Weird behaviour with dialog boxes and variables in Lua script for Lightroom plugin?

Question

I'm writing a Lightroom plugin using the Lightroom SDK/API in the Lua language. I'm new to Lua. I've found a situation where my script only works if a Lightroom dialog box (LrDialogs.message("random message")) is present in one function. Without it the function falls over at a later point claiming a string variable (Image.dr in the last LrDialogs.message) is 'nil' as opposed to the normal value it has when the plugin is working properly. Anyone know what's going wrong? Here's the relevant code segment: ------ read output file for exif and write to LR metadata ------ function parseOutput(outputFilePath)  LrDialogs.message("random message")    local tblOutput = {} --to hold the output exif (1 column table, i.e. an array)  local tblImages = {} --to hold the images and their relevant metadata    for line in io.lines(outputFilePath) do   line = removeWhitespaces(line)   table.insert(tblOutput, line)  end    local str = table.remove(tblOutput) --remove last line in table/file (it's log info, not exif)  tblImages = extractExif(tblOutput) --pick out the exif key/value pairs and add to Image objects end function extractExif(tblOutput)  local Image = {} --pseudo object to hold metadata for each image  local tblImages = {}  local blnFlag = false  local intCount = 0    for k,v in pairs(tblOutput) do --iterate through each value in the table   if string.find(v, "^=.+") then    --test if new image other than the first one    if blnFlag == true then     --add Image to tblImages and then clear Image object     table.insert(tblImages, Image)     --Image = {} --don't technically need this     blnFlag = false     --LrDialogs.message("inside blnFlag test")    end        i, j = string.find(v, "/") -- **** MAC ONLY. Back slash for Windows *****    Image.filePath = string.sub(v, i) --returns the file path    Image.name = string.match(v, "([^/]+)$") --return the file name    blnFlag = true       elseif string.find(v, "ISO") ~= nil then    Image.iso = string.match(v, "%a+:(.+)") --get text (i.e value) to right of colon   elseif string.find(v, "Film") ~= nil then    Image.filmSim = string.match(v, "%a+:(.+)")   elseif string.find(v, "Setting") ~= nil then    Image.drMode = string.match(v, "%a+:(.+)")   elseif (string.find(v, "Auto") ~= nil) or (string.find(v, "Development") ~= nil) then    Image.dr = string.match(v, "%a+:(.+)")   else   end  end    LrDialogs.message(Image.name .. Image.iso .. Image.filmSim .. Image.drMode .. Image.dr)    return tblImages end function removeWhitespaces(str)  return string.gsub(str, "%s", "") end

Answer 1

The dialog box is probably not fixing the real problem. It is most likely slowing the script down or changing the timing just enough that the bug does not show up. That is a common trap when using message boxes for debugging.

In your code, I would first look at the Image table and the way it is being reused. You create Image once before the loop, then keep adding values to the same table. When you do table.insert(tblImages, Image), Lua inserts a reference to that same table, not a fresh copy. So if you are processing multiple images, you should create a new Image table each time you detect a new image, especially after inserting the previous one.

I would also check this line:

LrDialogs.message(Image.name .. Image.iso .. Image.filmSim .. Image.drMode .. Image.dr)

If Image.dr is nil, Lua cannot concatenate it with strings. The safer test is:

LrDialogs.message(

tostring(Image.name) ..

tostring(Image.iso) ..

tostring(Image.filmSim) ..

tostring(Image.drMode) ..

tostring(Image.dr)

)

That will show you which value is actually missing instead of crashing. Then you can trace back and ask: did the file actually contain the “Auto” or “Development” line that sets Image.dr? If not, Image.dr will stay nil.

So I would fix it in this order:

1. Do not rely on LrDialogs.message as the fix. Use it only as a clue.
2. Create a new Image = {} for each new image record.
3. Use ipairs instead of pairs if the output lines must stay in order.
4. Use tostring() while debugging so nil values do not crash the message.
5. Add a default value for Image.dr if that field is optional.

The core issue is likely not the Lightroom dialog box itself. It is probably a combination of table reuse, order of parsing, and one field not being assigned before it is concatenated.