List

Jump to Section

Last update on February 28 2020 12:07:32 (UTC/GMT +8 hours)

List

A list is a container which holds comma-separated values (items or elements) between square brackets where items or elements need not all have the same type.

In general, we can define a list as an object that contains multiple data items (elements). The contents of a list can be changed during program execution. The size of a list can also change during execution, as elements are added or removed from it.

Python List Structure

Note: There are much programming languages which allow us to create arrays, which are objects similar to lists. Lists serve the same purpose as arrays and have many more built-in capabilities. Traditional arrays can not be created in Python.

Examples of lists:

numbers = [10, 20, 30, 40, 50]
names = [“Sara”, “David”, “Warner”, “Sandy”]
student_info = [“Sara”, 1, “Chemistry”]

Contents:

Lists: Commands

<list> = <list>[from_inclusive : to_exclusive : ±step_size]  
<list>.append(<el>)         
# Or: <list> += [<el>]
<list>.extend(<collection>) 
 # Or: <list> += <collection>  
<list>.sort()
<list>.reverse()
<list> = sorted(<collection>)
<iter> = reversed(<list>)  
sum_of_elements  = sum(<collection>)
elementwise_sum  = [sum(pair) for pair in zip(list_a, list_b)]
sorted_by_second = sorted(<collection>, key=lambda el: el[1])
sorted_by_both   = sorted(<collection>, key=lambda el: (el[1], el[0]))
flatter_list     = list(itertools.chain.from_iterable(<list>))
product_of_elems = functools.reduce(lambda out, x: out * x, <collection>)
list_of_chars    = list(<str>)  
# Returns number of occurrences. Also works on strings.
  <int> = <list>.count(<el>)
# Returns index of first occurrence or raises ValueError.
  index = <list>.index(<el>)
# Inserts item at index and moves the rest to the right.
  <list>.insert(index, <el>)
# Removes and returns item at index or from the end. 
  <el> = <list>.pop([index])
# Removes first occurrence of item or raises ValueError. 
  <list>.remove(<el>)
# Removes all items. Also works on dictionary and set.  
  <list>.clear()

Create a Python list

Following list contains all integer values:

my_list1 = [5, 12, 13, 14] # the list contains all integer values
print(my_list1)
[5, 12, 13, 14]

Following list contains all string:

my_list2 = ['red', 'blue', 'black', 'white'] # the list contains all string
# values
print(my_list2)
['red', 'blue', 'black', 'white']

Following list contains a string, an integer and a float values:

my_list3 = ['red', 12, 112.12] # the list contains a string, an integer and
# a float values
print(my_list3)
# ['red', 12, 112.12]

A list without any element is called an empty list. See the following statements.

my_list=[]
print(my_list)
# []

Use + operator to create a new list that is a concatenation of two lists and use * operator to repeat a list. See the following statements.

color_list1 = ["White", "Yellow"]
color_list2 = ["Red", "Blue"]
color_list3 = ["Green", "Black"]
color_list = color_list1 + color_list2 + color_list3
print(color_list)
# ['White', 'Yellow', 'Red', 'Blue', 'Green', 'Black']
number = [1,2,3]
print(number[0]*4)
# 4
print(number*4)
# [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]

List indices

List indices work the same way as string indices, list indices start at 0. If an index has a positive value it counts from the beginning and similarly it counts backward if the index has a negative value. As positive integers are used to index from the left end and negative integers are used to index from the right end, so every item of a list gives two alternatives indices. Let create a list called color_list with four items.

color_list=["RED", "Blue", "Green", "Black"]

Item	RED	Blue	Green	Black
Index (from left)	0	1	2	3
Index (from right)	-4	-3	-2	-1

If you give any index value which is out of range then interpreter creates an error message. See the following statements.

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements
# indices start at 0 and end at 3
color_list[0] # Return the First Element
# 'Red'
print(color_list[0],color_list[3]) # Print First and Last Elements
# Red Black
color_list[-1] # Return Last Element
# 'Black'
print(color_list[4]) # Creates Error as the indices is out of range
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# IndexError: list index out of range

Add an item to the end of the list

Python List: Add item at the end of the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.append("Yellow")
print(color_list)
# ['Red', 'Blue', 'Green', 'Black', 'Yellow']

Insert an item at a given position

Python List: Insert an item at a given position

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.insert(2, "White") #Insert an item at third position
print(color_list)
# ['Red', 'Blue', 'White', 'Green', 'Black']

Modify an element by using the index of the element

Python List: modify an element in the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list[2]="Yellow"  #Change the third color
print(color_list)
# ['Red', 'Blue', 'Yellow', 'Black']

Remove an item from the list

Python List: remove an element from the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.remove("Black")
print(color_list)
# ['Red', 'Blue', 'Green']

Remove all items from the list

Python List: remove all elements from the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.clear()
print(color_list)
# []

List Slices

Lists can be sliced like strings and other sequences.

Syntax:

sliced_list = List_Name[startIndex:endIndex]

This refers to the items of a list starting at index startIndex and stopping just before index endIndex. The default values for list are 0 (startIndex) and the end (endIndex) of the list. If you omit both indices, the slice makes a copy of the original list.

Cut first two items from a list:

Python List: Cut first two items of a list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements 
# indices start at 0 and end at 3
print(color_list[0:2]) # cut first two items
# ['Red', 'Blue']

Cut second item from a list:

Python List: Cut second element from a list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements 
# indices start at 0 and end at 3
print(color_list[1:2])
# ['Blue']
print(color_list[1:-2])
# ['Blue']

Cut second and third elements from a list:

Python List: Cut second and third elements from a list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements 
# indices start at 0 and end at 3
print(color_list[1:-1])
# ['Blue', 'Green']

Cut first three items from a list:

Python List: First three items from a list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements 
# indices start at 0 and end at 3
print(color_list[:3]) # cut first three items
# ['Red', 'Blue', 'Green']

Creates copy of original list:

Python List: Creates copy of original list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"] # The list have four elements 
# indices start at 0 and end at 3
print(color_list[:]) # Creates copy of original list
# ['Red', 'Blue', 'Green', 'Black']

Remove the item at the given position in the list, and return it

Python List: Remove the item at the given position in the list, and return it

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.pop(2) # Remove second item and return it
# 'Green'
print(color_list)
# ['Red', 'Blue', 'Black']

Return the index in the list of the first item whose value is x

Python List: Return the number of times ‘x’ appear in the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.index("Red")
# 0
color_list.index("Black")
# 3

Return the number of times ‘x’ appear in the list

Python List: Return the index in the list of the first item whose value is x

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list=["Red", "Blue", "Green", "Black", "Blue"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black', 'Blue']
color_list.count("Blue")
# 2

Sort the items of the list in place

Python List: Return the number of times ‘x’ appear in the list

See the following statements:

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.sort(key=None, reverse=False)
print(color_list)
# ['Black', 'Blue', 'Green', 'Red']

Reverse the elements of the list in place

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
['Red', 'Blue', 'Green', 'Black']
# >> color_list.reverse()
print(color_list)
# ['Black', 'Green', 'Blue', 'Red']

Return a shallow copy of the list

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.copy()
# ['Red', 'Blue', 'Green', 'Black']

Search the Lists and find Elements

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.index("Green")
# 2

Lists are Mutable

Items in the list are mutable i.e. after creating a list you can change any item in the list. See the following statements.

color_list=["Red", "Blue", "Green", "Black"]
print(color_list[0])
# Red
color_list[0]="White" # Change the value of first item "Red" to "White"
print(color_list)
# ['White', 'Blue', 'Green', 'Black']
print(color_list[0])
# White

Convert a list to a tuple in Python

listx = [1, 2, 3, 4]  
print(listx)  
# [1, 2, 3, 4]  
tuplex = tuple(listx)  
print(tuplex)  
# (1, 2, 3, 4)

How to use the double colon [ : : ]?

listx=[1, 5, 7, 3, 2, 4, 6]
print(listx)
# [1, 5, 7, 3, 2, 4, 6]
sublist=listx[2:7:2] #list[start:stop:step], #step specify an increment
# between the elements to cut of the list.
print(sublist)
# [7, 2, 6]
sublist=listx[::3] #returns a list with a jump every 2 times.
print(sublist)
# [1, 3, 6]
sublist=listx[6:2:-2] #when step is negative the jump is made back
print(sublist)
# [6, 2]

Find the largest and the smallest item in a list

listx=[5, 10, 3, 25, 7, 4, 15]
print(listx)
# [5, 10, 3, 25, 7, 4, 15]
print(max(listx))	# the max() function of built-in allows to know the highest
value in the list.
# 25
print(min(listx)) #the min() function of built-in allows to know the lowest
value in the list.
# 3

Compare two lists in Python

listx1, listx2=[3, 5, 7, 9], [3, 5, 7, 9]
print (listx1 == listx2)
# True
listx1, listx2=[9, 7, 5, 3], [3, 5, 7, 9]	#create two lists equal, but unsorted.
print(listx1 == listx2)
# False
listx1, listx2 =[2, 3, 5, 7], [3, 5, 7, 9]	#create two different list
print(listx1 == listx2)
# False
print(listx1.sort() == listx2.sort())	#order and compare
# True

Nested lists in Python

listx = [["Hello", "World"], [0, 1, 2, 3, 4, 5]]
print(listx)
# [['Hello', 'World'], [0, 1, 2, 3, 4, 5]]
listx = [["Hello", "World"], [0, 1, 2, 3, 3, 5]]
print(listx)
# [['Hello', 'World'], [0, 1, 2, 3, 3, 5]]
print(listx[0][1])		#The first [] indicates the index of the outer list.
# World
print(listx[1][3])		#the second [] indicates the index nested lists.
# 3
listx.append([True, False])		#add new items
print(listx)
# [['Hello', 'World'], [0, 1, 2, 3, 3, 5], [True, False]]		
listx[1][2]=4
print(listx)
# [['Hello', 'World'], [0, 1, 4, 3, 3, 5], [True, False]]		#update value items

How can I get the index of an element contained in the list?

listy = list("HELLO WORLD")
print(listy)
# ['H', 'E', 'L', 'L', 'O', ' ', 'W', 'O', 'R', 'L', 'D']
index = listy.index("L")	#get index of the first item whose value is passed as parameter
print(index)
# 2
index = listy.index("L", 4)	#define the index from which you want to search
print(index)
# 9
index = listy.index("O", 3, 5)	#define the segment of the list to be searched
print(index)
# 4

Using Lists as Stacks

color_list=["Red", "Blue", "Green", "Black"]
print(color_list)
# ['Red', 'Blue', 'Green', 'Black']
color_list.append("White")
color_list.append("Yellow")
print(color_list)
# ['Red', 'Blue', 'Green', 'Black', 'White', 'Yellow']
color_list.pop()
# 'Yellow'
color_list.pop()
# 'White'
color_list.pop()
# Black'
color_list
# ['Red', 'Blue', 'Green']

Using Lists as Queues

from collections import deque
color_list = deque(["Red", "Blue", "Green", "Black"])
color_list.append("White")      # White arrive
print(color_list)
# deque(['Red', 'Blue', 'Green', 'Black', 'White'])
color_list.append("Yellow")     # Yellow arrive
print(color_list)
# deque(['Red', 'Blue', 'Green', 'Black', 'White', 'Yellow'])
color_list.popleft()            # The first to arrive now leaves
# 'Red'
print(color_list)
# deque(['Blue', 'Green', 'Black', 'White', 'Yellow'])
color_list.popleft()            # The second to arrive now leaves
# 'Blue'
print(color_list)
# deque(['Green', 'Black', 'White', 'Yellow'])
print(color_list)               # Remaining queue in order of arrival
# deque(['Green', 'Black', 'White', 'Yellow'])

Source : .

Dictionary