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Human-readable representation of Lua tables

branch: master


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This function transform any Lua table into a human-readable representation of that table.

The objective here is human understanding (i.e. for debugging), not serialization or compactness.

Examples of use

inspect has the following declaration: str = inspect(value, <options>).

value can be any Lua value. inspect transforms simple types (like strings or numbers) into strings. Tables, on the other hand, are rendered in a way a human can understand.

"Array-like" tables are rendered horizontally:

inspect({1,2,3,4}) == "{ 1, 2, 3, 4 }"

"dictionary-like" tables are rendered with one element per line:

inspect({a=1,b=2}) == [[{
  a = 1,
  b = 2

The keys will be sorted alphanumerically when possible.

"Hybrid" tables will have the array part on the first line, and the dictionary part just below them:

inspect({1,2,3,b=2,a=1}) == [[{ 1, 2, 3,
  a = 1,
  b = 2

Tables can be nested, and will be indented with two spaces per level.

inspect({a={b=2}}) == [[{
  a = {
    b = 2

Functions, userdata and any other custom types from Luajit are simply as <function x>, <userdata x>, etc.:

inspect({ f = print, ud = some_user_data, thread = a_thread} ) == [[{
  f = <function 1>,
  u = <userdata 1>,
  thread = <thread 1>

If the table has a metatable, inspect will include it at the end, in a special field called <metatable>:

inspect(setmetatable({a=1}, {b=2}) == [[{
  a = 1
  <metatable> = {
    b = 2

inspect can handle tables with loops inside them. It will print <id> right before the table is printed out the first time, and replace the whole table with <table id> from then on, preventing infinite loops.

a = {1, 2}
b = {3, 4, a}
a[3] = b -- a references b, and b references a
inspect(a) = "<1>{ 1, 2, { 3, 4, <table 1> } }"

Notice that since both a appears more than once in the expression, it is prefixed by <1> and replaced by <table 1> every time it appears later on.


inspect's second parameter allows controlling the maximum depth that will be printed out. When the max depth is reached, it'll just return {...}:

local t5 = {a = {b = {c = {d = {e = 5}}}}}

inspect(t5, {depth = 4}) == [[{
  a = {
    b = {
      c = {
        d = {...}

inspect(t5, {depth = 2}) == [[{
  a = {
    b = {...}

options.depth defaults to infinite (math.huge).


Sometimes it might be convenient to "filter out" some parts of the output. The options.filter option can do that.

options.filter accepts a table of values. Any value on that table will be rendered as <filtered>. This is useful for hiding things like long complex tables that are not interesting for the task at hand, for example an unuseful complex metatable.

local person = {name = 'peter'}
setmetatable(person, complex_mt)
inspect(x, {filter = {complex_mt}}) == [[{
  name = "peter",
  <metatable> = <filtered>

options.filter can also be a function. The function must return true for the values that must be filtered out.

local isEvenNumber = function(x) return type(x) == 'number' and x % 2 == 0 end

inspect({1,2,3,4,5}, {filter = isEvenNumber}) == "{ 1, <filtered>, 3, <filtered>, 5 }"

Gotchas / Warnings

This method is not appropriate for saving/restoring tables. It is meant to be used by the programmer mainly while debugging a program.


Just copy the inspect.lua file somewhere in your projects (maybe inside a /lib/ folder) and require it accordingly.

Remember to store the value returned by require somewhere! (I suggest a local variable named inspect, although others might like table.inspect)

local inspect = require 'inspect'
      -- or --
table.inspect = require 'inspect'

Also, make sure to read the license file; the text of that license file must appear somewhere in your projects' files.


This project uses busted for its specs. If you want to run the specs, you will have to install busted first. Then just execute the following from the root inspect folder:

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