Python List

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.

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"]

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"]

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

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

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

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

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

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:

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:

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:

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:

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:

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

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

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

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

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'])
>>>

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