Magic / Dunder Methods

Why this matters

Pandas' DataFrame.__getitem__, SQLAlchemy's operator overloading, and pathlib's / operator are all dunder tricks. Staff-level interviews often probe __eq__/__hash__ contracts and the __init__ vs __new__ split — knowing when to override __new__ matters for singletons, immutables, and metaclass work.

Common pitfalls

• Overriding __eq__ without __hash__ — silently makes the class unhashable and breaks set/dict keys.
• Writing __repr__ that isn't a valid eval() target — good __repr__ should reconstruct the object for debug logs.
• Confusing __str__ (human) with __repr__ (developer) — print() falls back to __repr__ only when __str__ is absent.