Question 1

What is a list in Python? What is its distinction from a tuple?

Accepted Answer

A list in Python is an array that stores multiple items into a single variable. Lists are ordered and mutable, allowing the addition, deletion or modification of entries in that list even after the creation of the list. Lists are declared with a square bracket [].

Conversely, a tuple is ordered but is immutable. Once a tuple is created, it cannot be modified. Tuples are created by enclosing values in parentheses ().

Key Differences:
List → Mutable
Tuple → Immutable

Example:
# This list can be modified
my_list = [10, 20, 30]
# This tuple cannot be changed
my_tuple = (10, 20, 30)
Whenever your data changes use lists. When you have fixed data remain unchanged use tuples.

Question 2

How does one create an empty list in Python?

Accepted Answer

An empty list in Python can be defined simply by using square brackets as shown below: empty_list = []. The other way of defining the empty list is by using the list() function as shown below: empty_list = list(). Both the implementations create a list that does not contain any element at the beginning. Typically, this is useful for capturing or holding values through the life of your program. For example, the program may later want to add user input, loop results or filtered items into this list.

Question 3

How do you add an element at the end of the list? (.append())

Accepted Answer

For a list you want to add a new item to the very end of you can always go ahead and use the method .append() in Python. This is by far one of the commonest as well as the simplest ways to increase the size of a list within the running program.

Example:
fruits = ["apple", "banana"]
fruits.append("orange")
print(fruits)

Output:
['apple', 'banana', 'orange']
Here, "orange" is appended to the end of the fruits collection. This method is particularly important when you create a list one item at a time, like collecting results from a loop or pooling data from user input.

Question 4

How do you add an element at a specific index in a list? (.insert())

Accepted Answer

The list method .insert() is used to insert an element at a particular place in a list.
According to the index (or place) chosen, it enables you to insert an item everywhere in the list.

How it works:
my_list = [1, 2, 3, 4]
my_list.insert(2, 5)
print(my_list)

Explanation:
insert(2, 5)- Means to put the 5 number in the 2nd location index that is in front of the 2nd element in the list.
As a result, the list looks like [1, 2, 5, 3, 4].
Some Key Points:
The first argument tells at what index you want the item to end up.
The second argument is what you want to insert.
If necessary, the list will shift elements to the right in order to create space for the added one.
This is useful when you need to fine-tune the location of an item in your list!

Question 5

How do you remove an element in a list by its value? (.remove())

Accepted Answer

To remove value from a list, you can use the .remove() method, which will remove the very first occurrence of the value specified in the list.
So, that's how you use it:

my_list = [1, 2, 3, 4, 2, 5]
my_list.remove(2)
print(my_list)

Explanation:
remove(2) will take the first occurrence of the value 2 out of my list.
Thus, this would give us this list: [1, 3, 4, 2, 5].

Key Points:
If the value is not found in the given list, it raises ValueError.
Removes the first occurrence of the value only not all of them.
The .remove() method makes any changes directly to the original list.
This is useful in cases where you are sure of the value but are unsure where the element to delete is in that particular index.

Question 6

How do you remove an element from a list by its position? (.pop())

Accepted Answer

Pop() is used to delete elements from a list by position or index. According to the particular index supplied while applying this method, the method removes the object from the list and returns the removed object against that index of that particular index supplied while applying for this method.
If no index is supplied to the method, in that case, the last element will be removed as well with the return.

This is how pop() works:
my_list = [10, 20, 30, 40, 50]
removed_element = my_list.pop(2) # 30 is removed from index 2
print(my_list) # Output now is [10, 20, 40, 50]
print(f"Removed element: {removed_element}" ) # Output will be Removed element: 30

Explanation:
Here pop(2) has the effect of removing and returning the element at index 2, which is 30 in this instance.
The revised list now will be [10, 20, 40, 50].
Had you not mentioned any index say my_list.pop() that means it removes and returns the last element of a list.
Key Points
The .pop() method removes an element from a list and returns it back. Thus it is a lot of use if you plan to work with the deleted item later on.
Popping an element from an index that does not exist would raise an IndexError (for an out-of-range index-specific).
It, like .remove(), modifies the self list in place.

Question 7

How do you access the list item by index?

Accepted Answer

In Python, getting the access to an item of the list can be done via the index by using square brackets [] with the index number written inside. An index starts from 0 for the first item, 1 for the second item and so on. In case you wish to access any item at the desired position just type the index number inside the square brackets.

The following is how an item can be accessed from a list by index:
my_list = [10, 20, 30, 40, 50]
item = my_list[2] # Accesses the element at index 2, which is 30
print(item) # Output will be 30

Explanation:
my_list[2] accesses the third item in the list (since indexing starts from 0), which is 30 in this case.
If you use a negative index (for example my_list[-1]), it accesses the last element from the list, which means that my_list[-1] would return 50, the last item.
Summary:
Index starts from 0, which means, for example, my_list[0] is the first item.
If you attempt to access an index out of range, you will receive an IndexError from Python.

Question 8

What happens if an index is out-of-bounds while attempting to access a list?

Accepted Answer

An IndexError in python is encountered when a list is accessed with an index that is out of bounds. This means that an index contemplated does not exist in the list.

For the sake of clarity:
my_list = [10, 20, 30]
print(my_list[5]) # Trying to access index 5, which is out of bounds.

Explanation:
The index 0 is given to the first element (10), index 1 to the second element (20) and finally index 2 to the last element (30) in my_list.
Here trying to access index 5 that is more than 2 is not index-out-of-bound hence IndexError will be raised:
The first indication may be the error user typically will see: "IndexError: list index out of range".
You can either verify the validity of the index before access or wrap it up in a try/catch that will catch the error.

Question 9

How many elements does a list contain? (len())

Accepted Answer

To find the number of elements in a list using Python, you can use its built-in function named len(). The function returns the number of items in a list.
Example:
my_list = [10, 20, 30, 40]
print(len(my_list)) # Output will be 4
For example,
my_list consists of four numeric elements which are 10, 20, 30 and 40.
When you pass my_list to the function len(), it will return 4 as there are some 4 elements in the list.

Points to note:
The built-in len() function is a global function in Python to be used with lists, strings, tuples, dictionaries and all other iterable objects.
It gives the number of values (total count) present in a collection such as a list or any other.

Question 10

How do you slice a list? Give an example.

Accepted Answer

In Python, slicing a list means getting a portion or a subset of the list by giving a starting index and an ending index. A step can be mentioned alternatively.
Here the slicing is done inside square brackets [] by putting a colon :.

Syntax:
list[start:end:step]
start: The index where slicing starts from (inclusive).
end: The index where slicing ends (exclusive).
step: Optional, it indicates the step or gap between each element.

Example:
my_list = [10, 20, 30, 40, 50, 60, 70]
sliced_list = my_list[2:5]
print(sliced_list) # Output: [30, 40, 50]

Explanation:
In this example, my_list[2:5] slices up starting from index 2 (which is 30) and slices up to but not including index 5 (which is 50).
Hence, the sliced list will be [30, 40, 50].

Next Examples:
Slicing with Step:
my_list = [10, 20, 30, 40, 50, 60, 70]
sliced_list = my_list[1:6:2]
print(sliced_list) # Output: [20, 40, 60]
So slicing is done from index 1 to index 6 having the value of 2 as a step to measure every second element.

Omitting Start or End:
Omitting start would mean here for slicing from the beginning:
sliced_list = my_list[:4] # Output: [10, 20, 30, 40]

Omit end to slice till the end:
sliced_list = my_list[3:] # Output: [40, 50, 60, 70]
Negative Indexing: Negative indices can also be used to slice from the rear of the list.
sliced_list = my_list[-4:-1] # Output: [40, 50, 60]

Key Points:
Slicing does not modify the original list, instead it returns a new list. In any case, if an index goes out of bounds, Python will take care of it automatically (for example, negative indices will count from the end of the list).

Question 11

How do you reverse the order of all items in a list? (.reverse() or slicing)

Accepted Answer

Reversal of lists is done in Python using the .reverse() method or slicing. Both methods give reversed lists, but they differ in how they work.
1. The .reverse() method: This modifies the original list and changes the order of elements to reversed order.
fruits = ['apple', 'banana', 'cherry']
fruits.reverse()
print(fruits) # Output: ['cherry', 'banana', 'apple']

2. Slicing method ([::-1]): This makes a new list, which is reversed in respect to the original.
fruits = ['apple', 'banana', 'cherry']
reversed_fruits = fruits[::-1]
print(reversed_fruits) # Output: ['cherry', 'banana', 'apple']

One must remember the point of importance:
- The .reverse() function deprecated changes to the original list.
- Slicing ([::-1]) creates a new reversed list leaving the original one intact.
Either way, this is very easy depending on the case for displaying the items in reverse order.

Question 12

How do you sort a list in Python? (.sort())

Accepted Answer

In the Python programming language, the ascending sort for a list's contents can be efficiently carried out using the built in .sort() method to arrange the items and modify the initial instance of the list itself.

The general syntax:
list.sort().

Example:
numbers = [5, 2, 9, 1, 7]
numbers.sort()
print(numbers) # Output: [1, 2, 5, 7, 9]

Explanation:
.sort() organizes list in place which means it does not return a new list.
Sorting is done in ascending order by default.

In order to sort in descending order use reverse=True option:
numbers.sort(reverse=True)
print(numbers) # Output: [9, 7, 5, 2, 1]
It works for lists in either numbers or strings giving you an easy way to keep your data in order.

Question 13

How do you create a new sorted list from an existing list? (sorted())

Accepted Answer

In Python, built-in function sorted() allows the sorting of a new list to be returned without modification of the old list.

This is how it works:
numbers = [5, 2, 9, 1, 7]
new_list = sorted(numbers)
print(new_list) # [1, 2, 5, 7, 9]
print(numbers) # [5, 2, 9, 1, 7] (original list is untouched)

Point to Note:
sorted() gives a brand new sorted list.
The original list keeps its order.
In addition, if you wish to sort in reverse order, just add the reverse=True argument, like this:
sorted(numbers, reverse=True)
This is sometimes useful when you wish to keep the original data untouched and work with a sorted version.

Question 14

What is list comprehension? Give an example of list comprehension that creates a list of squares.

Accepted Answer

In one line, list comprehension gives a concise way to tell how a new list is created — an operation performed on each one of another list or a range.
List comprehension is often used in cases where you want to build up a list on the fly using some loop like syntax.

For example, let’s create a list of squares from 1 to 5:
squares = [x**2 for x in range(1, 6)]
print(squares) # Output : [1, 4, 9, 16, 25]

Here, x**2 is the operation which is performed for each x in the range from 1 to 5. It looks like a for loop but much cleaner and easier to read.

Question 15

How do you add a new key-value pair to a dictionary?

Accepted Answer

You can add a new key-value pair to a dictionary in Python using simple assignment (e.g., `my_dict['city'] = 'New York'`), the `update()` method to add multiple pairs (e.g., `my_dict.update({'country': 'USA', 'job': 'Engineer'})`), the `setdefault()` method to add a pair only if the key doesn't exist (e.g., `my_dict.setdefault('city', 'Los Angeles')`), or by using the `dict()` constructor when creating a new dictionary (e.g., `my_dict = dict(name='Alice', age=25, city='New York')`).

Question 16

What happens if you try to access a value in a dictionary using a key that doesn't exist?

Accepted Answer

If you try to access a value using a non-existent key with square brackets `[]`, Python will raise a `KeyError` and the program will halt. However, if you use the `.get()` method with a non-existent key, it will return `None` by default, or a specified default value if provided, without raising an error.

Question 17

How do you tell if a key exists in a dictionary? (using in operator)

Accepted Answer

You can check if a key exists in a dictionary using the `in` operator. It returns `True` if the key is present in the dictionary's keys and `False` otherwise. Example: `if 'age' in my_dict: print("Key exists")`.

Question 18

How do you delete key-value pairs in a dictionary? (.pop(), del)

Accepted Answer

You can delete key-value pairs from a dictionary using the `.pop()` method, which removes the pair and returns the value (raising a `KeyError` if the key doesn't exist unless a default value is provided), or the `del` keyword, which removes the pair without returning a value (raising a `KeyError` if the key doesn't exist). Example: `age = my_dict.pop('age')` or `del my_dict['city']`.

Question 19

How do you obtain a list of all keys in a dictionary? (.keys())

Accepted Answer

The `.keys()` method returns a view object that displays a list of all the keys in the dictionary. You can convert this view object to a list using `list(my_dict.keys())`. Example: `keys = my_dict.keys()`, `keys_list = list(keys)`.

Question 20

How do you obtain a list of all values in a dictionary? (.values())

Accepted Answer

The `.values()` method returns a view object that displays a list of all the values in the dictionary. You can convert this view object to a list using `list(my_dict.values())`. Example: `values = my_dict.values()`, `values_list = list(values)`.

Question 21

How do you iterate over a dictionary and retrieve both keys and values? (.items())

Accepted Answer

You can iterate over a dictionary and retrieve both keys and values using the `.items()` method. This method returns a view object that displays a list of a dictionary's key-value tuple pairs. You can then unpack these pairs in a `for` loop. Example: `for key, value in my_dict.items(): print(f"Key: {key}, Value: {value}")`.

Question 22

What is dictionary comprehension? Give an example of using it to construct a new dictionary with altered values.

Accepted Answer

Dictionary comprehension is a concise way to create new dictionaries based on existing iterables. It uses a similar syntax to list comprehension but creates key-value pairs. Example: `old_dict = {'a': 1, 'b': 2, 'c': 3}`, `new_dict = {key: value * 2 for key, value in old_dict.items()}` will result in `new_dict` being `{'a': 2, 'b': 4, 'c': 6}`.

Question 23

Can you apply comprehensions for filtering items from a list or a dictionary? Provide examples.

Accepted Answer

Yes, you can use comprehensions to filter items from both lists and dictionaries by including an `if` condition within the comprehension syntax. List filtering example: `numbers = [1, 2, 3, 4, 5, 6]`, `even_numbers = [num for num in numbers if num % 2 == 0]` results in `[2, 4, 6]`. Dictionary filtering example: `marks = {'Alice': 45, 'Bob': 75, 'Carol': 60}`, `passed = {name: score for name, score in marks.items() if score > 50}` results in `{'Bob': 75, 'Carol': 60}`.

Question 24

Is it possible to have multiple instances of the same key in a Python dictionary? Give reasons if yes or no.

Accepted Answer

No, it is not possible to have multiple instances of the same key in a Python dictionary. Keys in a dictionary must be unique. If you try to assign a value to an existing key again, the old value will be overwritten by the new value. Python uses keys for efficient lookup, and duplicate keys would create ambiguity.

Question 25

Is it possible to have multiple instances of the same value in a Python dictionary? Give reasons if yes or no.

Accepted Answer

Yes, it is possible to have multiple instances of the same value in a Python dictionary. While keys must be unique, values can be duplicated across different keys. Python's dictionary structure enforces uniqueness only for keys, as they serve as the primary means of accessing the associated values.

Question 26

How do you create a copy of a dictionary? (.copy())

Accepted Answer

You can create a shallow copy of a dictionary in Python using the `.copy()` method. This method returns a new dictionary object with the same key-value pairs as the original. Modifying the copy will not affect the original dictionary (unless the values themselves are mutable objects). Example: `copy_dict = original_dict.copy()`.

Learning Python Lists and Dictionaries Interview Questions

1.What is a list in Python? What is its distinction from a tuple?

2. How does one create an empty list in Python?

3. How do you add an element at the end of the list? (.append())

4. How do you add an element at a specific index in a list? (.insert())

5. How do you remove an element in a list by its value? (.remove())

6. How do you remove an element from a list by its position? (.pop())

7. How do you access the list item by index?

8. What happens if an index is out-of-bounds while attempting to access a list?

9. How many elements does a list contain? (len())

10. How do you slice a list? Give an example.

11. How do you reverse the order of all items in a list? (.reverse() or slicing)

12. How do you sort a list in Python? (.sort())

13. How do you create a new sorted list from an existing list? (sorted())

14. What is list comprehension? Give an example of list comprehension that creates a list of squares.

15. How do you check for membership of an item in a list? (using in operator)

16. What is a dictionary in Python? What is its difference from a list?

17. How do you create an empty dictionary in Python?

18. How do you add a new key-value pair to a dictionary?

19. How do you access a value from a dictionary when you have the corresponding key?

20. What happens if you try to access a value in a dictionary using a key that doesn't exist?

21. How do you tell if a key exists in a dictionary? (using in operator)

22.How do you delete key-value pairs in a dictionary? (.pop(), del)

23.How do you obtain a list of all keys in a dictionary? (.keys())

24.How do you obtain a list of all values in a dictionary? (.values())

25.How do you iterate over a dictionary and retrieve both keys and values? (.items())

26.What is dictionary comprehension? Give an example of using it to construct a new dictionary with altered values.

27.Can you apply comprehensions for filtering items from a list or a dictionary? Provide examples.

28.Is it possible to have multiple instances of the same key in a Python dictionary? Give reasons if yes or no.

29. Is it possible to have multiple instances of the same value in a Python dictionary? Give reasons if yes or no.

30.How do you create a copy of a dictionary? (.copy())