Matias Guijarro wrote:
I would like to change the default behaviour of Lua when
representing a table through the tostring() function ; I
would prefer to have something more like in Python for
example (instead of having "table: 0x8062ac0", I would
like to see the contents, for example { 1, 2, 3 }).
So first of all I tried to set a new metatable for tables,
As Shmuel pointed out, table metatables are per-table, so
they don't really work for what you want to do.
Lua 5.1.1 Copyright (C) 1994-2006 Lua.org, PUC-Rio
> Tbl = setmetatable({}, {__tostring = function(t)
>> return "Hello!"
>> end})
> print(Tbl) -- Works for this table,
Hello!
> print({}) -- but doesn't work for other tables.
table: 0x493378
A way that will work is to simply replace the tostring
function itself. Replace it with a function that calls your
table-to-string function for tables, and calls the original
tostring for everything else.
Roberto's Programming in Lua book explains how to write the
table-to-string function. (The first edition is available
online: <http://www.lua.org/pil/12.1.html>.)
As it happens, I happened to write an implementation of just
the functionality you're looking for a few days ago. It is
below, but I encourage you to try whipping together at least
a simple version of your own before looking at mine.
- Aaron
-- This script makes tostring convert tables to a
-- representation of their contents.
-- The real tostring:
_tostring = _tostring or tostring
-- Characters that have non-numeric backslash-escaped versions:
local BsChars = {
["\a"] = "\\a",
["\b"] = "\\b",
["\f"] = "\\f",
["\n"] = "\\n",
["\r"] = "\\r",
["\t"] = "\\t",
["\v"] = "\\v",
["\""] = "\\\"",
["\\"] = "\\\\"}
-- Is Str an "escapeable" character (a non-printing character other than
-- space, a backslash, or a double quote)?
local function IsEscapeable(Char)
return string.find(Char, "[^%w%p]") -- Non-alphanumeric, non-punct.
and Char ~= " " -- Don't count spaces.
or string.find(Char, '[\\"]') -- A backslash or quote.
end
-- Converts an "escapeable" character (a non-printing character,
-- backslash, or double quote) to its backslash-escaped version; the
-- second argument is used so that numeric character codes can have one
-- or two digits unless three are necessary, which means that the
-- returned value may represent both the character in question and the
-- digit after it:
local function EscapeableToEscaped(Char, FollowingDigit)
if IsEscapeable(Char) then
local Format = FollowingDigit == ""
and "\\%d"
or "\\%03d" .. FollowingDigit
return BsChars[Char]
or string.format(Format, string.byte(Char))
else
return Char .. FollowingDigit
end
end
-- Quotes a string in a Lua- and human-readable way. (This is a
-- replacement for string.format's %q placeholder, whose result
-- isn't always human readable.)
local function StrToStr(Str)
return '"' .. string.gsub(Str, "(.)(%d?)",
EscapeableToEscaped) .. '"'
end
-- Lua keywords:
local Keywords = {["and"] = true, ["break"] = true, ["do"] = true,
["else"] = true, ["elseif"] = true, ["end"] = true, ["false"] = true,
["for"] = true, ["function"] = true, ["if"] = true, ["in"] = true,
["local"] = true, ["nil"] = true, ["not"] = true, ["or"] = true,
["repeat"] = true, ["return"] = true, ["then"] = true,
["true"] = true, ["until"] = true, ["while"] = true}
-- Is Str an identifier?
local function IsIdent(Str)
return not Keywords[Str] and string.find(Str, "^[%a_][%w_]*$")
end
-- Converts a non-table to a Lua- and human-readable string:
local function ScalarToStr(Val)
local Ret
local Type = type(Val)
if Type == "string" then
Ret = StrToStr(Val)
elseif Type == "function" or Type == "userdata" or Type == "thread" then
-- Punt:
Ret = "<" .. _tostring(Val) .. ">"
else
Ret = _tostring(Val)
end -- if
return Ret
end
-- Converts a table to a Lua- and human-readable string.
local function TblToStr(Tbl, Seen)
Seen = Seen or {}
local Ret = {}
if not Seen[Tbl] then
Seen[Tbl] = true
local LastArrayKey = 0
for Key, Val in pairs(Tbl) do
if type(Key) == "table" then
Key = "[" .. TblToStr(Key, Seen) .. "]"
elseif not IsIdent(Key) then
if type(Key) == "number" and Key == LastArrayKey + 1 then
-- Don't mess with Key if it's an array key.
LastArrayKey = Key
else
Key = "[" .. ScalarToStr(Key) .. "]"
end
end
if type(Val) == "table" then
Val = TblToStr(Val, Seen)
else
Val = ScalarToStr(Val)
end
Ret[#Ret + 1] =
(type(Key) == "string"
and (Key .. " = ") -- Explicit key.
or "") -- Implicit array key.
.. Val
end
Ret = "{" .. table.concat(Ret, ", ") .. "}"
else
Ret = "<cycle to " .. _tostring(Tbl) .. ">"
end
return Ret
end
-- A replacement for tostring that prints tables in Lua- and
-- human-readable format:
function tostring(Val)
return type(Val) == "table"
and TblToStr(Val)
or _tostring(Val)
end
-- A test function:
local function TblToStrTest()
local Fnc = function() end
local Tbl = {}
Tbl[Tbl] = Tbl
local Tbls = {
{},
{1, true, 3},
{Foo = "Bar"},
{["*Foo*"] = "Bar"},
{Fnc},
{"\0\1\0011\a\b\f\n\r\t\v\"\\"},
{[{}] = {}},
Tbl,
}
local Strs = {
"{}",
"{1, true, 3}",
'{Foo = "Bar"}',
'{["*Foo*"] = "Bar"}',
'{<' .. _tostring(Fnc) .. '>}',
[[{"\0\1\0011\a\b\f\n\r\t\v\"\\"}]],
"{[{}] = {}}",
"{[<cycle to " .. _tostring(Tbl) .. ">] = <cycle to "
.. _tostring(Tbl) .. ">}",
}
for I, Tbl in ipairs(Tbls) do
assert(tostring(Tbl) == Strs[I],
"Assertion failed on " .. _tostring(tostring(Tbl)))
end
end
TblToStrTest()
