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❓ Python Language - Q&A

Pairs with Memory & Runtime and OOP & Language Concepts. Format and tag legend: Overview.

Q1. What is the GIL and what are its implications? The Global Interpreter Lock lets only one thread execute Python bytecode at a time. So threads don't speed up CPU-bound work - use multiprocessing or native libs for that - but they're fine for I/O-bound work since the GIL is released during blocking I/O. Follow-ups: How does asyncio differ from threads for I/O? Med · MAANG

Q2. Is Python pass-by-value or pass-by-reference? Neither exactly - it's pass-by-object-reference ("call by sharing"). Rebinding a parameter doesn't affect the caller, but mutating a mutable argument (list, dict) does. Immutable args (int, str, tuple) appear value-like because you can't mutate them in place. Follow-ups: Show a bug this causes. (mutating a shared default list.) Med · MAANG

Q3. Why is a mutable default argument dangerous? The default is evaluated once at function definition and shared across all calls, so def f(x=[]) accumulates state between calls. Use def f(x=None) and create the list inside. Follow-ups: When is the default expression evaluated? (at definition, not call.) Med · MAANG

Q4. Which Python types are mutable vs immutable? Mutable: list, dict, set, most objects. Immutable: int, float, str, tuple, frozenset, bytes. Only immutable (hashable) types can be dict keys or set members. Follow-ups: Why can't a list be a dict key? (unhashable - mutable.) Easy · MAANG

Q5. What is a generator and why use one? A function using yield that produces values lazily, one at a time, without building the whole sequence in memory. Ideal for large or infinite streams and memory-efficient pipelines. Follow-ups: Difference between a generator and a list comprehension? (lazy vs eager.) Med · MAANG

Q6. What is a decorator? A callable that wraps a function to add behavior (logging, caching, auth, timing) without changing its body. @lru_cache and @property are stdlib examples. It's syntactic sugar for f = decorator(f). Follow-ups: How would you write one that times a function? Med · MAANG

Q7. What is a context manager? An object usable with with that guarantees setup and teardown (via __enter__/__exit__), even on exceptions - closing files, releasing locks, committing/rolling back transactions. The Pythonic answer to deterministic resource cleanup. Follow-ups: How do you write one with contextlib? Med · MAANG

Q8. How does Python manage memory / garbage collection? Primarily reference counting (objects freed when their count hits zero) plus a generational cycle collector to reclaim reference cycles that counting alone can't. Old generations are collected less often (most objects die young). Follow-ups: Why does reference counting alone leak cycles? Hard · Google

Q9. is vs ==? == compares values (calls __eq__); is compares identity (same object in memory). Use is only for singletons like None. Small-int and string interning can make is deceptively true - don't rely on it for value equality. Follow-ups: Why does 256 is 256 but 257 is 257 may vary? (small-int caching.) Med · MAANG

Q10. What's the difference between a list and a tuple? A list is mutable and general-purpose; a tuple is immutable and often used for fixed, heterogeneous records or as dict keys. Tuples are slightly more memory-efficient and hashable. Follow-ups: When would you deliberately choose a tuple? (fixed record, dict key, safety.) Easy · MAANG

Q11. What are *args and **kwargs? *args collects extra positional arguments into a tuple; **kwargs collects extra keyword arguments into a dict. They let functions accept a variable number of arguments and forward them. Follow-ups: How do you forward them to another function? (f(*args, **kwargs).) Easy · Startups

Q12. How would you speed up CPU-bound Python code? Move the hot work off the GIL: use multiprocessing, native/C extensions or NumPy (which release the GIL), or a different runtime. Profile first to find the real hotspot rather than guessing. Follow-ups: Why won't threads help here? (GIL serializes bytecode.) Med · MAANG

Q13. What is a shallow copy vs a deep copy? A shallow copy duplicates the container but shares the nested objects; a deep copy recursively duplicates everything. Mutating a nested object through a shallow copy affects the original - a common bug with lists of lists. Follow-ups: Which does list(x) or x[:] give? (shallow.) Med · Google

Related: Memory & Runtime · Concurrency Q&A · OOP & Design Q&A