Require source container to be a row-wise iterator

src/Table.jl

@@ -214,18 +214,17 @@ function append_columnaccess!(t::Table, t2)
     return t
 end
 
-append_rowaccess!(t::Table, t2) =
-    mapfoldl(_asnamedtuple(NamedTuple{columnnames(t)}), push!, Tables.rows(t2); init = t)
-
-function Base.append!(t::Table, t2)
-    if Tables.istable(t2)
-        if Tables.columnaccess(t2)
-            return append_columnaccess!(t, t2)
-        elseif Tables.rowaccess(t2)
-            return append_rowaccess!(t, t2)
-        end
+append_rows!(t::Table, rows) =
+    mapfoldl(_asnamedtuple(NamedTuple{columnnames(t)}), push!, rows; init = t)
+
+isrowiterator(t) =
+    Tables.istable(t) && Tables.rowaccess(t) && Tables.rows(t) === t
+
+function Base.append!(t::Table, rows)
+    if isrowiterator(rows) && Tables.columnaccess(rows)
+        return append_columnaccess!(t, rows)
     end
-    throw(ArgumentError(string("Cannot handle non-table type ", typeof(t2))))
+    return append_rows!(t, rows)
 end
 
 function Base.popfirst!(t::Table)

test/Table.jl

@@ -188,19 +188,24 @@
         @test isequal(Table(t |> rowtable), t)
     end
 
-    @testset "append!(_, ::$(typeof(t2)))" for t2 in Any[
+    @testset "append!(_, rows(::$(typeof(t2))))" for t2 in Any[
         [(a=3, b=4)],
         (a=[3], b=[4]),
     ]
         t = Table(a=[1], b=[2])
-        @test append!(t, t2) == Table(a = [1, 3], b = [2, 4])
+        @test append!(t, Tables.rows(t2)) == Table(a = [1, 3], b = [2, 4])
     end
 
     @testset "append! error handling" begin
-        @test_throws(
-            ArgumentError("Cannot handle non-table type Nothing"),
+        err = nothing
+        @test try
             append!(Table(a = [1], b = [2]), nothing)
-        )
+            false
+        catch err
+            true
+        end
+        @test err isa MethodError
+        @test err.f === iterate
     end
 
     @testset "group" begin