TCS Python Developer Interview Questions & Coding Guide (2026 Edition)

Q1. What is the difference between a List and a Tuple in Python?

Lists are mutable (can append, remove, modify elements), defined with square brackets [], and consume slightly more memory due to dynamic over-allocation. Tuples are immutable (cannot be altered after creation), defined with parentheses (), have faster iteration performance, and can be used as dictionary keys because they are hashable.

Q2. What are Decorators in Python and how do you write one?

A decorator is a function that takes another function as an argument, extends its behavior without modifying its source code, and returns the modified function.

def timing_decorator(func): def wrapper(*args, **kwargs): import time start = time.time() result = func(*args, **kwargs) print(f"Executed {func.__name__} in {time.time() - start:.4f}s") return result return wrapper @timing_decorator
def calculate_squares(n): return [i**2 for i in range(n)]

Q3. What is the Global Interpreter Lock (GIL) and how does it affect multithreading?

The GIL is a mutex in CPython that prevents multiple native OS threads from executing Python bytecodes simultaneously. It protects CPython memory management from race conditions. Because of the GIL, Python multithreading is effective for I/O-bound tasks (network/file operations) but does NOT provide true parallelism for CPU-bound computations (which require the multiprocessing module).

Problem 1: Reverse Words in a String without built-in reverse

def reverse_words(sentence: str) -> str: words = sentence.strip().split() return " ".join(words[::-1]) # Test
print(reverse_words("TCS Digital Python Interview"))
# Output: "Interview Python Digital TCS"

Problem 2: Count Character Frequencies and Output in Order

def char_frequency(s: str) -> dict: freq = {} for char in s: freq[char] = freq.get(char, 0) + 1 return freq print(char_frequency("intervio"))
# Output: {'i': 2, 'n': 1, 't': 1, 'e': 1, 'r': 1, 'v': 1, 'o': 1}

Executing SQL from Python (sqlite3 / SQLAlchemy)

Connecting to DB engines, executing parameterized queries with prepared statements (preventing SQL injection), and using Context Managers (with conn:) for auto-commit/rollback.

Common Join & Aggregation Questions

Writing INNER JOIN vs LEFT JOIN to find employees with no managers, and using GROUP BY with HAVING COUNT(*) > 1 to detect duplicate records.