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My Primers

Path resolver

Prompt: Given a current working directory and a path (absolute or relative, with ., .., possibly symlinks), return the resolved absolute path.

What’s tested: string handling, edge cases, and whether you handle symlinks correctly.

The mechanical part: split path on /. If absolute, start from /; else start from cwd. For each component: skip empty and ., pop for .., push otherwise. Join the result with /. Easy.

The actual interview, which is symlinks. A symlink in the middle of the path replaces the resolution stack at that point, and the replacement itself may contain .. or further symlinks. You cannot just resolve the final string and then deref the final symlink — that’s wrong. Real realpath() resolves component by component.

def resolve(path, cwd, fs):
    parts = (path if path.startswith("/") else cwd + "/" + path).split("/")
    stack = []
    seen = 0
    for part in parts:
        if part in ("", "."): continue
        if part == "..":
            if stack: stack.pop()
            continue
        stack.append(part)
        node = fs.lookup("/".join(stack))
        if node.is_symlink:
            seen += 1
            if seen > 40: raise TooManySymlinks    # Linux limit
            target = node.link_target
            # symlink replaces last component; resolve recursively
            stack.pop()
            stack = resolve(target, "/" + "/".join(stack), fs).split("/")[1:]
    return "/" + "/".join(stack)

Staff signal moves:

  • Volunteer the symlink-loop protection (Linux caps at 40). Without it, a → b, b → a blows the stack.
  • Mention .. after a symlink is genuinely weird — POSIX says .. is the parent of the resolved path, but shells (bash) often track it lexically. Different answers are defensible; state which you’re implementing and why.
  • Trailing slash semantics: foo/ only resolves if foo is a directory. Real realpath enforces this.
  • Don’t reach for os.path.realpath even as a joke. They’re testing whether you can be realpath.