fix coverage

python/cudf_polars/cudf_polars/experimental/parallel.py

@@ -35,7 +35,7 @@ def __init__(self, count: int):
 
 
 LowerIRTransformer: TypeAlias = (
-    "GenericTransformer[IR, MutableMapping[IR, PartitionInfo]]"
+    "GenericTransformer[IR, tuple[IR, MutableMapping[IR, PartitionInfo]]]"
 )
 """Protocol for Lowering IR nodes."""
 
@@ -278,22 +278,22 @@ def _(
 
     nrows = max(ir.df.shape()[0], 1)
     count = math.ceil(nrows / rows_per_partition)
-    length = math.ceil(nrows / count)
-
-    slices = [
-        DataFrameScan(
-            ir.schema,
-            ir.df.slice(offset, length),
-            ir.projection,
-            ir.predicate,
-            ir.config_options,
-        )
-        for offset in range(0, nrows, length)
-    ]
-    new_node = Union(ir.schema, None, *slices)
-    return new_node, {slice: PartitionInfo(count=1) for slice in slices} | {
-        new_node: PartitionInfo(count=count)
-    }
+
+    if count > 1:
+        length = math.ceil(nrows / count)
+        slices = [
+            DataFrameScan(
+                ir.schema,
+                ir.df.slice(offset, length),
+                ir.projection,
+                ir.predicate,
+                ir.config_options,
+            )
+            for offset in range(0, nrows, length)
+        ]
+        return rec(Union(ir.schema, None, *slices))
+
+    return rec.state["default_mapper"](ir)
 
 
 ##
@@ -307,7 +307,7 @@ def _(
 ) -> tuple[IR, MutableMapping[IR, PartitionInfo]]:
     # zlice must be None
     if ir.zlice is not None:
-        return rec.state["default_mapper"](ir)
+        return rec.state["default_mapper"](ir)  # pragma: no cover
 
     # Lower children
     children, _partition_info = zip(*(rec(c) for c in ir.children), strict=False)