introspection - Print all local variables accessible to the current scope in Lua

Question

I know how to print "all" global variables using the following code

for k,v in pairs(_G) do
    print("Global key", k, "value", v)
end

So my question is how to do that for all variables that are accessible from the currently executing function, something that can do what locals() does for Python.

Answer 1

Here is an implementation of a locals() function. It will return a table of locals from the calling scope:

function locals()
  local variables = {}
  local idx = 1
  while true do
    local ln, lv = debug.getlocal(2, idx)
    if ln ~= nil then
      variables[ln] = lv
    else
      break
    end
    idx = 1 + idx
  end
  return variables
end

Notice that in the lua REPL, each line is a separate chunk with separate locals. Also, internal variables are returned (names start with '(' if you want to remove them):

> local a = 2; for x, v in pairs(locals()) do print(x, v) end
a   2
(*temporary)    function: 0x10359b38

---

Thanks for the accept. You have unlocked the last piece of the puzzle! ;-)

Upvalues are local variables from outer scopes, that are used in the current function. They are neither in _G nor in locals()

function upvalues()
  local variables = {}
  local idx = 1
  local func = debug.getinfo(2, "f").func
  while true do
    local ln, lv = debug.getupvalue(func, idx)
    if ln ~= nil then
      variables[ln] = lv
    else
      break
    end
    idx = 1 + idx
  end
  return variables
end

Example (notice you have to use a for it to show up):

> local a= 2; function f() local b = a; for x,v in pairs(upvalues()) do print(x,v) end end; f()
a   2