Merge pull request #55 from DeaMariaLeon/message

Adding error message when different libraries are used

narwhals/dataframe.py

@@ -12,6 +12,7 @@
 from narwhals.translate import get_modin
 from narwhals.translate import get_pandas
 from narwhals.translate import get_polars
+from narwhals.utils import validate_same_library
 
 if TYPE_CHECKING:
     import numpy as np
@@ -133,6 +134,7 @@ def join(
     ) -> Self:
         if how != "inner":
             raise NotImplementedError("Only inner joins are supported for now")
+        validate_same_library([self, other])
         return self._from_dataframe(
             self._dataframe.join(
                 self._extract_native(other),

narwhals/functions.py

@@ -4,6 +4,8 @@
 from typing import Iterable
 from typing import Literal
 
+from narwhals.utils import validate_same_library
+
 if TYPE_CHECKING:
     from narwhals.dataframe import DataFrame
     from narwhals.dataframe import LazyFrame
@@ -21,6 +23,7 @@ def concat(
     if not items:
         raise ValueError("No items to concatenate")
     items = list(items)
+    validate_same_library(items)
     first_item = items[0]
     plx = first_item.__narwhals_namespace__()
     return first_item.__class__(

narwhals/utils.py

@@ -42,3 +42,13 @@ def parse_version(version: Sequence[str | int]) -> tuple[int, ...]:
 
 def isinstance_or_issubclass(obj: Any, cls: Any) -> bool:
     return isinstance(obj, cls) or issubclass(obj, cls)
+
+
+def validate_same_library(items: Iterable[Any]) -> None:
+    if all(item._is_polars for item in items):
+        return
+    if all(hasattr(item._dataframe, "_implementation") for item in items) and (
+        len({item._dataframe._implementation for item in items}) == 1
+    ):
+        return
+    raise NotImplementedError("Cross-library comparisons aren't supported")

tests/test_common.py

@@ -630,3 +630,24 @@ def test_reindex(df_raw: Any) -> None:
     compare_dicts(result, expected)
     with pytest.raises(ValueError, match="Multi-output expressions are not supported"):
         nw.to_native(df.with_columns(nw.all() + nw.all()))
+
+
+@pytest.mark.parametrize(
+    ("df_raw", "df_raw_right"),
+    [(df_pandas, df_polars), (df_polars, df_pandas)],
+)
+def test_library(df_raw: Any, df_raw_right: Any) -> None:
+    df_left = nw.LazyFrame(df_raw)
+    df_right = nw.LazyFrame(df_raw_right)
+    with pytest.raises(
+        NotImplementedError, match="Cross-library comparisons aren't supported"
+    ):
+        nw.concat([df_left, df_right], how="horizontal")
+    with pytest.raises(
+        NotImplementedError, match="Cross-library comparisons aren't supported"
+    ):
+        nw.concat([df_left, df_right], how="vertical")
+    with pytest.raises(
+        NotImplementedError, match="Cross-library comparisons aren't supported"
+    ):
+        df_left.join(df_right, left_on=["a"], right_on=["a"], how="inner")