Steam. IDs... IDs... IDs... Let's clean this mess up (GMod).

print_id.lua

-- Made/Published to clear up misinformation, for you to possibly learn something new from it. - CaptainPRICE
-- Tell me in the comments (below) if you have spot bug(s), or if you have any suggestions or improvements as I would like to hear them!
-- See my comment here: https://github.com/wiremod/wire/pull/1217#issuecomment-246211364

local game = game
local IS_DEDICATED, IS_SINGLEPLAYER, MAX_NUM_PLAYERS = game.IsDedicated(), game.SinglePlayer(), game.MaxPlayers()

print(CLIENT and "CLIENT" or (SERVER and "SERVER" or "MENU"), IS_SINGLEPLAYER and "SP" or "MP", IS_DEDICATED and "DEDICATED" or "LISTEN", "MAX=" .. MAX_NUM_PLAYERS, "v" .. VERSION, BRANCH)

if not debug then return print("debug library is missing") end
local debug = debug
if not string then return print("string library is missing") end
local string = string
if not player then return print("player library is missing") end
local player = player

local _R = debug.getregistry()

local ENTITY = FindMetaTable("Entity")
if not ENTITY then
    ENTITY = _R.Entity
    if not ENTITY then return end
end
local PLAYER = FindMetaTable("Player")
local PLAYER_BASE
if not PLAYER then
    PLAYER = _R.Player
    if not PLAYER then return end
end
PLAYER_BASE = PLAYER.MetaBaseClass
if not PLAYER_BASE then return end
--print(PLAYER.MetaName .. " inherits from " .. PLAYER_BASE.MetaName) -- Prints: "Player inherits from Entity"

local print = print
local tostring = tostring
local typeof = type
local gsub = string.gsub
local formatvar = function(str, var)
    if not isstring(str) then return end
    str = gsub(gsub(str, "{:tostring:}", (isstring(var) and ('"' .. var .. '"')) or tostring(var)), "{:typeof:}", typeof(var))
    return str
end
local FormatVar = function(name, var)
    return formatvar(name .. " = {:tostring:} ({:typeof:})", var)
end

local PrintValue = function(actor, name)
    if not IsValid(actor) or not isstring(name) then return end
    local is_bot = actor:IsBot()
    local func = is_bot and ENTITY[name] or (PLAYER[name] or PLAYER_BASE[name]) -- Use Entity for bots; otherwise use Player, if it is not found in Player, then search the Player's base class (Entity)
    if not isfunction(func) then return end
    local value = func(actor)
    name = (is_bot and "Bot:" or "Ply:") .. name .. "()"
    print(FormatVar(name, value))
end

local CAN_HAVE_BOTS = not IS_SINGLEPLAYER or (MAX_NUM_PLAYERS > 1)
local humans = player.GetHumans()
local Bot = CAN_HAVE_BOTS and player.GetBots()[1] or NULL
local Ply = (CAN_HAVE_BOTS and CLIENT) and LocalPlayer() or humans[math.random(1, #humans)]

print(FormatVar("Bot", Bot), FormatVar("Ply", Ply))

if CLIENT then
    local GetPlayerInfo = PLAYER.GetPlayerInfo or _R.Player.GetPlayerInfo
    if isfunction(GetPlayerInfo) then
        if IsValid(Bot) then
            print(FormatVar("Bot:GetPlayerInfo().friendid", GetPlayerInfo(Bot).friendid))
        end
        if IsValid(Ply) then
            print(FormatVar("Ply:GetPlayerInfo().friendid", GetPlayerInfo(Ply).friendid))
        end
        --print(FormatVar("NULL:GetPlayerInfo().friendid", GetPlayerInfo(NULL).friendid)) -- ERROR!
    end
end

for _, func_name in next, { "AccountID", "EntIndex", "SteamID", "SteamID64", "UniqueID", "UserID" } do
    PrintValue(Bot, func_name)
    PrintValue(Ply, func_name)
end

-- First run in singleplayer (SP) on serverside, then clientside.
-- Second run in multiplayer (MP) on serverside, then clientside.
-- Script output (at version: 160905 - dev branch):
--[[
SERVER  SP  LISTEN  MAX=1   v160905 dev
Bot = [NULL Entity] (Entity)    Ply = Player [1][CaptainPRICE] (Player)
Ply:AccountID() = nil (nil)
Ply:EntIndex() = 1 (number)
Ply:SteamID() = "STEAM_0:0:0" (string)
Ply:SteamID64() = nil (nil)
Ply:UniqueID() = "1" (string)
Ply:UserID() = 2 (number)

CLIENT  SP  LISTEN  MAX=1   v160905 dev
Bot = [NULL Entity] (Entity)    Ply = Player [1][CaptainPRICE] (Player)
Ply:GetPlayerInfo().friendid = 131959820 (number)
Ply:AccountID() = 131959820 (number)
Ply:EntIndex() = 1 (number)
Ply:SteamID() = "STEAM_0:0:65979910" (string)
Ply:SteamID64() = "76561198092225548" (string)
Ply:UniqueID() = "1" (string)
Ply:UserID() = 2 (number)

SERVER  MP  LISTEN  MAX=128 v160905 dev
Bot = Player [2][Bot01] (Player)    Ply = Player [1][CaptainPRICE] (Player)
Bot:AccountID() = 0 (number)
Ply:AccountID() = 131959820 (number)
Bot:EntIndex() = 2 (number)
Ply:EntIndex() = 1 (number)
Bot:SteamID() = "BOT" (string)
Ply:SteamID() = "STEAM_0:0:65979910" (string)
Bot:SteamID64() = "90071996842377216" (string)
Ply:SteamID64() = "76561198092225548" (string)
Bot:UniqueID() = "1513418020" (string)
Ply:UniqueID() = "445831337" (string)
Bot:UserID() = 4 (number)
Ply:UserID() = 3 (number)

CLIENT  MP  LISTEN  MAX=128 v160905 dev
Bot = Player [2][Bot01] (Player)    Ply = Player [1][CaptainPRICE] (Player)
Bot:GetPlayerInfo().friendid = 0 (number)
Ply:GetPlayerInfo().friendid = 131959820 (number)
Bot:AccountID() = nil (nil)
Ply:AccountID() = 131959820 (number)
Bot:EntIndex() = 2 (number)
Ply:EntIndex() = 1 (number)
Bot:SteamID() = "NULL" (string)
Ply:SteamID() = "STEAM_0:0:65979910" (string)
Bot:SteamID64() = nil (nil)
Ply:SteamID64() = "76561198092225548" (string)
Bot:UniqueID() = "1513418020" (string)
Ply:UniqueID() = "445831337" (string)
Bot:UserID() = 4 (number)
Ply:UserID() = 3 (number)
]]

print_id_2.lua

-- Made/Published to clear up misinformation, for you to possibly learn something new from it. - CaptainPRICE
-- Tell me in the comments (below) if you have spot bug(s), or if you have any suggestions or improvements as I would like to hear them!

local game = game
local IS_DEDICATED, IS_SINGLEPLAYER, MAX_NUM_PLAYERS = game.IsDedicated(), game.SinglePlayer(), game.MaxPlayers()

print(CLIENT and "CLIENT" or (SERVER and "SERVER" or "MENU"), IS_SINGLEPLAYER and "SP" or "MP", IS_DEDICATED and "DEDICATED" or "LISTEN", "MAX=" .. MAX_NUM_PLAYERS, "v" .. VERSION, BRANCH)

if not debug then return print("debug library is missing") end
local debug = debug
if not math then return print("math library is missing") end
local math = math
if not string then return print("string library is missing") end
local string = string
if not player then return print("player library is missing") end
local player = player
--if not team then return print("team library is missing") end
--local team = team

local _R = debug.getregistry()

local ENTITY = FindMetaTable("Entity")
if not ENTITY then
    ENTITY = _R.Entity
    if not ENTITY then return end
end
local PLAYER = FindMetaTable("Player")
local PLAYER_BASE
if not PLAYER then
    PLAYER = _R.Player
    if not PLAYER then return end
end
PLAYER_BASE = PLAYER.MetaBaseClass
if not istable(PLAYER_BASE) then return end -- It must have a base class
--print(PLAYER.MetaName .. " inherits from " .. PLAYER_BASE.MetaName) -- Prints: "Player inherits from Entity"
local VEHICLE = FindMetaTable("Vehicle")
local VEHICLE_BASE
if not VEHICLE then
    VEHICLE = _R.Vehicle
    if not VEHICLE then return end
end
VEHICLE_BASE = VEHICLE.MetaBaseClass
if not istable(VEHICLE_BASE) then return end -- It must have a base class
--print(VEHICLE.MetaName .. " inherits from " .. VEHICLE_BASE.MetaName) -- Prints: "Vehicle inherits from Entity"

local print = print
local tostring = tostring
local typeof = type
local gsub = string.gsub
-- A small function to make variable display nicer
local formatvar = function(var, str)
    if not isstring(str) then return end
    str = gsub(gsub(str, "{:tostring:}", (isstring(var) and ('"' .. var .. '"')) or tostring(var)), "{:typeof:}", typeof(var))
    return str
end
local FormatVar = function(var, name)
    return formatvar(var, name .. " = {:tostring:} ({:typeof:})")
end
-- Attempts to find a function in a class (from metatable and registry) for the given Entity/Player/Vehicle (it is specifically for entities)
local GetFunctionFromClass = function(actor, name, force)
    if not force and (not IsValid(actor) or not isentity(actor)) then return end
    if actor:IsPlayer() then
        -- Use Player class, if it is not found in Player, then search the Player's base class (Entity)
        return ((PLAYER[name] or _R.Player[name]) or PLAYER_BASE[name]) or _R.Entity[name], ((isfunction(PLAYER[name]) and PLAYER.MetaName) or isfunction(PLAYER_BASE[name]) and PLAYER_BASE.MetaName) or --[[typeof(actor)]]"Player"
    end
    if actor:IsVehicle() then
        -- Use Vehicle class, if it is not found in Vehicle, then search the Vehicle's base class (Entity)
        return ((VEHICLE[name] or _R.Vehicle[name]) or VEHICLE_BASE[name]) or _R.Entity[name], ((isfunction(VEHICLE[name]) and VEHICLE.MetaName) or isfunction(VEHICLE_BASE[name]) and VEHICLE_BASE.MetaName) or --[[typeof(actor)]]"Vehicle"
    end
    -- Else, it is some other type of entity such as CSEnt (we don't really care about here, or do we?)
    -- But, just look up Entity class for it
    return ENTITY[name] or _R.Entity[name], ENTITY.MetaName or --[[typeof(actor)]]"Entity" -- Entity is the base class
end
-- Addition to FormatVar function above, returns nicely formatted string of var. var is expected to be Entity/Player/Vehicle (it will evaluate a (name) function on its class)
local FormatClassVar = function(var, name, force)
    local class_func, class_name = GetFunctionFromClass(var, name, force)
    if isfunction(class_func) and isstring(class_name) then
        return FormatVar(class_func(var), class_name .. ":" .. name .. "()") -- Like it? :)
    end
    -- Else, it is a failure - should not happen, please report if it does
    ErrorNoHalt("typeof(class_func) = " .. typeof(class_func), '\n', "typeof(class_name) = " .. typeof(class_name), '\n')
    return var, name, class_func, class_name
end
-- Prints it out nicely, use specifically for entities
local PrintEvalEntityValue = function(actor, name, force)
    if force or (IsValid(actor) and isstring(name)) then
        print(FormatClassVar(actor, name, force))
    end
end

---------------------------------------------------------------------------------------------------------------------------------------

local CAN_HAVE_BOTS = not IS_SINGLEPLAYER or (MAX_NUM_PLAYERS > 1)
local humans = player.GetHumans()
local Bot = CAN_HAVE_BOTS and player.GetBots()[1] or NULL
local Ply = (CAN_HAVE_BOTS and CLIENT) and LocalPlayer() or humans[math.random(1, #humans)]

print(FormatVar(Bot, "Bot"), FormatVar(Ply, "Ply"))

local FORCE = true -- Set to true to bypass validation checks; otherwise false
-- Simply alter the data table as you please  ˘\_(^^)_/˘
local data = {
    {
        "Ply",               -- 1. (n)  Text which is printed before evaluation
        Ply,                 -- 2. (e)  Entity value to use (it will automatically detect its class, but it has to inherit from Entity class as that is the "ultimate" base class for any entity)
        "EntIndex",          -- 3. (fn) Name of the function to evaluate on the class with above entity value (it is smart about finding it)
        Color(70, 200, 70)   -- 4. (c)  *OPTIONAL* Color to use for text specified at slot/index 1
    },
    {
        "Ply",
        Ply,
        "UserID",
        team.GetColor(Ply:Team()) -- Use Ply's team color
    },
    {
        "NULL",
        NULL,                     -- Note: NULL is an instance of Entity class (not Player/Vehicle/etc...)
        "EntIndex"
    }
}
if CAN_HAVE_BOTS and IsValid(Bot) then
    table.Merge(data, {
        {
            "Bot",
            Bot,
            "EntIndex",
            Color(0, 100, 240)
        },
        {
            "Bot",
            Bot,
            "UserID",
            team.GetColor(Bot:Team()) -- Use Bot's team color
        }
    })
end
for i, v in next, data do
    local n, e, fn, c = v[1], v[2], v[3], v[4]
    if not isentity(e) or not isstring(fn) then
        ErrorNoHalt("data table contains values of wrong types (possibly incorrect order at index " .. i .. "?)", '\n', "typeof(n) = " .. typeof(n), '\n', "typeof(e) = " .. typeof(e), '\n', "typeof(fn) = " .. typeof(fn), '\n', "typeof(c) = " .. typeof(c), '\n')
        print(i, v, n, e, fn, c)
        break
    end
    if not isstring(n) or #n < 1 then n = "-" end -- Default text
    --print(n, FormatClassVar(e, fn)) -- Display results (FormatClassVar function will run the named function and print it nicely)
    if IsColor(c) then
        MsgC(c, n)
    else
        Msg(n)
    end
    Msg('\t')
    PrintEvalEntityValue(e, fn, FORCE)
end

for k, v in next, player.GetAll() do -- Print all
    print(k .. " - " .. tostring(v) .. " { EntIndex = " .. v:EntIndex() .. ", UserID = " .. v:UserID() .. " }")
end

-- _G.Entity function expects Entity EntIndex - not Player UserID! (No matter what.)
-- _G.Player function expects Player UserID - not Entity EntIndex! (No matter what.)
-- Screenshot (proof): https://cloud.githubusercontent.com/assets/9789070/18476543/c67d332a-79ca-11e6-9d1e-d3ff5dc64e15.png
-- First run in singleplayer (SP) on serverside, then clientside.
-- Second run in multiplayer (MP) on serverside, then clientside.
-- Script output (at version: 160905 - dev branch):
--[[
SERVER  SP  LISTEN  MAX=1   v160905 dev
Bot = [NULL Entity] (Entity)    Ply = Player [1][CaptainPRICE] (Player)
Ply Entity:EntIndex() = 1 (number)
Ply Player:UserID() = 2 (number)
NULL    Entity:EntIndex() = 0 (number)
1 - Player [1][CaptainPRICE] { EntIndex = 1, UserID = 2 }

CLIENT  SP  LISTEN  MAX=1   v160905 dev
Bot = [NULL Entity] (Entity)    Ply = Player [1][CaptainPRICE] (Player)
Ply Entity:EntIndex() = 1 (number)
Ply Player:UserID() = 2 (number)
NULL    Entity:EntIndex() = 0 (number)
1 - Player [1][CaptainPRICE] { EntIndex = 1, UserID = 2 }

SERVER  MP  LISTEN  MAX=128 v160905 dev
Bot = Player [2][Bot01] (Player)    Ply = Player [1][CaptainPRICE] (Player)
Bot Entity:EntIndex() = 2 (number)
Bot Player:UserID() = 4 (number)
NULL    Entity:EntIndex() = 0 (number)
1 - Player [1][CaptainPRICE] { EntIndex = 1, UserID = 3 }
2 - Player [2][Bot01] { EntIndex = 2, UserID = 4 }

CLIENT  MP  LISTEN  MAX=128 v160905 dev
Bot = Player [2][Bot01] (Player)    Ply = Player [1][CaptainPRICE] (Player)
Bot Entity:EntIndex() = 2 (number)
Bot Player:UserID() = 4 (number)
NULL    Entity:EntIndex() = 0 (number)
1 - Player [1][CaptainPRICE] { EntIndex = 1, UserID = 3 }
2 - Player [2][Bot01] { EntIndex = 2, UserID = 4 }
]]