Interview Q&A / Python

1. How does a dictionary work internally in Python, and why must keys be hashable?

   A dict is a hash table. Python calls hash() on the key to pick a slot, then uses == to confirm the match, which makes lookups O(1) on average. Keys must be hashable, meaning their hash never changes, otherwise the dict would look in the wrong slot and lose them. That is why lists cannot be keys but tuples of immutable values can.

   IntermediateIn-depth4 min read
2. What is a decorator in Python and how do you write one?

   A decorator is a function that takes another function, wraps it with extra behaviour, and returns the wrapped version. Writing @my_decorator above a function is shorthand for func = my_decorator(func). They are used for logging, timing, caching and access checks without changing the original function's code.

   IntermediateIn-depth3 min read
3. What is a generator in Python, and how is yield different from return?

   A generator is a function that uses yield to produce values one at a time, pausing after each one and resuming where it left off on the next request. return ends the function and gives back one result, while yield hands out a value and keeps the function's state. Generators save memory because they never build the full list.

   IntermediateIn-depth3 min read