Designing a File System

The Idea

Name one contract — an entry that can report its size. A File answers from its own bytes; a Folder answers by asking everything it holds and adding up.

Nesting then costs nothing: a folder of folders is just a folder whose children answer the same question.

Real-World Example

A shipping dock. A pallet's weight is the sum of the crates on it, and a crate may itself be a loaded pallet. The scale asks one question and never cares which it got.

The Code

class File:
    def __init__(self, name, size): self.name, self._size = name, size
    def size(self): return self._size

class Folder:
    def __init__(self, name): self.name, self.items = name, []
    def add(self, entry): self.items.append(entry); return self
    def size(self): return sum(e.size() for e in self.items)   # ask, do not test

root = Folder("/").add(File("a.txt", 12)).add(Folder("logs").add(File("x.log", 30)))
print(root.size(), len(root.items))       # 42 2

The Tradeoff

Uniformity is bought with traversal: every size read walks the whole subtree, and a leaf-only operation needs a type test after all. A flat path-to-record table inverts both costs.