feat: Add empty constructor for GNNHeteroGraph (#358)

* add empty heterograph constructor

* update docs

* Update src/GNNGraphs/convert.jl

Co-authored-by: Carlo Lucibello <carlo.lucibello@gmail.com>

* Update docs/src/heterograph.md

Co-authored-by: Carlo Lucibello <carlo.lucibello@gmail.com>

* add tests

---------

Co-authored-by: Carlo Lucibello <carlo.lucibello@gmail.com>

docs/src/heterograph.md

@@ -12,8 +12,13 @@ the type [`GNNHeteroGraph`](@ref).
 
 ## Creating a Heterograph
 
-A heterograph can be created by passing pairs `edge_type => data` to the constructor.
+A heterograph can be created empty or by passing pairs `edge_type => data` to the constructor.
 ```jldoctest
+julia> g = GNNHeteroGraph()
+GNNHeteroGraph:
+  num_nodes: Dict()
+  num_edges: Dict()
+  
 julia> g = GNNHeteroGraph((:user, :like, :actor) => ([1,2,2,3], [1,3,2,9]),
                           (:user, :rate, :movie) => ([1,1,2,3], [7,13,5,7]))
 GNNHeteroGraph:

src/GNNGraphs/convert.jl

@@ -4,26 +4,28 @@ function to_coo(data::EDict; num_nodes = nothing, kws...)
     graph = EDict{COO_T}()
     _num_nodes = NDict{Int}()
     num_edges = EDict{Int}()
-    for k in keys(data)
-        d = data[k]
-        @assert d isa Tuple
-        if length(d) == 2
-            d = (d..., nothing)
+    if !isempty(data)
+        for k in keys(data)
+            d = data[k]
+            @assert d isa Tuple
+            if length(d) == 2
+                d = (d..., nothing)
+            end
+            if num_nodes !== nothing
+                n1 = get(num_nodes, k[1], nothing)
+                n2 = get(num_nodes, k[3], nothing)
+            else
+                n1 = nothing
+                n2 = nothing
+            end
+            g, nnodes, nedges = to_coo(d; hetero = true, num_nodes = (n1, n2), kws...)
+            graph[k] = g
+            num_edges[k] = nedges
+            _num_nodes[k[1]] = max(get(_num_nodes, k[1], 0), nnodes[1])
+            _num_nodes[k[3]] = max(get(_num_nodes, k[3], 0), nnodes[2])
         end
-        if num_nodes !== nothing
-            n1 = get(num_nodes, k[1], nothing)
-            n2 = get(num_nodes, k[3], nothing)
-        else
-            n1 = nothing
-            n2 = nothing
-        end
-        g, nnodes, nedges = to_coo(d; hetero = true, num_nodes = (n1, n2), kws...)
-        graph[k] = g
-        num_edges[k] = nedges
-        _num_nodes[k[1]] = max(get(_num_nodes, k[1], 0), nnodes[1])
-        _num_nodes[k[3]] = max(get(_num_nodes, k[3], 0), nnodes[2])
+        graph = Dict([k => v for (k, v) in pairs(graph)]...) # try to restrict the key/value types
     end
-    graph = Dict([k => v for (k, v) in pairs(graph)]...) # try to restrict the key/value types
     return graph, _num_nodes, num_edges
 end
 

src/GNNGraphs/gnnheterograph.jl

@@ -100,6 +100,8 @@ end
 GNNHeteroGraph(data; kws...) = GNNHeteroGraph(Dict(data); kws...)
 GNNHeteroGraph(data::Pair...; kws...) = GNNHeteroGraph(Dict(data...); kws...)
 
+GNNHeteroGraph() = GNNHeteroGraph(Dict{Tuple{Symbol,Symbol,Symbol}, Any}())
+
 function GNNHeteroGraph(data::Dict; kws...)
     all(k -> k isa EType, keys(data)) || throw(ArgumentError("Keys of data must be tuples of the form `(source_type, edge_type, target_type)`"))
     return GNNHeteroGraph(Dict([k => v for (k, v) in pairs(data)]...); kws...)
@@ -135,10 +137,17 @@ function GNNHeteroGraph(data::EDict;
     num_graphs = !isnothing(graph_indicator) ?
                  maximum([maximum(gi) for gi in values(graph_indicator)]) : 1
 
-    ndata = normalize_heterographdata(ndata, default_name = :x, ns = num_nodes)
-    edata = normalize_heterographdata(edata, default_name = :e, ns = num_edges,
-                                      duplicate_if_needed = true)
-    gdata = normalize_graphdata(gdata, default_name = :u, n = num_graphs)
+
+    if length(keys(graph)) == 0
+        ndata = Dict{Symbol, DataStore}()
+        edata = Dict{Tuple{Symbol, Symbol, Symbol}, DataStore}()
+        gdata = DataStore()
+    else
+        ndata = normalize_heterographdata(ndata, default_name = :x, ns = num_nodes)
+        edata = normalize_heterographdata(edata, default_name = :e, ns = num_edges,
+                                          duplicate_if_needed = true)
+        gdata = normalize_graphdata(gdata, default_name = :u, n = num_graphs)
+    end
 
     return GNNHeteroGraph(graph,
                           num_nodes, num_edges, num_graphs,

test/GNNGraphs/gnnheterograph.jl

@@ -1,4 +1,14 @@
 
+
+@testset "Empty constructor" begin
+    g = GNNHeteroGraph()
+    @test isempty(g.num_nodes)
+    g = add_edges(g, (:user, :like, :actor) => ([1,2,3,3,3], [3,5,1,9,4]))
+    @test g.num_nodes[:user] == 3
+    @test g.num_nodes[:actor] == 9
+    @test g.num_edges[(:user, :like, :actor)] == 5
+end
+
 @testset "Constructor from pairs" begin
     hg = GNNHeteroGraph((:A, :e1, :B) => ([1,2,3,4], [3,2,1,5]))
     @test hg.num_nodes == Dict(:A => 4, :B => 5)