JSON Flattener

def flatten(obj, prefix="", sep="."):
    """Flatten a nested dict into dot-notation keys."""
    result = {}
    if isinstance(obj, dict):
        for key, value in obj.items():
            new_key = f"{prefix}{sep}{key}" if prefix else key
            if isinstance(value, (dict, list)):
                result.update(flatten(value, new_key, sep))
            else:
                result[new_key] = value
    elif isinstance(obj, list):
        for i, value in enumerate(obj):
            new_key = f"{prefix}{sep}{i}" if prefix else str(i)
            if isinstance(value, (dict, list)):
                result.update(flatten(value, new_key, sep))
            else:
                result[new_key] = value
    else:
        result[prefix] = obj
    return result

def unflatten(flat, sep="."):
    """Reconstruct nested dict from dot-notation keys."""
    result = {}
    for key, value in flat.items():
        parts = key.split(sep)
        d = result
        for part in parts[:-1]:
            d = d.setdefault(part, {})
        d[parts[-1]] = value
    return result

# Example
nested = {"user": {"name": "Alice", "address": {"city": "Seoul"}}}
flat = flatten(nested)
print(flat)  # {'user.name': 'Alice', 'user.address.city': 'Seoul'}
print(unflatten(flat))  # back to nested