CODEWITHSUNDEEP
luaintrospection
Edu Felipe
Edu Felipe

Reputation: 10387

Print all local variables accessible to the current scope in Lua

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.

Upvotes: 32

Views: 38257

Answers (5)

Sirmabus
Sirmabus

Reputation: 736

The problem with Judge Maygarden's loop version above is simply local i = 0. It does nothing because the first indexed with '0' will always return nil.

Remember Lua indexes by default start with '1', not '0' like C/C++. You can use '0' for an index with your own types of course, but the default functions are expecting the default of '1' as the first index.

Just change it to local i = 1 and his loop will work fine.

Upvotes: 4

u0b34a0f6ae
u0b34a0f6ae

Reputation: 49843

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

Upvotes: 42

Nick Dandoulakis
Nick Dandoulakis

Reputation: 43158

You can use getfenv to get a local environment.

getfenv ([f]) Returns the current environment in use by the function. f can be a Lua function or a number that specifies the function at that stack level: Level 1 is the function calling getfenv. If the given function is not a Lua function, or if f is 0, getfenv returns the global environment. The default for f is 1.

Edit: sorry, I was wrong.

I just checked Lua source code. debug.getlocal() is the only way to get the local variables.
Lua uses an internal Proto structure and doesn't give us access to that.
(Proto holds local properties plus a parent Proto reference. Iterating function's Proto, by using getfenv,
we also iterate inherited properties, not what we wanted)

Users can define their Proto s either with environments and the set/getfenv functions or by using metatables.

Upvotes: 0

Judge Maygarden
Judge Maygarden

Reputation: 27613

See debug.getlocal:

local foobar = 1

local i = 0
repeat
    local k, v = debug.getlocal(1, i)
    if k then
        print(k, v)
        i = i + 1
    end
until nil == k

Output:

foobar  1
i       2

Upvotes: 3

lhf
lhf

Reputation: 72422

Use debug.getlocal.

Upvotes: 8

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