Overriding Lua libraries with stub versions, that have access to the originals

app.lua

-- add my custom lib directory to the path
package.path = 'lib/?.lua;'..package.path

-- add this line only to switch to using the wrappers;
-- do this after making any other path adjustments.
package.real,package.path = package.path,'debug/?.lua;'..package.path

local mylib = require 'mylib'

mylib.lua

-- This is debug/mylib.lua, but Gist won't let me name it that :p

-- hide the debug folder from the path temporarily
package.orig,package.path = package.path,package.real

-- get the real version of the library
local mylib = require 'mylib'

-- undo the damage we did to the path
package.path = package.orig

--> make my debug modifications to lib
local realfoo = mylib.foo
function mylib.foo(...)
  print("Ha ha, I'm debugging!")
  local result = realfoo(...) 
  return changeinusefulways(result)
end

return mylib