Get path of parent folder of script location : Applescript

The open handler’s parameter gets a list of alias objects¹. The curly braces ({}) in your error message indicate that the error happend while it was trying to operate on a list object.

Thus, you need to use something like set FolderPath to first item of the_dropped_folder to work with a single item instead of the list (and probably give the parameter a plural name while you are at it so it “reads better”). That should let your set ParentPath to container of FolderPath statement work properly.

The next statement will probably fail though. ParentPath will be a Finder folder object which does not have a POSIX path property. Usually the easiest way around this problem is to have Finder convert its item object (folder is a subclass of item) into an alias object and then extract its POSIX path (alias objects do have a POSIX path property).

If you put all this together, you might end up with something like this:

on open someDroppedAliases
    set theAlias to first item of someDroppedAliases
    tell application "Finder"
        set parentFolder to container of theAlias
        set parentFolderAlias to parentFolder as alias
    end tell
    set parentFolderPath to POSIX path of parentFolderAlias
    display dialog "Path of container:" default answer parentFolderPath
end open

Without all the intermediate variables:

on open someDroppedAliases
    tell application "Finder" to ¬
        set parentFolderPath to POSIX path of ¬
            (container of first item of someDroppedAliases as alias)
    display dialog "Path of container:" default answer parentFolderPath
end open

Or, with System Events (whose item objects actually have a POSIX path property):

on open someDroppedAliases
    tell application "System Events" to ¬
        set parentFolderPath to POSIX path of ¬
            container of first item of someDroppedAliases
    display dialog "Path of container:" default answer parentFolderPath
end open

Note: Nothing in my versions (or in your original formulation) is specific to processing folders. The same program will process a dropped file and yield its container.

¹ Technically they are «class bmrk» objects in Snow Leopard, which seem to work mostly like proper alias objects (though there may be some differences from true alias objects).

From the Library help of Standard Additions:

POSIX file n : A file object specified with a POSIX (slash)-style pathname.

• POSIX path (text, r/o) : the POSIX (slash)-style path of a file or alias object

There is no container property.

You need what Finder considers an item to be able to do this. One way to get it:

set b to container of (a as alias)