Question

Is there a way to implement a mechanism similar to python’s __getitem__?

for instance, having the following:

local t1 = {a=1, b=2, c=3, d=4} 

if in code, t1.e will be called, then I wish to have something else returned rather than nil

Answer 1

You can use setmetatable and the __index metamethod:

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end
})

print(t1.hi) -- prints "something"

Note that this will not be called when you do t.nonexistant = something. For that, you need the __newindex metamethod:

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end,

    __newindex = function(table, key, value)
        rawset(table, tostring(key) .. '_nope', value)
    end
})

print(t1.hi) -- prints "something"
t1.hi = 'asdf'
print(t1.hi) -- prints "something"
print(t1.hi_nope) -- prints "asdf"