add check for spec dataframe

Author: samuel.oranyeli

janitor/polars/__init__.py

@@ -2,7 +2,7 @@
 
 from polars.type_aliases import ColumnNameOrSelector
 
-from janitor.utils import import_message
+from janitor.utils import check, import_message
 
 from .pivot_longer import _pivot_longer, _pivot_longer_dot_value
 
@@ -467,6 +467,7 @@ def pivot_longer_spec(
     Returns:
         A polars DataFrame/LazyFrame.
     """
+    check("spec", spec, [pl.DataFrame])
     if ".name" not in spec.columns:
         raise KeyError(
             "Kindly ensure the spec DataFrame has a `.name` column."

tests/polars/functions/test_pivot_longer_spec.py

@@ -2,7 +2,6 @@
 import pytest
 from polars.testing import assert_frame_equal
 
-import janitor.polars  # noqa: F401
 from janitor.polars import pivot_longer_spec
 
 
@@ -23,6 +22,15 @@ def df_checks():
 spec = pl.DataFrame(spec)
 
 
+def test_spec_is_a_dataframe(df_checks):
+    """Raise Error if spec is not a DataFrame."""
+    with pytest.raises(
+        TypeError,
+        match="spec should be one of.+",
+    ):
+        df_checks.pipe(pivot_longer_spec, spec={".name": "name"})
+
+
 def test_spec_columns_has_dot_name(df_checks):
     """Raise KeyError if '.name' not in spec's columns."""
     with pytest.raises(