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Python : List

Tutorial by:Rizwan Abdullah      Date: 2016-06-13 03:58:10

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Python offers a range of compound datatypes often referred to as sequences. List is one of the most frequently used and very versatile datatype used in Python.

Creating a List

In Python programming, a list is created by placing all the items (elements) inside a square bracket [ ], separated by commas. It can have any number of items and they may be of different types (integer, float, string etc.). A list can even have another list as an item. These are called nested list.


# empty list
my_list = []

# list of integers
my_list = [1, 2, 3]

# list with mixed datatypes
my_list = [1, "Hello", 3.4]

# nested list
my_list = ["mouse", [8, 4, 6]]

Accessing Elements in a List

There are various ways in which we can access the elements of a list.

Indexing

We can use the index operator [] to access an item in a list. Index starts from 0. So, a list having 5 elements will have index from 0 to 4. Trying to access an element other that this will raise an IndexError. The index must be an integer. We can't use float or other types, this will result into TypeError. Nested list are accessed using nested indexing.


>>> my_list = ['p','r','o','b','e']
>>> my_list[0]
'p'
>>> my_list[2]
'o'
>>> my_list[4]
'e'
>>> my_list[4.0]
...
TypeError: list indices must be integers, not float
>>> my_list[5]
...
IndexError: list index out of range

>>> n_list = ["Happy", [2,0,1,5]]
>>> n_list[0][1]    # nested indexing
'a'
>>> n_list[1][3]    # nested indexing
5

Negative indexing

Python allows negative indexing for its sequences. The index of -1 refers to the last item, -2 to the second last item and so on.


>>> my_list = ['p','r','o','b','e']
>>> my_list[-1]
'e'
>>> my_list[-5]
'p'

Changing or Adding Elements to a List

List are mutable, meaning, their elements can be changed unlike string or tuple. We can use assignment operator (=) to change an item or a range of items.


>>> odd = [2, 4, 6, 8]    # mistake values
>>> odd[0] = 1            # change the 1st item
>>> odd
[1, 4, 6, 8]
>>> odd[1:4] = [3, 5, 7]  # change 2nd to 4th items
>>> odd                   # changed values
[1, 3, 5, 7]

We can add one item to a list using append() method or add several items using extend() method.


>>> odd
[1, 3, 5]
>>> odd.append(7)
>>> odd
[1, 3, 5, 7]
>>> odd.extend([9, 11, 13])
>>> odd
[1, 3, 5, 7, 9, 11, 13]

We can also use + operator to combine two lists. This is also called concatenation. The * operator repeats a list for the given number of times.


>>> odd
[1, 3, 5]
>>> odd + [9, 7, 5]
[1, 3, 5, 9, 7, 5]
>>> ["re"] * 3
['re', 're', 're']

Furthermore, we can insert one item at a desired location by using the method insert() or insert multiple items by squeezing it into an empty slice of a list.


>>> odd
[1, 9]
>>> odd.insert(1,3)
>>> odd
[1, 3, 9]
>>> odd[2:2] = [5, 7]
>>> odd
[1, 3, 5, 7, 9]

Deleting or Removing Elements from a List

 

 
 

We can delete one or more items from a list using the keyword del. It can even delete the list entirely.


>>> my_list = ['p','r','o','b','l','e','m']
>>> del my_list[2]    # delete one item
>>> my_list
['p', 'r', 'b', 'l', 'e', 'm']
>>> del my_list[1:5]  # delete multiplt items
>>> my_list
['p', 'm']
>>> del my_list       # delete entire list
>>> my_list
...
NameError: name 'my_list' is not defined

We can use remove() method to remove the given item or pop() method to remove an item at the given index. The pop() method removes and returns the last item if index is not provided. This helps us implement lists as stacks (first in, last out data structure). We can also use the clear() method to empty a list.


>>> my_list = ['p','r','o','b','l','e','m']
>>> my_list.remove('p')
>>> my_list
['r', 'o', 'b', 'l', 'e', 'm']
>>> my_list.pop(1)
'o'
>>> my_list
['r', 'b', 'l', 'e', 'm']
>>> my_list.pop()
'm'
>>> my_list
['r', 'b', 'l', 'e']
>>> my_list.clear()
>>> my_list
[]

Finally, we can also delete items in a list by assigning an empty list to a slice of elements.


>>> my_list = ['p','r','o','b','l','e','m']
>>> my_list[2:3] = []
>>> my_list
['p', 'r', 'b', 'l', 'e', 'm']
>>> my_list[2:5] = []
>>> my_list
['p', 'r', 'm']

Python List Methods

Methods that are available with list object in Python programming are tabulated below. They are accessed as list.method(). Some of the methods have already been used above.

Python List Methods
Method Description
append(x) Add item x at the end of the list
extend(L) Add all items in given list L to the end
insert(i, x) Insert item x at position i
remove(x) Remove first item that is equal to x, from the list
pop([i]) Remove and return item at position i (last item if i is not provided)
clear() Remove all items and empty the list
index(x) Return index of first item that is equal to x
count(x) Return the number of items that is equal to x
sort() Sort items in a list in ascending order
reverse() Reverse the order of items in a list
copy() Return a shallow copy of the list

>>> my_list = [3, 8, 1, 6, 0, 8, 4]
>>> my_list.index(8)
1
>>> my_list.count(8)
2
>>> my_list.sort()
>>> my_list
[0, 1, 3, 4, 6, 8, 8]
>>> my_list.reverse()
>>> my_list
[8, 8, 6, 4, 3, 1, 0]

Python List Comprehension

List comprehension is an elegant and concise way to create new list from an existing list in Python. List comprehension consists of an expression followed by for statement inside square brackets. Here is an example to make a list with each item being increasing power of 2.


>>> pow2 = [2 ** x for x in range(10)]
>>> pow2
[1, 2, 4, 8, 16, 32, 64, 128, 256, 512]

This code is equivalent to


pow2 = []
for x in range(10):
   pow2.append(2 ** x)

A list comprehension can optionally contain more for or if statements. An optional if statement can filter out items for the new list. Here are some examples.


>>> pow2 = [2 ** x for x in range(10) if x > 5]
>>> pow2
[64, 128, 256, 512]
>>> odd = [x for x in range(20) if x % 2 == 1]
>>> odd
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
>>> [x+y for x in ['Python ','C '] for y in ['Language','Programming']]
['Python Language', 'Python Programming', 'C Language', 'C Programming']

Other List Operations

List Membership Test

We can test if an item exists in a list or not, using the keyword in.


>>> my_list = ['p','r','o','b','l','e','m']
>>> 'p' in my_list
True
>>> 'a' in my_list
False
>>> 'c' not in my_list
True

Iterating Through a List

Using a for loop we can iterate though each item in a list.


>>> for fruit in ['apple','banana','mango']:
...     print("I like",fruit)
...    
I like apple
I like banana
I like mango

Built-in Functions with List

Built-in functions like all(), any(), enumerate(), len(), max(), min(), list(), sorted() etc. are commonly used with list to perform different tasks.

Built-in Functions with List
Function Description
all() Return True if all elements of the list are true (or if the list is empty).
any() Return True if any element of the list is true. If the list is empty, return False.
enumerate() Return an enumerate object. It contains the index and value of all the items of list as a tuple.
len() Return the length (the number of items) in the list.
list() Convert an iterable (tuple, string, set, dictionary) to a list.
max() Return the largest item in the list.
min() Return the smallest item in the list
sorted() Return a new sorted list (does not sort the list itself).
sum() Retrun the sum of all elements in the list.
 

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