Top Python Interview Questions and Expert Answers Explained

1. How does Python manage memory?

Answer: It uses three mechanisms—reference counting, garbage collection, and a memory pool.

Reference Counting

Every object has a reference count. The count increases when a new name is bound to the object or when it is placed in a container (list, tuple, dict). It decreases when del is used, when the reference goes out of scope, or when reassigned. sys.getrefcount() returns the current count. Immutable objects like numbers and strings are often shared to save memory.

Garbage Collection

When an object's reference count drops to zero, it is reclaimed. For reference cycles, Python periodically runs a cyclic detector to break and collect unreachable objects.

Memory Pool (pymalloc)

Python keeps freed memory in its own pools instead of returning it to the OS. Objects smaller than 256 bytes use the pymalloc allocator; larger objects use the system malloc. Separate private pools exist for integers, floats, and lists, so memory freed by one type cannot be reused by another.

2. What is a lambda function and its benefits?

Answer: A lambda expression creates an anonymous (unnamed) function, useful for short callbacks.

Syntax: lambda [arguments]: expression Example:

a = lambda x, y: x + y

a(3, 11)

3. How to convert between tuple and list in Python?

Answer: Use the built‑in tuple() and list() functions; type() can check the object type.

4. Code to remove duplicate elements from a list

Answer:

Method 1: set(my_list) Method 2 using a dictionary:

a = [1,2,4,2,4,5,6,5,7,8,9,0]
b = {}
b = b.fromkeys(a)
c = list(b.keys())
print(c)

5. Sort a list and remove consecutive duplicates from the end

a = [1,2,4,2,4,5,7,10,5,5,7,8,9,0,3]
a.sort()
last = a[-1]
for i in range(len(a)-2, -1, -1):
    if last == a[i]:
        del a[i]
    else:
        last = a[i]
print(a)

6. How to copy objects in Python (assignment, shallow copy, deep copy)?

Assignment ( =) creates a new reference; changes affect both names.

Shallow copy creates a new container but references the original items. Methods: slicing, list(), copy.copy().

Deep copy recursively copies all nested objects, so modifications are independent. Use copy.deepcopy().

7. Usage and purpose of except

Syntax: try … except … [else …] [finally …]. If an exception occurs in the try block, control jumps to the matching except. The else runs if no exception, and finally always runs.

8. What does the pass statement do?

It does nothing; it serves as a placeholder in empty code blocks.

9. How to use the range() function?

It generates a sequence of numbers, commonly used in for i in range(...) loops.

10. How to search and replace text using Python?

Use the re module’s sub() or subn() functions.

import re
p = re.compile('blue|white|red')
print(p.sub('colour', 'blue socks and red shoes'))
print(p.sub('colour', 'blue socks and red shoes', count=1))
print(p.subn('colour', 'blue socks and red shoes'))

11. Difference between match() and search() in re

re.match()

checks only at the start of the string; re.search() scans the whole string for the first occurrence.

print(re.match('super', 'superstition').span())   # (0, 5)
print(re.match('super', 'insuperable'))          # None
print(re.search('super', 'superstition').span()) # (0, 5)
print(re.search('super', 'insuperable').span())  # (2, 7)

12. Greedy vs. non‑greedy matching with <.*> and <.*?>

<.*>

is greedy (matches the longest possible string); <.*?> is non‑greedy (matches the shortest).

13. Generating random numbers in Python

Use the random module:

Integers: random.randint(a, b) Range: random.randrange(start, stop, step) Floats: random.random() (0‑1) or

random.uniform(a, b)

14. Tools for static code analysis and bug detection

PyChecker and Pylint can analyze code for bugs, complexity, and style.

15. Setting a global variable inside a function

def f():
    global x
    x = 10

16. Differences among single, double, and triple quotes

Single and double quotes are equivalent; use a backslash to escape characters. Triple quotes allow multi‑line strings and can contain both single and double quotes without escaping.