Python Interview Questions

Default values are evaluated once, when the def statement runs, not on each call. A mutable default such as a list or dict is therefore shared across calls and accumulates state. Use None as the default and create the object inside the function. The same applies to defaults like datetime.now().

*args collects extra positional arguments into a tuple and **kwargs collects extra keyword arguments into a dict. The same operators unpack sequences and dicts into calls. They are used for wrappers and decorators that forward arguments, and for builders that accept field overrides.

Fixtures are functions decorated with @pytest.fixture that pytest injects into tests by parameter name. Scope (function, class, module, package, session) controls how often they are created. Code after yield is teardown. conftest.py shares fixtures across a directory without imports.

A decorator is a function that takes a function and returns a wrapper. A configurable retry decorator needs three levels: a factory taking options, the decorator, and a wrapper that loops, catches only the listed exceptions, sleeps with exponential backoff, and re-raises after the last attempt. Use functools.wraps.

In CPython the Global Interpreter Lock lets only one thread execute Python bytecode at a time, so threads help I/O-bound work but not CPU-bound work. Use asyncio or threads for I/O, multiprocessing for CPU. pytest-xdist runs tests in separate processes. Python 3.13+ offers an experimental free-threaded build.