Python Lists, Nested Lists & List Comprehension

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Python Lists, Nested Lists & List Comprehension Interview Questions

What is a list in Python?
A list is an ordered, mutable collection of items. Lists can contain elements of different data types and are defined using square brackets []. Example: my_list = [1, 2, 3, "hello", True].
What is list slicing in Python?
List slicing extracts parts of a list using the syntax list[start:stop:step]. Example: numbers[1:4] gets elements from index 1 to 3. Negative indices count from the end: numbers[-3:] gets last 3 elements.
What are common list methods in Python?
append() - adds element at end
extend() - adds multiple elements
insert() - inserts at specific position
remove() - removes first matching element
pop() - removes and returns element at index
sort() - sorts list in place
reverse() - reverses list in place
What is list comprehension in Python?
List comprehension is a concise way to create lists. Syntax: [expression for item in iterable if condition]. Example: squares = [x**2 for x in range(10)] creates list of squares from 0 to 81.
What is a nested list in Python?
A nested list is a list containing other lists as elements. Example: matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]. Access elements using multiple indices: matrix[0][1] returns 2.
How to flatten a nested list?
Multiple ways:
# Using nested loops flat_list = [] for sublist in nested_list: for item in sublist: flat_list.append(item) # Using list comprehension flat_list = [item for sublist in nested_list for item in sublist] # Using itertools.chain from itertools import chain flat_list = list(chain.from_iterable(nested_list))
What is the difference between append() and extend() methods?
append() adds its argument as a single element to the end of a list. extend() iterates over its argument and adds each element. Example: list.append([1,2]) adds list as element, list.extend([1,2]) adds 1 and 2 as separate elements.
How to create a list with list comprehension with if condition?
Syntax: [expression for item in iterable if condition]. Example: even_numbers = [x for x in range(20) if x % 2 == 0] creates list of even numbers from 0 to 18.
What is the difference between sort() and sorted()?
sort() is a list method that sorts the list in place and returns None. sorted() is a built-in function that returns a new sorted list from any iterable, leaving the original unchanged.
How to copy a list in Python?
Shallow copy: new_list = old_list.copy() or new_list = old_list[:]
Deep copy (for nested lists): import copy; new_list = copy.deepcopy(old_list)
Assignment (new_list = old_list) creates a reference, not a copy.
How to use list comprehension with if-else conditions?
Syntax: [expression1 if condition else expression2 for item in iterable]. Example: result = ["even" if x%2==0 else "odd" for x in range(5)] creates ['even', 'odd', 'even', 'odd', 'even'].
What is the difference between remove() and pop() methods?
remove() removes the first matching value and returns None. pop() removes and returns the element at a specific index (default last). remove() takes a value, pop() takes an index.
How to create a 2D list (matrix) using list comprehension?
Example: matrix = [[i+j for j in range(3)] for i in range(3)] creates:
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
This creates a 3x3 matrix with values based on row and column indices.
What is the time complexity of list operations in Python?
append(): O(1)
insert(): O(n)
pop() (end): O(1)
pop() (middle): O(n)
remove(): O(n)
index(): O(n)
in operator: O(n)
How to transpose a matrix (2D list) using list comprehension?
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] transpose = [[row[i] for row in matrix] for i in range(len(matrix[0]))] # Result: [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
Or using zip: transpose = list(map(list, zip(*matrix)))
What is list unpacking in Python?
List unpacking assigns elements of a list to multiple variables. Example: a, b, c = [1, 2, 3] assigns 1 to a, 2 to b, 3 to c. Use * to capture remaining elements: first, *middle, last = [1, 2, 3, 4, 5].
How to find common elements between two lists?
# Using list comprehension common = [x for x in list1 if x in list2] # Using sets (faster for large lists) common = list(set(list1) & set(list2))
Set method is O(1) average for lookups vs O(n) for list method.
What is the difference between list and array?
Lists are built-in Python data structures that can hold heterogeneous data types. Arrays (from array module or NumPy) are more memory efficient for homogeneous data types and support mathematical operations. Lists are more flexible, arrays are faster for numerical computations.
How to remove duplicates from a list?
# Preserving order (Python 3.7+) from collections import OrderedDict unique_list = list(OrderedDict.fromkeys(my_list)) # Using set (doesn't preserve order) unique_list = list(set(my_list)) # Using list comprehension (preserves order) unique_list = [] [unique_list.append(x) for x in my_list if x not in unique_list]
How to use nested list comprehension?
Nested list comprehension creates lists of lists. Syntax: [[expression for inner_item in inner_iterable] for outer_item in outer_iterable]. Example: matrix = [[0 for _ in range(3)] for _ in range(3)] creates 3x3 matrix of zeros.
Note: Lists are one of Python's most versatile data structures. List comprehension provides a more readable and efficient way to create and manipulate lists. Understanding list operations, slicing, and comprehension is essential for writing clean, efficient Python code. Lists are mutable, ordered collections that can contain heterogeneous elements, making them suitable for many programming scenarios.
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