Question 1

deque vs list for front removal

Accepted Answer

A Python list is backed by a contiguous array. Removing from position 0 requires shifting every remaining element one slot to the left, which is O(n). A deque uses a doubly-linked block structure so it can remove from either end in O(1). For BFS queues or any pattern that repeatedly pulls from the front, this difference matters a lot at scale.

Question 2

__name__ == '__main__' guard

Accepted Answer

When Python runs a file directly, it sets `__name__` to `'__main__'`. When the same file is imported as a module, `__name__` becomes the module's name instead. The guard lets you write code - like `main()` calls or test scaffolding - that only fires during direct execution. Without it, that code would run every time someone imported the file, which can cause unexpected side effects in tests or other scripts.

Question 3

Counter missing key behavior

Accepted Answer

Counter is designed for counting, so accessing a missing key returns 0 instead of raising a KeyError. The letter 'h' appears once in 'hello', so `c['h']` is 1. 'z' is not in the string, but Counter silently returns 0 rather than throwing an error - unlike a regular dict, which would raise KeyError. This behavior makes frequency counting code much cleaner.