add from_native

narwhals/__init__.py

@@ -15,6 +15,7 @@
 from narwhals.expression import sum
 from narwhals.expression import sum_horizontal
 from narwhals.series import Series
+from narwhals.translate import from_native
 from narwhals.translate import to_native
 
 __version__ = "0.6.7"
@@ -26,6 +27,7 @@
     "is_pandas",
     "get_implementation",
     "to_native",
+    "from_native",
     "all",
     "col",
     "len",

narwhals/translate.py

@@ -3,6 +3,8 @@
 from typing import TYPE_CHECKING
 from typing import Any
 
+from narwhals.dataframe import DataFrame
+from narwhals.dataframe import LazyFrame
 from narwhals.dependencies import get_cudf
 from narwhals.dependencies import get_modin
 from narwhals.dependencies import get_pandas
@@ -30,6 +32,29 @@ def to_native(obj: BaseFrame | Series) -> Any:
     raise TypeError(msg)
 
 
+def from_native(df: Any) -> BaseFrame:
+    if (pl := get_polars()) is not None and isinstance(df, pl.DataFrame):
+        return DataFrame(df)
+    elif (pl := get_polars()) is not None and isinstance(df, pl.LazyFrame):
+        return LazyFrame(df)
+    elif (
+        (pd := get_pandas()) is not None
+        and isinstance(df, pd.DataFrame)
+        or (mpd := get_modin()) is not None
+        and isinstance(df, mpd.DataFrame)
+        or (cudf := get_cudf()) is not None
+        and isinstance(df, cudf.DataFrame)
+    ):
+        return DataFrame(df)
+    elif hasattr(df, "__narwhals_dataframe__"):  # pragma: no cover
+        return DataFrame(df.__narwhals_dataframe__())
+    elif hasattr(df, "__narwhals_lazyframe__"):  # pragma: no cover
+        return LazyFrame(df.__narwhals_lazyframe__())
+    else:
+        msg = f"Expected pandas-like dataframe, Polars dataframe, or Polars lazyframe, got: {type(df)}"
+        raise TypeError(msg)
+
+
 __all__ = [
     "get_pandas",
     "get_polars",

tpch/q1.py

@@ -1,4 +1,5 @@
 # ruff: noqa
+import polars as pl
 from typing import Any
 from datetime import datetime
 import narwhals as nw
@@ -10,7 +11,7 @@
 
 def q1(df_raw: Any) -> Any:
     var_1 = datetime(1998, 9, 2)
-    df = nw.LazyFrame(df_raw)
+    df = nw.from_native(df_raw)
     result = (
         df.filter(nw.col("l_shipdate") <= var_1)
         .with_columns(
@@ -36,10 +37,14 @@ def q1(df_raw: Any) -> Any:
         )
         .sort(["l_returnflag", "l_linestatus"])
     )
-    return nw.to_native(result.collect())
+    return nw.to_native(result)
 
 
 df = pd.read_parquet(
     "../tpch-data/s1/lineitem.parquet", dtype_backend="pyarrow", engine="pyarrow"
 )
 print(q1(df))
+df = pl.read_parquet("../tpch-data/s1/lineitem.parquet")
+print(q1(df))
+df = pl.scan_parquet("../tpch-data/s1/lineitem.parquet")
+print(q1(df).collect())