Q & A – Python List

(1) Check If A List Contains An Element.

The in operator will return True if a specific element is in a list.

lst = ['Subrat','Arpita','Abhispa','Subhada']
'Subhada' in lst

True

(2) How To Iterate Over 2+ Lists At The Same Time.

You can zip() lists and then iterate over the zip object.
A zip object is an iterator of tuples.
Below we iterate over 3 lists simultaneously and interpolate the values into a string.

name = ['Snowball', 'Chewy', 'Bubbles', 'Gruff']
animal = ['Cat', 'Dog', 'Fish', 'Goat']
age = [1, 2, 2, 6]
z = zip(name, animal, age)
for i, j, k in z:
    print(f"Name: {i} Animal: {j} Age: {k}")

(3) Is A List Mutable?

Yes.
Notice in the code below how the value associated with the same identifier in memory has not changed.

x = [1,2]
print(f"List: {x}    Address: {id(x)}")
x.append(3)
print(f"List: {x} Address: {id(x)}")

List: [1, 2]    Address: 1837796991424
List: [1, 2, 3] Address: 1837796991424

(4) Does A List Need To Be Homogeneous?

No.
Different types of object can be mixed together in a list.

a = [1,'a',1.0,[]]
print(a)

a = [1,'a',1.0,[]]
print(a)

(5) What Is The Difference Between Append And Extend?

.append() adds an object to the end of a list.
.extend() adds each value from a 2nd list as its own element. So extending a list with another list combines their values.

Append Method

a = [1,2,3]
print(a)
a.append([4])
print(a)

a = [1,2,3]
print(a)
a.append([4])
print(a)

Extend Method

a = [1,2,3]
print(a)
a.extend([4])
print(a)

[1, 2, 3]
[1, 2, 3, 4]

(5) What Does “del” Do?

‘del’ keyword can delete the entire list.
Or it can be used to delete an element from the list.

Example-1: Deleting An Element

a = [1,2,3]
print(a)
del a[0]
print(a)

[1, 2, 3]
[2, 3]

Example-2: Deleting Entire List

a = [1,2,3]
print(a)
del a
print(a)

[1, 2, 3]
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Input In [20], in <cell line: 4>()
      2 print(a)
      3 del a
----> 4 print(a)

NameError: name 'a' is not defined

(6) What Is The Difference Between “remove” and “pop”?

.remove() removes the first instance of a matching object.
.pop() removes an object by its index.

Example-1: remove() Method

a = ['a', 'a', 'b', 'b', 'c', 'c']
print(a)
a.remove('b')
print(a)

['a', 'a', 'b', 'b', 'c', 'c']
['a', 'a', 'b', 'c', 'c']

Example-2: pop() Method

a = ['a', 'a', 'b', 'b', 'c', 'c']
print(a)
a.pop(4)
print(a)

['a', 'a', 'b', 'b', 'c', 'c']
['a', 'a', 'b', 'b', 'c']

Note:

The difference between pop and del is that pop returns the popped element.

(7) Remove Duplicates From A List

If you’re not concerned about maintaining the order of a list, then converting to a set and back to a list will achieve this.

Example-1: Using Set

li = [3, 2, 2, 1, 1, 1]
list(set(li))

[1, 2, 3]

Example-1: Using List Comprehinsion

lst = [1,2,1,3,4,7,6,1,3,4,5,5]
print('Original List:', lst)
unique = []
[unique.append(i) for i in lst if i not in unique]
print("Unique List: ",unique)

Original List: [1, 2, 1, 3, 4, 7, 6, 1, 3, 4, 5, 5]
Unique List:  [1, 2, 3, 4, 7, 6, 5]

(8) Find The Index Of The 1st Matching Element.

We can use the index() method to get the index of first matching element.

Example-1:

fruit = ['pear', 'orange', 'apple', 'grapefruit', 'apple', 'pear']
fruit.index('apple')

Example-2:

fruit = ['pear', 'orange', 'apple', 'grapefruit', 'apple', 'pear']
fruit.index('kkk')

ValueError                                Traceback (most recent call last)
Input In [41], in <cell line: 2>()
      1 fruit = ['pear', 'orange', 'apple', 'grapefruit', 'apple', 'pear']
----> 2 fruit.index('kkk')

ValueError: 'kkk' is not in list

(9) Remove All Elements From A List.

.clear() method is used to remove all the element from the list.

Example-1:

fruit = ['pear', 'orange', 'apple']
print(fruit)
fruit.clear()
print(fruit)

['pear', 'orange', 'apple']
[]

(10) Iterate Over Both The Values Indices Of A List.

enumerate() method is used to get the index and values of the list.

Example-1:

name_list = ['Subrat','Subghada','Arpita']

for idx, val in enumerate(name_list):
    print(f'Id: {idx} Value: {val}')

Id: 0 Value: Subrat
Id: 1 Value: Subghada
Id: 2 Value: Arpita

(11) How To Concatenate Two Lists.

The + operator will concatenate 2 lists.

Example-1:

one = ['a', 'b', 'c']
two = [1, 2, 3]
three = one + two
print(three)

['a', 'b', 'c', 1, 2, 3]

(12) Count The Occurrence Of A Specific Object In A List

The count() method returns the number of occurrences of a specific object.

Example-1:

pets = ['dog','cat','fish','fish','cat']
pets.count('fish')

(13) How To Shallow Copy A List?

The count() method returns the number of occurrences of a specific object.

Example-1:

names = ['Subrat','Subghada','Arpita','Anuradha']

copy_names = names.copy()

copy_names.remove('Anuradha')

print(f'Names: {names}')

print(f'Copy Names: {copy_names}')

Names: ['Subrat', 'Subghada', 'Arpita', 'Anuradha']
Copy Names: ['Subrat', 'Subghada', 'Arpita']

Note:

If you change the “copy_name” variable it won’t affect the original variable “names”.

(14) What Is The Difference Between A Deep Copy And A Shallow Copy?

(1) Deep Copy:

In the case of deep copy, a copy of the object is copied into another object.
It means that any changes made to a copy of the object do not reflect in the original object.

Syntax Of Deep Copy:

import copy
copy.deepcopy(x)

Examples Of Deep Copy:

import copy

names = ['Subrat','Subghada','Arpita','Anuradha']

copy_names = copy.deepcopy(names)

copy_names.remove('Anuradha')

print(f'Names: {names}')

print(f'Copy Names: {copy_names}')

Names: ['Subrat', 'Subghada', 'Arpita', 'Anuradha']
Copy Names: ['Subrat', 'Subghada', 'Arpita']

(2) Shallow Copy:

In the case of shallow copy, a reference of an object is copied into another object.
It means that any changes made to a copy of an object do reflect in the original object.
In python, this is implemented using the “copy()” function.

Syntax Of Deep Copy:

import copy
copy.copy(x)

Examples Of Deep Copy:

import copy

names = ['Subrat','Subghada','Arpita','Anuradha']

copy_names = names 

copy_names.remove('Anuradha')

print(f'Names: {names}')

print(f'Copy Names: {copy_names}')

Names: ['Subrat', 'Subghada', 'Arpita']
Copy Names: ['Subrat', 'Subghada', 'Arpita']

(15) Multiply Every Element In A List By 5 With The Map Function.

In the case of deep copy, a copy of the object is copied into another object.
It means that any changes made to a copy of the object do not reflect in the original object.

Example-1:

def multiply_5(val):
    return val * 5
a = [10,20,30,40,50]
b = [val for val in map(multiply_5, a)] 
print(b)

[50, 100, 150, 200, 250]

(16) Insert A Value At A Specific Index In An Existing List

The insert() method takes an object to insert and the index to insert it at.

Example-1:

li = ['a','b','c','d','e']
li.insert(2, 'HERE')
print(li)

['a', 'b', 'HERE', 'c', 'd', 'e']

(17) Subtract Values In A List From The First Element With The “reduce” Function.

reduce() needs to be imported from functools.
Given a function, reduce iterates over a sequence and calls the function on every element.
The output from the previous element is passed as an argument when calling the function on the next element.

Example-1:

from functools import reduce

def subtract(a,b):
    return a - b

numbers = [100,10,5,1,2,7,5]

reduce(subtract, numbers)

Note:

Above we subtracted 10, 5, 1, 2, 7 and 5 from 100.

Example-1:

from functools import reduce

def subtract(a,b):
    return a + b

numbers = [100,10,5,1,2,7,5]

reduce(subtract, numbers)

Note:

Above we add 100, 10, 5, 1, 2, 7 .

(18) Remove Negative Values From A List With The Filter Function

Given a function, filter() will remove any elements from a sequence on which the function doesn’t return True.
Below we remove elements less than zero.

Example-1:

def remove_negatives(x):
    return True if x >= 0 else False

a = [-10, 27, 100, -1, 0, -30]

b = [x for x in filter(remove_negatives, a)]

print(b)

[27, 100, 0]

(19) Convert A List Into A Dictionary Where List Elements Are Keys.

For this we can use a dictionary comprehension.

Example-1:

li = ['The', 'quick', 'brown', 'fox', 'was', 'quick']
d = {k:1 for k in li}
print(d)

{'The': 1, 'quick': 1, 'brown': 1, 'fox': 1, 'was': 1}

(20) Modify An Existing List With A Lambda Function.

Let’s use the map() function to modify the list.

Example-1:

a = [10,20,30,40,50]

b = [x for x in map(lambda x: x*5, a)]

print(b)

[50, 100, 150, 200, 250]

(21) Remove Elements In A List After A Specific Index.

Using the slice syntax, we can return a new list with only the elements up to a specific index.

Example-1:

li = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,10]
li[:10]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

(22) Remove Elements In A List Before A Specific Index.

Using the slice syntax, we can return a new list with only the elements up to a specific index.

Example-1:

li = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,10]
li[15:]

[16, 17, 18, 19, 10]

(23) Remove Elements In A List Between 2 Indices

We can mention the start and end index to do this operation.

Example-1:

li = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,10]
li[12:17]

[13, 14, 15, 16, 17]

(24) Sort A List Of Integers In Ascending Order.

The sort() method mutates a list into ascending order.

Example-1:

li = [10,1,9,2,8,3,7,4,6,5]

li.sort()

li

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

(25) Sort A List Of Integers In Descending Order.

It’s also possible to sort in descending order with sort() by adding the argument reverse=True.

Example-1:

li = [10,1,9,2,8,3,7,4,6,5]

li.sort(reverse=True)

print(li)

[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

(26) Filter Even Values Out Of A List With List Comprehension.

You can add conditional logic inside a list comprehension to filter out values following a given pattern.
Here we filter out values divisible by 2.

Example-1:

def even(x):
    return True if x % 2 == 0 else False

li = [10,1,9,2,8,3,7,4,6,5]

[x for x in filter(even, li)]

[10, 2, 8, 4, 6]

(27) Count Occurrences Of Each Value In A List.

We can use dictionary comprehension to do this task
We can also use the Counter() method to do this operation.

Example-1:

lst = [1, 2, 3, 4, 1, 4, 1]

count = {k:lst.count(lst[v]) for v,k in enumerate(lst) }

print(count)

{1: 3, 2: 1, 3: 1, 4: 2}

Example-2:

from collections import Counter

li = ['blue', 'pink', 'green', 'green', 'yellow', 'pink', 'orange']

Counter(li)

Counter({'blue': 1, 'pink': 2, 'green': 2, 'yellow': 1, 'orange': 1})

(28) Combine Elements In A List Into A Single String.

This can be done with the join() function.

Example-1:

names = ['Subrat','Subghada','Arpita','Anuradha']

joined = "-".join(names)

print(joined)

Subrat-Subghada-Arpita-Anuradha

(29) Use The “any” Function To Return True If Any Value In A List Is Divisible By 2.

We can combine any() with a list comprehension to return True if any values in the returned list evaluate to True.
Below the 1st list comprehension returns True because the list has a 2 in it, which is divisible by 2.

Example-1:

li1 = [1,2,3]

truth_table = [i % 2 == 0 for i in li1]

print(truth_table)

any(truth_table)

[False, True, False]
True

Example-2:

li2 = [1,3]

truth_table = [i % 2 == 0 for i in li2]

print(truth_table)

any(truth_table)

[False, False]
False

(30) Use The “all” Function To Return True If All Values In A List Are Negative.

Similar to the any() function, all() can also be used with a list comprehension to return True only if all values in the returned list are True.

Example-1:

li1 = [1,2,3]

truth_table = [i > 0 for i in li1]

print(truth_table)

all(truth_table)

[True, True, True]
True

Example-2:

li2 = [1,-2,3]

truth_table = [i > 0 for i in li2]

print(truth_table)

all(truth_table)

[True, False, True]
False

(31) Reverse The Order Of A List.

A list can be mutated into reverse order with the reverse() method.

Example-1:

li = [1,2,3,4,5,6,7,8,9,10]

li.reverse()

print(li)

[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

(32) What Is The Difference Between Reverse And Reversed?

reverse() reverses the list in place.
reversed() returns an iterable of the list in reverse order.

Example-1:

li = [1,2,3,4,5,6,7,8,9,10]

li.reverse()

print(li)

[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Example-2:

li = [1,2,3,4,5,6,7,8,9,10]

rev = reversed(li)

print(rev)

list(rev)

<list_reverseiterator object at 0x000001ABE49721F0>
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

(33) What Is The Difference Between sort and sorted?

sort() modifies the list in place.
sorted() returns a new list in reverse order.

Example-1:

li = [10,1,9,2,8,3,7,4,6,5]

li.sort()

print(li)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Example-2:

li = [10,1,9,2,8,3,7,4,6,5]

new_lst = sorted(li)

print(new_lst)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

(34) Return The Minimum Value In A List.

The min() function returns the minimum value in a list.

Example-1:

li = [10,1,9,2,8,3,-7,4,6,5]

min(li)

-7

(35) Return The Maximum Value In A List.

The max() function returns the maximum value in a list.

Example-1:

li = [10,1,9,2,8,3,7,4,6,5]

max(li)

(36) Return The Sum Of Value In A List.

The sum() function returns the sum of all values in a list.

Example-1:

li = [10,1,9,2,8,3,7,4,6,5]

sum(li)

(37) Find The Intersection Of 2 Lists

The sum() function returns the sum of all values in a list.

Example-1:

li1 = [1,2,3]
li2 = [2,3,4]

set(li1) & set(li2)

{2, 3}

(38) Find The Difference Between A Set And Another Set.

We can’t subtract lists, but we can subtract sets.

Example-1:

li1 = [1,2,3]
li2 = [2,3,4]

set(li1) - set(li2)

{1}

Example-1:

li1 = [1,2,3]
li2 = [2,3,4]

set(li1) - set(li2)

{1}

(39) Flatten A List Of Lists With A List Comprehensions.

Unlike Ruby, Python3 doesn’t have an explicit flatten function. But we can use list comprehension to flatten a list of lists.

Example-1:

li = [[1,2,3],[4,5,6]]

[i for x in li for i in x]

[1, 2, 3, 4, 5, 6]

(40) Generate A List Of Every Integer Between 2 Values

We can create a range between 2 values and then convert that to a list.

Example-1:

list(range(1,10))

[1, 2, 3, 4, 5, 6, 7, 8, 9]

(41) Combine 2 Lists Into A Dictionary

Using zip() and the list() constructor we can combine 2 lists into a dictionary where one list becomes the keys and the other list becomes the values.

Example-1:

name = ['Snowball', 'Chewy', 'Bubbles', 'Gruff']

animal = ['Cat', 'Dog', 'Fish', 'Goat']

dict(zip(name, animal))

{'Snowball': 'Cat', 'Chewy': 'Dog', 'Bubbles': 'Fish', 'Gruff': 'Goat'}

(42) Reverse The Order Of A List Using The Slice Syntax

While we can reverse a list with reverse() and reversed(), it can also be done with the slice syntax.
This returns a new list by iterating back over the list from end to beginning.

Example-1:

li = [1,2,3,4,5,6,7,8]

li[::-1]

[8, 7, 6, 5, 4, 3, 2, 1]

(43) Find The Smallest Number In A List Using For Loop

We can use the for loop to iterate over the list and find the smallest number.

Example-1:

lst = [24, 10, 5, 20, 15]

current_min = lst[0]

for num in lst:
    if num < current_min:
        current_min = num
        
print(current_min)

(44) Sort A List Using For Loop

We can use the for loop to iterate over the list and find the Sort The Elements.

Example-1:

lst = [24, 10, 5, 20, 15]

temp = 0

for i in range(0, len(lst)):
    for j in range (i+1, len(lst)):
        if(lst[i] > lst[j]):
            temp = lst[i]
            lst[i] = lst[j]
            lst[j] = temp
print(lst)

[5, 10, 15, 20, 24]

Example-2:

lst = [24, 10, 5, 20, 15]

temp = 0

for i in range(0, len(lst)):
    for j in range (i+1, len(lst)):
        if(lst[i] < lst[j]):
            temp = lst[i]
            lst[i] = lst[j]
            lst[j] = temp
print(lst)

[24, 20, 15, 10, 5]