, 81, 100]
>>> squares
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>>

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Nested Lists

Using nested lists means creating other lists within lists, for example:

>>>a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

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Python List Functions & Methods

Python includes the following functions:

Sequence Number
1	len(list) Number of elements in a list
2	max(list) Returns the maximum value of a list
3	min(list) Returns the minimum value of a list
4	list(seq) Converts a tuple to a list

Python includes the following methods:

Sequence number
1	list.append(obj) Adds a new object to the end of a list
2	list.count(obj) Counts the number of times an element appears in a list
3	list.extend(seq) Appends multiple values from another sequence to the end of a list (extends the original list with the new list)
4	list.index(obj) Finds the index of the first occurrence of a value in a list
5	list.insert(index, obj) Insert an object into a list
6	list.pop([index=-1]) Remove an element from a list (default is the last element) and return the value of that element
7	list.remove(obj) Remove the first occurrence of a value from a list
8	list.reverse() Reverse the elements in a list
9	list.sort( key=None, reverse=False) Sort the original list
10	list.clear() Clear the list
11	list.copy() Copy the list