Lua serializer and pretty printer


Lua serializer and pretty printer.


  • Human readable:
    • Provides single-line and multi-line output.
    • Nested tables are properly indented in the multi-line output.
    • Numerical keys are listed first.
    • Keys are (optionally) sorted alphanumerically.
    • Array part skips keys ({'a', 'b'} instead of {[1] = 'a', [2] = 'b'}).
    • nil values are included when expected ({1, nil, 3} instead of {1, [3]=3}).
    • Keys use short notation ({foo = 'foo'} instead of {['foo'] = 'foo'}).
    • Shared references and self-references are marked in the output.
  • Machine readable: provides reliable deserialization using loadstring().
  • Supports deeply nested tables.
  • Supports tables with self-references.
  • Keeps shared tables and functions shared after de/serialization.
  • Supports function serialization using string.dump().
  • Supports serialization of global functions.
  • Supports __tostring and __serialize metamethods.
  • Escapes new-line \010 and end-of-file control \026 characters in strings.
  • Configurable with options and custom formatters.


local serpent = require("serpent")
local a = {1, nil, 3, x=1, ['true'] = 2, [not true]=3}
a[a] = a -- self-reference with a table as key and value

print(serpent.dump(a)) -- full serialization
print(serpent.line(a)) -- single line, no self-ref section
print(serpent.block(a)) -- multi-line indented, no self-ref section

local fun, err = loadstring(serpent.dump(a))
if err then error(err) end
local copy = fun()

-- or using serpent.load:
local ok, copy = serpent.load(serpent.dump(a))
print(ok and copy[3] == a[3])


Serpent provides three functions that are shortcuts to the same internal function, but set different options by default:

  • dump(a[, {...}]) -- full serialization; sets name, compact and sparse options;
  • line(a[, {...}]) -- single line pretty printing, no self-ref section; sets sortkeys and comment options;
  • block(a[, {...}]) -- multi-line indented pretty printing, no self-ref section; sets indent, sortkeys, and comment options.

Note that line and block functions return pretty-printed data structures and if you want to deserialize them, you need to add return before running them through loadstring. For example: loadstring('return '..require('mobdebug').line("foo"))() == "foo".

While you can use loadstring or load functions to load serialized fragments, Serpent also provides load function that adds safety checks and reports an error if there is any executable code in the fragment.

  • ok, res = serpent.load(str[, {safe = true}]) -- loads serialized fragment; you need to pass {safe = false} as the second value if you want to turn safety checks off.

Similar to pcall and loadstring calls, load returns status as the first value and the result or the error message as the second value.


  • indent (string) -- indentation; triggers long multi-line output
  • comment (true/false/maxlevel) -- provide stringified value in a comment (up to maxlevel of depth)
  • sortkeys (true/false/function) -- sort keys
  • sparse (true/false) -- force sparse encoding (no nil filling based on #t)
  • compact (true/false) -- remove spaces
  • fatal (true/False) -- raise fatal error on non-serilizable values
  • nocode (true/False) -- disable bytecode serialization for easy comparison
  • nohuge (true/False) -- disable checking numbers against undefined and huge values
  • maxlevel (number) -- specify max level up to which to expand nested tables
  • maxnum (number) -- specify max number of elements in a table
  • numformat (string; "%.17g") -- specify format for numeric values (shortest possible round-trippable double)
  • valignore (table) -- allows to specify a list of values to ignore (as keys)
  • keyallow (table) -- allows to specify the list of keys to be serialized. Any keys not in this list are not included in final output (as keys)
  • keyignore (table) -- allows to specity the list of keys to ignore in serialization.
  • valtypeignore (table) -- allows to specify a list of value types to ignore (as keys)
  • custom (function) -- provide custom output for tables
  • name (string) -- name; triggers full serialization with self-ref section

These options can be provided as a second parameter to Serpent functions.

block(a, {fatal = true})
line(a, {nocode = true, valignore = {[arrayToIgnore] = true}})
function todiff(a) return dump(a, {nocode = true, indent = ' '}) end

Serpent functions set these options to different default values:

  • dump sets compact and sparse to true;
  • line sets sortkeys and comment to true;
  • block sets sortkeys and comment to true and indent to ' '.

Metatables with __tostring and __serialize methods

If a table or a userdata value has __tostring or __serialize method, the method will be used to serialize the value. If __serialize method is present, it will be called with the value as a parameter. if __serialize method is not present, but __tostring is, then tostring will be called with the value as a parameter. In both cases, the result will be serialized, so __serialize method can return a table, that will be serialize and replace the original value.


A custom sort function can be provided to sort the contents of tables. The function takes 2 parameters, the first being the table (a list) with the keys, the second the original table. It should modify the first table in-place, and return nothing. For example, the following call will apply a sort function identical to the standard sort, except that it will not distinguish between lower- and uppercase.

local mysort  = function(k, o) -- k=keys, o=original table
  local maxn, to = 12, {number = 'a', string = 'b'}
  local function padnum(d) return ("%0"..maxn.."d"):format(d) end
  local sort = function(a,b)
    -- this -vvvvvvvvvv- is needed to sort array keys first
    return ((k[a] and 0 or to[type(a)] or 'z')..(tostring(a):gsub("%d+",padnum))):upper()
         < ((k[b] and 0 or to[type(b)] or 'z')..(tostring(b):gsub("%d+",padnum))):upper()
  table.sort(k, sort)

local content = { some = 1, input = 2, To = 3, serialize = 4 }
local result = require('serpent').block(content, {sortkeys = mysort})


Serpent supports a way to provide a custom formatter that allows to fully customize the output. The formatter takes four values:

  • tag -- the name of the current element with '=' or an empty string in case of array index,
  • head -- an opening table bracket { and associated indentation and newline (if any),
  • body -- table elements concatenated into a string using commas and indentation/newlines (if any), and
  • tail -- a closing table bracket } and associated indentation and newline (if any).

For example, the following call will apply `Foo{bar} notation to its output (used by Metalua to display ASTs):

print((require "serpent").block(ast, {comment = false, custom =
    local out = head..body..tail
    if tag:find('^lineinfo') then
      out = out:gsub("\n%s+", "") -- collapse lineinfo to one line
    elseif tag == '' then
      body = body:gsub('%s*lineinfo = [^\n]+', '')
      local _,_,atag = body:find('tag = "(%w+)"%s*$')
      if atag then
        out = "`"..atag..head.. body:gsub('%s*tag = "%w+"%s*$', '')..tail
        out = out:gsub("\n%s+", ""):gsub(",}","}")
      else out = head..body..tail end
    return tag..out


  • Doesn't handle userdata (except filehandles in io.* table).
  • Threads, function upvalues/environments, and metatables are not serialized.


A simple performance test against serialize.lua from metalua, pretty.write from Penlight, and tserialize.lua from lua-nucleo is included in t/bench.lua.

These are the results from one of the runs:

  • nucleo (1000): 0.256s
  • metalua (1000): 0.177s
  • serpent (1000): 0.22s
  • serpent (1000): 0.161s -- no comments, no string escapes, no math.huge check
  • penlight (1000): 0.132s

Serpent does additional processing to escape \010 and \026 characters in strings (to address, which is already fixed in Lua 5.2) and to check all numbers for math.huge. The seconds number excludes this processing to put it on an equal footing with other modules that skip these checks (nucleo still checks for math.huge).


Paul Kulchenko (


See LICENSE file.


v0.28 (May 06 2015)

  • Switched to a method proposed by @SoniEx2 to disallow function calls (#15).
  • Added more tostring for Lua 5.3 support (pkulchenko/ZeroBraneStudio#401).
  • Updated environment handling to localize the impact (#15).
  • Added setting env to protect against assigning global functions (closes #15).
  • Updated tests to work with Lua 5.3.
  • Added explicit tostring for Lua 5.3 with LUA_NOCVTN2S set (pkulchenko/ZeroBraneStudio#401).
  • Fixed crash when not all Lua standard libraries are loaded (thanks to Tommy Nguyen).
  • Improved Lua 5.2 support for serialized functions.

v0.27 (Jan 11 2014)

  • Fixed order of elements in the array part with sortkeys=true (fixes #13).
  • Updated custom formatter documentation (closes #11).
  • Added load function to deserialize; updated documentation (closes #9).

v0.26 (Nov 05 2013)

  • Added load function that (safely) loads serialized/pretty-printed values.
  • Updated documentation.

v0.25 (Sep 29 2013)

  • Added maxnum option to limit the number of elements in tables.
  • Optimized processing of tables with numeric indexes.

v0.24 (Jun 12 2013)

  • Fixed an issue with missing numerical keys (fixes #8).
  • Fixed an issue with luaffi that returns getmetatable(ffi.C) as true.

v0.23 (Mar 24 2013)

  • Added support for cdata type in LuaJIT (thanks to Evan).
  • Added comment to indicate incomplete output.
  • Added support for metatables with __serialize method.
  • Added handling of metatables with __tostring method.
  • Fixed an issue with having too many locals in self-reference section.
  • Fixed emitting premature circular reference in self-reference section, which caused invalid serialization.
  • Modified the sort function signature to also pass the original table, so not only keys are available when sorting, but also the values in the original table.

v0.22 (Jan 15 2013)

  • Added ability to process __tostring results that may be non-string values.

v0.21 (Jan 08 2013)

  • Added keyallow and valtypeignore options (thanks to Jess Telford).
  • Renamed ignore to valignore.

v0.19 (Nov 16 2012)

  • Fixed an issue with serializing shared functions as keys.
  • Added serialization of metatables using __tostring (when present).

v0.18 (Sep 13 2012)

  • Fixed an issue with serializing data structures with circular references that require emitting temporary variables.
  • Fixed an issue with serializing keys pointing to shared references.
  • Improved overall serialization logic to inline values when possible.

v0.17 (Sep 12 2012)

  • Fixed an issue with serializing userdata that doesn't provide tostring().

v0.16 (Aug 28 2012)

  • Removed confusing --[[err]] comment from serialized results.
  • Added a short comment to serialized functions when the body is skipped.

v0.15 (Jun 17 2012)

  • Added ignore option to allow ignoring table values.
  • Added comment=num option to set the max level up to which add comments.
  • Changed all comments (except math.huge) to be controlled by comment option.

v0.14 (Jun 13 2012)

  • Fixed an issue with string keys with numeric values ['3'] getting mixed with real numeric keys (only with sortkeys option set to true).
  • Fixed an issue with negative and real value numeric keys being misplaced.

v0.13 (Jun 13 2012)

  • Added maxlevel option.
  • Fixed key sorting such that true and 'true' are always sorted in the same order (for a more stable output).
  • Removed addresses from names of temporary variables (for stable output).

v0.12 (Jun 12 2012)

  • Added options to configure serialization process.
  • Added goto to the list of keywords for Lua 5.2.
  • Changed interface to dump/line/block methods.
  • Changed math.huge to 1/0 for better portability.
  • Replaced \010 with \n for better readability.

v0.10 (Jun 03 2012)

  • First public release.