additional argument to appendlabel!

Project.toml

 name = "PlanarLarvae"
 uuid = "c2615984-ef14-4d40-b148-916c85b43307"
 authors = ["François Laurent", "Institut Pasteur"]
-version = "0.9.1"
+version = "0.9.2"
 
 [deps]
 DelimitedFiles = "8bb1440f-4735-579b-a4ab-409b98df4dab"

src/Datasets.jl

@@ -1138,16 +1138,21 @@ coerce(ref::AbstractVector, a) = coerce(eltype(ref), a)
 coerce(ref, a) = coerce(typeof(ref), a)
 
 """
-    appendlabel!(run, label; recordname=:labels, attrname=(:secondarylabels, :names), ignore=String[])
+    appendlabel!(run, label; recordname=:labels, attrname=(:secondarylabels, :names), ignore=String[], ifempty=true)
 
 Append a label to existing labels, at every track steps.
 Per default, the label to append is considered secondary, meaning it is represented appart in
 the metadata. Secondary labels are recorded as such in metadata, so that they can be
 distinguished from primary labels.
+
 Argument `ignore` lists uncompatible labels that prevent the label to be appended.
+
+Argument `ifempty`, if set to false, additionally allows preventing the specified label to be
+appended in cases the label is explicitly undefined with an empty list of labels.
 """
 function appendlabel!(run::Run, label::Union{Symbol, String};
-        recordname=:labels, attrname=(:secondarylabels, :names), ignore=String[])
+        recordname=:labels, attrname=(:secondarylabels, :names), ignore=String[],
+        ifempty=true)
     decodelabels!(run)
     topattr = attrname isa Symbol ? attrname : attrname[1]
     labels = if topattr== recordname
@@ -1178,7 +1183,9 @@ function appendlabel!(run::Run, label::Union{Symbol, String};
         for (i, existinglabel) in enumerate(labels)
             if isa(existinglabel, Vector)
                 if isempty(existinglabel)
-                    labels[i] = label # not a vector
+                    if ifempty
+                        labels[i] = label # not a vector
+                    end
                 elseif isempty(existinglabel ∩ ignore)
                     push!(existinglabel, label)
                 end