Declare dataset partition counts as unknown

src/dataset.jl

@@ -154,6 +154,8 @@ struct DatasetPartitions
     end
 end
 
+Base.IteratorSize(::Type{DatasetPartitions}) = Base.SizeUnknown()
+
 function iterated_partition(partitions::DatasetPartitions, cursor)
     partition = nothing
     walker, root, files, fileidx, step = cursor

test/runtests.jl

@@ -12,4 +12,5 @@ const julia_parcompat = joinpath(artifact"julia_parcompat", readdir(artifact"jul
     include("test_cursors.jl")
     include("test_writer.jl")
     include("test_dataset_types.jl")
+    include("test_dataset_partitions.jl")
 end

test/test_dataset_partitions.jl

@@ -0,0 +1,36 @@
+using Parquet, Tables, Test
+
+@testset "Collect dataset partitions" begin
+    fixture = joinpath(@__DIR__, "booltest", "alltypes_plain.snappy.parquet")
+    mktempdir() do path
+        for name in ("a.parquet", "b.parquet", "_common_metadata", "_metadata")
+            cp(fixture, joinpath(path, name))
+        end
+        for factory in (Parquet.Dataset, Parquet.read_parquet), (filter, expected) in (
+            (p -> true, 2),
+            (p -> endswith(p, "a.parquet"), 1),
+            (p -> false, 0),
+        )
+            dataset = factory(path; filter=filter)
+            try
+                nmanual = 0
+                for partition in Tables.partitions(dataset)
+                    nmanual += 1
+                    close(partition)
+                end
+                @test nmanual == expected
+                partitions = collect(Tables.partitions(dataset))
+                try
+                    @test partitions isa AbstractVector
+                    @test length(partitions) == expected
+                    @test all(p -> p isa Parquet.Table, partitions)
+                    @test all(p -> Tables.getcolumn(p, :id) == Int32[6, 7], partitions)
+                finally
+                    foreach(close, partitions)
+                end
+            finally
+                close(dataset)
+            end
+        end
+    end
+end