Unflatten dotted keys

Python data engineering interview problem. Difficulty: intermediate. Pattern: Recursion. About 16 minutes. Part of the Pro drill bank.

Reconstruct a nested dictionary and list structure from a flat dictionary of dotted path keys. Treat this as a production helper: match the contracted return shape, including empty and duplicate inputs.

Implement unflatten(flat: dict) -> dict to reverse a dotted key flattening operation. A flat dictionary contains keys like "a.b": 1 and "a.c.0": 2. Numeric path segments (such as "0", "1") represent zero-based list indices. Alphabetic string segments represent dictionary keys. Nested containers must be created as needed to place every value at its target path. An empty dictionary input must return {}.

Requirements

• Numeric segments must generate lists, not dicts.
• Do not mutate the input dictionary.

Constraints

• 0 <= len(flat) <= 1,000
• List indices are non-negative sequential integers.

Examples

Input: unflatten({'a.b': 1, 'a.c.0': 2, 'a.c.1': 3}) Output: {'a': {'b': 1, 'c': [2, 3]}} Path a.b becomes a nested dict entry, while a.c.0 and a.c.1 become a list.

Topics: lakebench, python, nested-dict, array-indices.

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