Python Interview Questions 2026

Tip: Lists are mutable and use more memory; tuples are immutable, hashable (so usable as dict keys), and slightly faster. Use tuples for fixed records and as dictionary keys; use lists for collections you will modify.

Tip: The GIL allows only one thread to execute Python bytecode at a time in CPython, so threads do not give true parallelism for CPU-bound work. Use multiprocessing (separate interpreters) for CPU-bound tasks and threading/asyncio for I/O-bound tasks where the GIL is released during blocking calls.

Tip: A decorator is a callable that takes a function and returns a new function, used to wrap behaviour (logging, caching, auth) without modifying the original. @decorator is syntactic sugar for func = decorator(func). Use functools.wraps to preserve the wrapped function's metadata.

Tip: copy.copy() (shallow) duplicates the outer object but shares references to nested objects, so mutating a nested element affects both. copy.deepcopy() recursively copies everything, producing fully independent objects. Shallow copy is faster but risky with nested mutable data.

Tip: CPython tracks each object's reference count and frees it when the count hits zero. A separate generational garbage collector breaks reference cycles (e.g. two objects referencing each other). The gc module exposes tuning and manual collection.

Tip: A generator yields items lazily one at a time, holding only the current state in memory; a list materialises everything. Use generators for large or streaming datasets to keep memory flat. They are single-pass — once exhausted you must recreate them.

Tip: *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 are commonly used to forward arguments to wrapped functions in decorators.

Tip: == compares values (calls __eq__); is compares identity (same object in memory). Use is only for singletons like None (if x is None). Small integers (-5..256) and interned strings are cached, which can make is appear to work — never rely on that for value comparison.