lua implementaiton of python's default dict

defaultdict.lua

function defaultdict(callable)
    local T = {}
    setmetatable(T, {
        __index = function(T, key)
            local val = rawget(T, key)
            if not val then
                rawset(T, key, callable())
            end
            return rawget(T, key)
        end
    })
    return T
end

a = defaultdict(function() return {} end)

a.foo.bar = "a"
a.foo2.bar = "b"

print (a.foo.bar) -- 'a'
print (a.foo2.bar) -- 'b'