Weights throws if called without key for multiple edge values

Author: simonschoelly

src/matrices.jl

@@ -315,12 +315,15 @@ without any edge values.
 """
 function weights end
 
-LG.weights(g::ZeroEdgeValGraph) = LG.DefaultDistance(nv(g))
-
-LG.weights(g::OneEdgeValGraph; kwargs...) = LG.weights(g, 1; kwargs...)
-
 function LG.weights(g::AbstractValGraph, key; zerovalue=zero(edgevals_type(g, key)))
 
     return ValMatrix(g, key, zerovalue)
 end
 
+function LG.weights(g::AbstractValGraph; kwargs...)
+
+    g isa ZeroEdgeValGraph && return LG.DefaultDistance(nv(g))
+    g isa OneEdgeValGraph && return weights(g, 1; kwargs...)
+
+    throw(ArgumentError("g has more than one edge value key. Must specify key."))
+end

test/matrices.jl

@@ -179,6 +179,12 @@ using LightGraphs: DefaultDistance
         @test weights(g) == DefaultDistance(nv(g))
     end
 
+    @testset "weights for graph with two edge values and no key specified" begin
+
+        @test_throws ArgumentError weights(ValGraph(3, edgeval_types=(a=Int, b=String)))
+        @test_throws ArgumentError weights(ValDiGraph(2, edgeval_types=(Int, String)); zerovalue=nothing)
+    end
+
     @testset "convert AdjacencyMatrix to SparseMatrixCSC" begin
 
         g1 = ValGraph{Int8}(4)