python: improve error when passing DataFrame to LazyFrame (#3482)

py-polars/polars/internals/lazy_frame.py

@@ -1149,6 +1149,8 @@ def join_asof(
         force_parallel
             Force the physical plan to evaluate the computation of both DataFrames up to the join in parallel.
         """
+        if not isinstance(ldf, LazyFrame):
+            raise ValueError(f"Expected a `LazyFrame` as join table, got {type(ldf)}")
 
         if isinstance(on, str):
             left_on = on
@@ -1296,6 +1298,9 @@ def join(
         └──────┴──────┴─────┴───────┘
 
         """
+        if not isinstance(ldf, LazyFrame):
+            raise ValueError(f"Expected a `LazyFrame` as join table, got {type(ldf)}")
+
         if how == "asof":
             warnings.warn(
                 "using asof join via LazyFrame.join is deprecated, please use LazyFrame.join_asof",

py-polars/tests/test_errors.py

@@ -1,4 +1,5 @@
 import io
+import typing
 
 import numpy as np
 import pytest
@@ -57,3 +58,26 @@ def test_panic_exception() -> None:
         match=r"""this operation is not implemented/valid for this dtype: .*""",
     ):
         pl.struct(pl.Series("a", [1, 2, 3]), eager=True).sort()
+
+
+@typing.no_type_check
+def test_join_lazy_on_df() -> None:
+    df_left = pl.DataFrame(
+        {
+            "Id": [1, 2, 3, 4],
+            "Names": ["A", "B", "C", "D"],
+        }
+    )
+    df_right = pl.DataFrame({"Id": [1, 3], "Tags": ["xxx", "yyy"]})
+
+    with pytest.raises(
+        ValueError,
+        match="Expected a `LazyFrame` as join table, got <class 'polars.internals.frame.DataFrame'>",
+    ):
+        df_left.lazy().join(df_right, on="Id")
+
+    with pytest.raises(
+        ValueError,
+        match="Expected a `LazyFrame` as join table, got <class 'polars.internals.frame.DataFrame'>",
+    ):
+        df_left.lazy().join_asof(df_right, on="Id")