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Python interview questions

69 Python interview questions with answers, from junior to mid level.

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Foundations

What is Python?
What does it mean that Python is an 'interpreted' language?
What does age = 30 do in Python?
What type is {"name": "Ana", "age": 30} in Python?
What does this function call print?
How does Python know which lines belong inside an if or a loop?
What does this loop print?
What does import json do?
What are pip and PyPI?
What is the difference between print and return in a function?

Core

What does this print?
What does this print?
Why is a bare except: considered harmful?
What is the main difference between a list [1, 2] and a tuple (1, 2)?
What does the last line print?
What is "python"[-1]?
What is nums[1:3] if nums = [10, 20, 30, 40, 50]?
What does this print?
What is [x * 2 for x in range(4)]?
What do 7 / 2 and 7 // 2 return in Python 3?
In a method def area(self):, what is self?
What is __init__ for in a class?
What is len({1, 2, 2, 3, 3, 3})?
What does this print?
Why is with open("data.txt") as f: preferred over f = open("data.txt")?
What does this print?
What is a virtual environment (python -m venv .venv) for?
Which exception does int("abc") raise?

Intermediate

In CPython (default build, GIL enabled), which workload gets a real speedup from threading?
What does this print?
What does this print?
sum(x * x for x in range(10**8)) versus sum([x * x for x in range(10**8)]). What is the key difference?
Why use functools.wraps inside a decorator?
print([obj]) where obj defines both __str__ and __repr__. Which one is used for obj?
Inside an async def FastAPI endpoint you call time.sleep(2) (or requests.get). What happens under load?
Roughly how long does main() take?
Inside a coroutine you write asyncio.sleep(1) without await. What happens?
What does this print?
Since which Python version is insertion order of dict guaranteed by the language spec?
What happens with f(1, 2)?
What does @dataclass(frozen=True) give you (with default eq=True)?
In try / except / else / finally, when does else run?
Which methods make an object an iterator?
What does this print?
What is the idiomatic use of @classmethod?
Building a large string with s += piece in a loop is discouraged because…
Which structure gives O(1) append and pop at both ends?
What happens?
You call a function decorated with @functools.lru_cache passing a list. What happens?
Explain the GIL and how you would speed up a CPU-bound Python job.
How do decorators work? Write a decorator that logs how long a function takes.
What is the difference between an iterable, an iterator and a generator?

Advanced

What does this print?
A typical implementation of a decorator used as @retry(times=3) needs how many nested callables?
What does this print?
What is the main effect of defining __slots__ = ("x", "y") in a class?
A class defines __eq__ but not __hash__. What happens when you add an instance to a set?
In a context manager, what happens if __exit__ returns True?
Two objects reference each other and nothing else references them. What frees them in CPython?
Which typing tool lets a type checker accept any object that has a read() method, without inheritance?
What does T achieve here?
@property is built on which mechanism?
What does type(type) return?
What does this print?
What is the main overhead of multiprocessing.Pool.map compared to threads?
What is the difference between __new__ and __init__?
Several threads run counter += 1 on a shared global, 1M times each. Is the final count guaranteed correct with the GIL?
What is the current state of the GIL in modern CPython (3.13+)?
Explain how asyncio works and when you would choose it over threads or processes.

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