60 Python List Questions

Question 1

Check if a list contains an element

Answer 1

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

li = [1,2,3,’a’,’b’,’c’]
‘a’ in li # True

Question 2

How to iterate over 2+ lists at the same time

Answer 2

You can zip() lists and then iterate over the zip obj. A zip obj is an iterator of tuples

name = ['Snowball', 'Chewy', 'Bubbles', 'Gruff']
animal = ['Cat', 'Dog','Fish', 'Goat']
age = [1,2,2,6]

z = zip(name, animal, age)
for name, animal, age in z:
print(“%s the %s is %s” % (name, animal, age))

Snowball the Cat is 1 Chewy the Dog is 2 Bubbles the Fish is 2 Gruff the Goat is 6

Question 3

When would you use a list vs dictionary?

Answer 3

Depends on use-case, dictionary searches are faster than lists

Use list when order matters
ids = [23,1,7,9]

Use dictionary when you want to tally occurances
pets = {‘dogs’: 2, ‘cats’: 1, ‘fish’: 5}

Question 4

Is a list mutable?

Answer 4

Yes, notice same memory address

x = [1]
print(id(x), ‘:’, x) # 2769545838976 : [1]

x.append(5)
x.extend([6,7])
print(id(x), ‘:’, x) # 2769545838976 : [1, 5, 6, 7, 5, 6, 7]

Question 5

Does a list need to be homogeneous?

Answer 5

Nope, different types of objects can be mixed together in a list

a = [1, ‘a’, 1.0, []]
a # [1, ‘a’, 1.0, []]

Question 6

What’s the difference between append and extend?

Answer 6

.append() adds a single obj to the end of a list

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

.extend() combines their values

b = [1,2,3]
b.extend([5,6])
b #=> [1, 2, 3, 5, 6] # [1, 2, 3, 5, 6]

Question 7

Do Python lists store values or pointers?

Answer 7

Lists are mutable they store pointers than actual values in memory

print(id(1)) # 140718920247088
print(id(2)) # 140718920247120

a = [1,2,3]
print(id(a)) # 2769551452544

print(id(a[0])) # 140718920247088
print(id(a[1])) # 140718920247120

Question 8

What does ‘del’ do?

Answer 8

del removes an item from a list given its index

a = [‘w’, ‘x’, ‘y’, ‘z’]
a #=> [‘w’, ‘x’, ‘y’, ‘z’]
del a[1]
a #=> [‘w’, ‘y’, ‘z’]

Question 9

What is the difference between ‘remove’ and ‘pop’?

Answer 9

.remove() removes the first instance of a matching obj

a = [‘a’, ‘a’, ‘b’, ‘b’, ‘c’, ‘c’]
a.remove(‘b’)
a # [‘a’, ‘a’, ‘b’, ‘c’, ‘c’]

pop() removes the last element of a list by default if an index isn’t specified pop(2)

a = [‘a’, ‘a’, ‘b’, ‘b’, ‘c’, ‘c’]
a.pop(4) #=> ‘c’

Question 10

Remove duplicates from a list

Answer 10

If you’re not concerned about maintaining the order of a list, then convert to a set then back to a list

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

Question 11

Find the idx of the 1st matching element

Answer 11

Use index() to search by value and return the index

fruit = [‘pear’, ‘orange’, ‘apple’, ‘grapefruit’, ‘apple’, ‘pear’]

fruit. index(‘apple’) # 2
fruit. index(‘pear’) # 0

Question 12

Remove all elements from a list

Answer 12

Rather than creating a new empty list, we can clear the elements from list with .clear() or del fruit[:]

fruit = [‘pear’, ‘orange’, ‘apple’]
print( fruit ) # [‘pear’, ‘orange’, ‘apple’]

print( id(fruit) ) #4581174216

fruit.clear()

print( fruit ) # []
print( id(fruit) ) # 4581174216

Question 13

Iterate over both the values in a list and their indices

Answer 13

enumerate() adds a counter to the list passed as an argument

grocery_list = [‘flour’,’cheese’,’carrots’]
for idx,val in enumerate(grocery_list):
print(“%s: %s” % (idx, val))

0: flour
# 1: cheese
# 2: carrots

Question 14

How to concatenate two list

Answer 14

the + operator will concatenate 2 lists

one = ['a', 'b', 'c']
two = [1, 2, 3]

one + two # [‘a’, ‘b’, ‘c’, 1, 2, 3]

Question 15

How to manipulate every element in a list with list comprehension

Answer 15

Below return a new list with 1 added to every element

li = [0, 25, 50, 100]

[i+1 for i in li] # [1, 26, 51, 101]

Question 16

Count the occurrence of a specific object in a list

Answer 16

The count() returne the number of occurrences of a specific object

pets = [‘dog’,’cat’,’fish’,’fish’,’cat’]
pets.count(‘fish’) # 2

Question 17

How to shallow copy a list?

Answer 17

.copy() can be used to shallow copy a list

round1 = [‘chuck norris’, ‘bruce lee’, ‘sonny chiba’]
round2 = round1.copy()
round2.remove(‘sonny chiba’)

print(round1) #[‘chuck norris’, ‘bruce lee’, ‘sonny chiba’]
print(round2)# [‘chuck norris’, ‘bruce lee’]

Question 18

Why create a shallow copy of a list?

Answer 18

round1 = [‘chuck norris’, ‘bruce lee’, ‘sonny chiba’]

round2 = round1
round2.remove(‘sonny chiba’)

print(round1) # [‘chuck norris’, ‘bruce lee’]
print(round2) # [‘chuck norris’, ‘bruce lee’]

Without a shallow copy, round1 and round2 are just names pointing to the same list in memory.

Question 19

How to deep copy a list?

Answer 19

For this we need to import the copy module, then call copy.deepcopy().

Below we create a deep copy of a list, round1 called round2, update a value in round2, then print both. In this case, round1 isn’t affected.

round1 = [[‘Arnold’, ‘Sylvester’, ‘Jean Claude’], [‘Buttercup’, ‘Bubbles’, ‘Blossom’] ]
import copy
round2 = copy.deepcopy(round1)round2[0][0] = ‘Jet Lee’

Question 20

What is the difference bewtween a deep copy and a shallow copy?

Answer 20

Creating a shallow copy does create a new object in memory, but it’s filled with the same references to existing objects that the previous list has.

Creating a deep copy creates copies of the original objects and points to these new versions. So the new list is completely unaffected by changes to the old list and vice versa.

Question 21

What is the difference between a list and a tuple?

Answer 21

Tuples cannot be updated after creation. Adding/removing/updating an existing tuple requires creating a new tuple.

Lists can be modified after creation.

Tuples often represent an object like a record loaded from a database where elements are of different datatypes.

Lists are generally used to store an ordered sequence of a specific type of object (but not always).

Both are sequences and allow duplicate values.

Question 22

Return the length of a list

Answer 22

len() can return the length of a list

li = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’]
len(li) #5

But note it counts top level objects, so a nested list of several integers will only be counted as a single object. Below, li has a length of 2, not 5.

li = [[1,2],[3,4,5]]
len(li) #2

Question 23

What is the difference between a list and a set?

Answer 23

While a list is ordered, a set is not. That’s why using set to find unique values in a list, like list( set([3, 3, 2, 1]) ) loses the order.

While lists are often used to track order, sets are often used to track existence.

Lists allow duplicates, but all values in a set are unique by definition.

Question 24

How to check if an element is not in a list?

Answer 24

For this we use the in membership operator, but prefix with not.

li = [1,2,3,4]
5 not in li # True
4 not in li # False

Question 25

Multiply every element in a list by 5 with the map function

Answer 25

.map() allows iterating over a sequence and updating each value with another function.

def multiply\_5(val):
return val \* 5

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

[val for val in map(multiply_5, a)] # [50, 100, 150, 200, 250]

Question 26

Combine 2 lists into a list of tuple with the zip function

Answer 26

zip() combines multiple sequences into an iterator of tuples, where values at the same sequence index are combined in the same tuple.

alphabet = [‘a’, ‘b’, ‘c’] integers = [1, 2, 3]

list(zip(alphabet, integers)) # [(‘a’, 1), (‘b’, 2), (‘c’, 3)]

Question 27

Insert a value at a specific index in an existing list

Answer 27

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

li = [‘a’,’b’,’c’,’d’,’e’]
li.insert(2, ‘HERE’)
li # [‘a’, ‘b’, ‘HERE’, ‘c’, ‘d’, ‘e’]

Question 28

Subtract values in a list from the first element with the reduce function

Answer 28

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.

from functools import reduce

def subtract(a,b):
return a - b
numbers = [100,10,5,1,2,7,5]
reduce(subtract, numbers) # 70

Question 29

Remove negative values from a list with the filter function

Answer 29

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.

def remove\_negatives(x):
return True if x \>= 0 else False

a = [-10, 27, 1000, -1, 0, -30]

[x for x in filter(remove_negatives, a)] # [27, 1000, 0]

Question 30

Convert a list into a dictionary where list elements are keys

Answer 30

For this we can use a dictionary comprehension

li = [‘The’, ‘quick’, ‘brown’, ‘fox’, ‘was’, ‘quick’]
d = {k:1 for k in li}
d #=> {‘The’: 1, ‘quick’: 1, ‘brown’: 1, ‘fox’: 1, ‘was’: 1}

Question 31

Modify an existing list with a lambda function

Answer 31

Let’s take the previous map function we wrote and turn it into a one-liner with a lambda.

a = [10,20,30,40,50]
list(map(lambda val:val*5, a)) # [50, 100, 150, 200, 250]

Question 32

Remove elements in a list after a specific index

Answer 32

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

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]

Question 33

Remove elements in a list before a specific index

Answer 33

The slice syntax can also return a new list with the values after a specified index.

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]

Question 34

Remove elements in a list between 2 indices

Answer 34

Or between two indices

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]

Question 35

Return every 2nd element in a list between 2 indices

Answer 35

Or before/after/between indices at a specific interval.

Here we return every 2nd value between the indices 10 and 16 using the slice syntax.

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

Question 36

Sort a list of integers in ascending order

Answer 36

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

li = [10,1,9,2,8,3,7,4,6,5]
li.sort()
li # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Question 37

Sort a list of integers in descending order

Answer 37

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

li = [10,1,9,2,8,3,7,4,6,5]
li.sort(reverse=True)
li # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Question 38

Filter even values out of a list with list comprehension

Answer 38

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.

li = [1,2,3,4,5,6,7,8,9,10]
[i for i in li if i % 2 != 0] # [1, 3, 5, 7, 9]

Question 39

Count occurrences of each value in a list

Answer 39

One option is to iterate over a list and add counts to a dictionary. But the easiest option is to import the Counter class from collections and pass the list to it.

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})

Question 40

Get the first element from each nested list in a list

Answer 40

A list comprehension is well suited for iterating over a list of other objects and grabbing an element from each nested object.

li = [[1,2,3],[4,5,6],[7,8,9],[10,11,12],[13,14,15]]
[i[0] for i in li] # [1, 4, 7, 10, 13]

Question 41

What is the time complexity of insert, find and delete for a list?

Answer 41

Insert is O(n). If an element is inserted at the beginning, all other elements must be shifted right.

Find by index is O(1). But find by value is O(n) because elements need to be iterated over until the value is found.

Delete is O(n). If an element is deleted at the beginning, all other elements must to be shifted left.

Question 42

Combine elements in a list into a single string

Answer 42

This can be done with the join() function

li = [‘The’,’quick’,’brown’, ‘fox’, ‘jumped’, ‘over’, ‘the’, ‘lazy’, ‘dog’]
“ “.join(li) # ‘The quick brown fox jumped over the lazy dog’

Question 43

What’s the affect of multiplying a list by an integer?

Answer 43

Multiplying a list by an integer is called multiple concatenation and has the same affect as concatenating a list to itself n-times.

[‘a’,’b’] * 5 # [‘a’, ‘b’, ‘a’, ‘b’, ‘a’, ‘b’, ‘a’, ‘b’, ‘a’, ‘b’]

Question 44

Use the “any” function to return True if any value in a list is divisible by 2

Answer 44

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.

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

any(i % 2 == 0 for i in li1) # True
any(i % 2 == 0 for i in li2) # False

Question 45

Use the “all” function to return True if all values in a list are negative

Answer 45

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.

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

all(i % 2 == 0 for i in li1) # False
all(i % 2 == 0 for i in li2) # True

Question 46

Can you sort a list with “None” in it?

Answer 46

You cannot sort a list with None in it because comparison operators (used by sort()) can’t compare an integer with None.

li = [10,1,9,2,8,3,7,4,6,None]
li.sort()
li # TypeError: ‘

Question 47

What kind of copy would the list constructor create from an existing list?

Answer 47

The list constructor creates a shallow copy of a passed in list. That said, this is less pythonic than using .copy().

li1 = [‘a’,’b’]
li2 = list(li1)
li2.append(‘c’)

print(li1) # [‘a’, ‘b’]
print(li2) # [‘a’, ‘b’, ‘c’]

Question 48

Reverse the order of a list

Answer 48

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

li = [1,2,3,4,5,6,7,8,9,10]
li.reverse() # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Question 49

What is the difference between reverse and reversed?

Answer 49

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

li = [1,2,3,4,5,6,7,8,9,10]
list(reversed(li)) # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Question 50

What is the difference between sort and sorted?

Answer 50

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

li = [10,1,9,2,8,3,7,4,6,5]
li.sort()
li # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

li = [10,1,9,2,8,3,7,4,6,5]
sorted(li) # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Question 51

Return the minimum value in a list

Answer 51

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

li = [10,1,9,2,8,3,7,4,6,5]
min(li) # 1

Question 52

Return the maximum value in a list

Answer 52

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

li = [10,1,9,2,8,3,7,4,6,5]
max(li) # 10

Question 53

Return the sum of values in a list

Answer 53

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

li = [10,1,9,2,8,3,7,4,6,5]
sum(li) # 55

Question 54

Use a list as a stack

Answer 54

You can use append() and pop() to treat a list like a stack. Stacks function per LIFO (last in first out).

stack = []

stack. append(‘Jess’)
stack. append(‘Todd’)
stack. append(‘Yuan’)

print(stack) # [‘Jess’, ‘Todd’, ‘Yuan’]
print(stack.pop()) # Yuan
print(stack) # [‘Jess’, ‘Todd’]

Question 55

Find the intersection of 2 lists

Answer 55

We can do this by utilizing set() with an ampersand.

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

set(li1) & set(li2) # {2, 3}

Question 56

Find the difference between a set and another set

Answer 56

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

li1 = [1,2,3]
li2 = [2,3,4]
set(li1) - set(li2) #{1}
set(li2) - set(li1) #{4}

Question 57

Flatten a list of lists with a list comprehensions

Answer 57

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

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

[i for x in li for i in x] # [1, 2, 3, 4, 5, 6]

Question 58

Generate a list of every integer between 2 values

Answer 58

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

list(range(5,10)) # [5, 6, 7, 8, 9]

Question 59

Combine two lists into a dictionary

Answer 59

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.

name = ['Snowball', 'Chewy', 'Bubbles', 'Gruff']
animal = ['Cat', 'Dog', 'Fish', 'Goat']

dict(zip(name, animal)) # {‘Snowball’: ‘Cat’, ‘Chewy’: ‘Dog’, ‘Bubbles’: ‘Fish’, ‘Gruff’: ‘Goat’}

Question 60

Reverse the order of a list using the slice syntax

Answer 60

While we can reverse a list with reverse() and reversed(), it can also be done with the slice syntax.

li = [‘a’,’b’,3,4]
li[::-1] # [4, 3, ‘b’, ‘a’]