feat: add from_arrow (which uses the PyCapsule Interface)

docs/api-reference/narwhals.md

@@ -14,6 +14,7 @@ Here are the top-level functions available in Narwhals.
         - concat_str
         - from_dict
         - from_native
+        - from_pycapsule
         - get_level
         - get_native_namespace
         - is_ordered_categorical

narwhals/__init__.py

@@ -45,6 +45,7 @@
 from narwhals.expr import when
 from narwhals.functions import concat
 from narwhals.functions import from_dict
+from narwhals.functions import from_pycapsule
 from narwhals.functions import get_level
 from narwhals.functions import new_series
 from narwhals.functions import show_versions
@@ -68,6 +69,7 @@
     "selectors",
     "concat",
     "from_dict",
+    "from_pycapsule",
     "get_level",
     "new_series",
     "to_native",

narwhals/functions.py

@@ -6,6 +6,7 @@
 from typing import Any
 from typing import Iterable
 from typing import Literal
+from typing import Protocol
 from typing import TypeVar
 from typing import Union
 
@@ -21,6 +22,7 @@
 # The rest of the annotations seem to work fine with this anyway
 FrameT = TypeVar("FrameT", bound=Union[DataFrame, LazyFrame])  # type: ignore[type-arg]
 
+
 if TYPE_CHECKING:
     from types import ModuleType
 
@@ -29,6 +31,9 @@
     from narwhals.series import Series
     from narwhals.typing import DTypes
 
+    class SupportsPyCapsule(Protocol):
+        def __arrow_c_stream__(self) -> Any: ...
+
 
 def concat(
     items: Iterable[FrameT],
@@ -406,6 +411,100 @@ def _from_dict_impl(
     return from_native(native_frame, eager_only=True)
 
 
+def from_pycapsule(
+    native_frame: SupportsPyCapsule, *, native_namespace: ModuleType
+) -> DataFrame[Any]:
+    """
+    Construct a DataFrame from an object which supports the PyCapsule Interface.
+
+    Arguments:
+        native_frame: Object which implements `__arrow_c_stream__`.
+        native_namespace: The native library to use for DataFrame creation.
+
+    Examples:
+        >>> import pandas as pd
+        >>> import polars as pl
+        >>> import pyarrow as pa
+        >>> import narwhals as nw
+        >>> data = {"a": [1, 2, 3], "b": [4, 5, 6]}
+
+        Let's define a dataframe-agnostic function which creates a PyArrow
+        Table.
+
+        >>> @nw.narwhalify
+        ... def func(df):
+        ...     return nw.from_pycapsule(df, native_namespace=pa)
+
+        Let's see what happens when passing pandas / Polars input:
+
+        >>> func(pd.DataFrame(data))  # doctest: +SKIP
+        pyarrow.Table
+        a: int64
+        b: int64
+        ----
+        a: [[1,2,3]]
+        b: [[4,5,6]]
+        >>> func(pl.DataFrame(data))  # doctest: +SKIP
+        pyarrow.Table
+        a: int64
+        b: int64
+        ----
+        a: [[1,2,3]]
+        b: [[4,5,6]]
+    """
+    if not hasattr(native_frame, "__arrow_c_stream__"):
+        msg = f"Given object of type {type(native_frame)} does not support PyCapsule interface"
+        raise TypeError(msg)
+    implementation = Implementation.from_native_namespace(native_namespace)
+
+    if implementation is Implementation.POLARS and parse_version(
+        native_namespace.__version__
+    ) >= (1, 3):
+        native_frame = native_namespace.DataFrame(native_frame)
+    elif implementation in {
+        Implementation.PANDAS,
+        Implementation.MODIN,
+        Implementation.CUDF,
+        Implementation.POLARS,
+    }:
+        # These don't (yet?) support the PyCapsule Interface for import
+        # so we go via PyArrow
+        try:
+            import pyarrow as pa  # ignore-banned-import
+        except ModuleNotFoundError as exc:  # pragma: no cover
+            msg = f"PyArrow>=14.0.0 is required for `from_pycapsule` for object of type {native_namespace}"
+            raise ModuleNotFoundError(msg) from exc
+        if parse_version(pa.__version__) < (14, 0):  # pragma: no cover
+            msg = f"PyArrow>=14.0.0 is required for `from_pycapsule` for object of type {native_namespace}"
+            raise ModuleNotFoundError(msg) from None
+
+        tbl = pa.table(native_frame)
+        if implementation is Implementation.PANDAS:
+            native_frame = tbl.to_pandas()
+        elif implementation is Implementation.MODIN:  # pragma: no cover
+            from modin.pandas.utils import from_arrow
+
+            native_frame = from_arrow(tbl)
+        elif implementation is Implementation.CUDF:  # pragma: no cover
+            native_frame = native_namespace.DataFrame.from_arrow(tbl)
+        elif implementation is Implementation.POLARS:  # pragma: no cover
+            native_frame = native_namespace.from_arrow(tbl)
+        else:  # pragma: no cover
+            msg = "congratulations, you entered unrecheable code - please report a bug"
+            raise AssertionError(msg)
+    elif implementation is Implementation.PYARROW:
+        native_frame = native_namespace.table(native_frame)
+    else:  # pragma: no cover
+        try:
+            # implementation is UNKNOWN, Narwhals extension using this feature should
+            # implement PyCapsule support
+            native_frame = native_namespace.DataFrame(native_frame)
+        except AttributeError as e:
+            msg = "Unknown namespace is expected to implement `DataFrame` class which accepts object which supports PyCapsule Interface."
+            raise AttributeError(msg) from e
+    return from_native(native_frame, eager_only=True)
+
+
 def _get_sys_info() -> dict[str, str]:
     """System information
 

narwhals/stable/v1/__init__.py

@@ -21,6 +21,7 @@
 from narwhals.expr import when as nw_when
 from narwhals.functions import _from_dict_impl
 from narwhals.functions import _new_series_impl
+from narwhals.functions import from_pycapsule as nw_from_pycapsule
 from narwhals.functions import show_versions
 from narwhals.schema import Schema as NwSchema
 from narwhals.series import Series as NwSeries
@@ -64,6 +65,7 @@
     from typing_extensions import Self
 
     from narwhals.dtypes import DType
+    from narwhals.functions import SupportsPyCapsule
     from narwhals.typing import IntoExpr
 
 T = TypeVar("T")
@@ -2181,6 +2183,52 @@ def new_series(
     )
 
 
+def from_pycapsule(
+    native_frame: SupportsPyCapsule, *, native_namespace: ModuleType
+) -> DataFrame[Any]:
+    """
+    Construct a DataFrame from an object which supports the PyCapsule Interface.
+
+    Arguments:
+        native_frame: Object which implements `__arrow_c_stream__`.
+        native_namespace: The native library to use for DataFrame creation.
+
+    Examples:
+        >>> import pandas as pd
+        >>> import polars as pl
+        >>> import pyarrow as pa
+        >>> import narwhals.stable.v1 as nw
+        >>> data = {"a": [1, 2, 3], "b": [4, 5, 6]}
+
+        Let's define a dataframe-agnostic function which creates a PyArrow
+        Table.
+
+        >>> @nw.narwhalify
+        ... def func(df):
+        ...     return nw.from_pycapsule(df, native_namespace=pa)
+
+        Let's see what happens when passing pandas / Polars input:
+
+        >>> func(pd.DataFrame(data))  # doctest: +SKIP
+        pyarrow.Table
+        a: int64
+        b: int64
+        ----
+        a: [[1,2,3]]
+        b: [[4,5,6]]
+        >>> func(pl.DataFrame(data))  # doctest: +SKIP
+        pyarrow.Table
+        a: int64
+        b: int64
+        ----
+        a: [[1,2,3]]
+        b: [[4,5,6]]
+    """
+    return _stableify(  # type: ignore[no-any-return]
+        nw_from_pycapsule(native_frame, native_namespace=native_namespace)
+    )
+
+
 def from_dict(
     data: dict[str, Any],
     schema: dict[str, DType] | Schema | None = None,
@@ -2304,5 +2352,6 @@ def from_dict(
     "show_versions",
     "Schema",
     "from_dict",
+    "from_pycapsule",
     "new_series",
 ]

tests/from_pycapsule_test.py

@@ -0,0 +1,45 @@
+import sys
+
+import pandas as pd
+import polars as pl
+import pyarrow as pa
+import pytest
+
+import narwhals.stable.v1 as nw
+from narwhals.utils import parse_version
+from tests.utils import compare_dicts
+
+
+@pytest.mark.xfail(parse_version(pa.__version__) < (14,), reason="too old")
+def test_from_pycapsule_to_arrow() -> None:
+    df = nw.from_native(pl.DataFrame({"ab": [1, 2, 3], "ba": [4, 5, 6]}), eager_only=True)
+    result = nw.from_pycapsule(df, native_namespace=pa)
+    assert isinstance(result.to_native(), pa.Table)
+    expected = {"ab": [1, 2, 3], "ba": [4, 5, 6]}
+    compare_dicts(result, expected)
+
+
+@pytest.mark.xfail(parse_version(pa.__version__) < (14,), reason="too old")
+def test_from_pycapsule_to_polars(monkeypatch: pytest.MonkeyPatch) -> None:
+    tbl = pa.table({"ab": [1, 2, 3], "ba": [4, 5, 6]})
+    monkeypatch.delitem(sys.modules, "pandas")
+    df = nw.from_native(tbl, eager_only=True)
+    result = nw.from_pycapsule(df, native_namespace=pl)
+    assert isinstance(result.to_native(), pl.DataFrame)
+    expected = {"ab": [1, 2, 3], "ba": [4, 5, 6]}
+    compare_dicts(result, expected)
+    assert "pandas" not in sys.modules
+
+
+@pytest.mark.xfail(parse_version(pa.__version__) < (14,), reason="too old")
+def test_from_pycapsule_to_pandas() -> None:
+    df = nw.from_native(pa.table({"ab": [1, 2, 3], "ba": [4, 5, 6]}), eager_only=True)
+    result = nw.from_pycapsule(df, native_namespace=pd)
+    assert isinstance(result.to_native(), pd.DataFrame)
+    expected = {"ab": [1, 2, 3], "ba": [4, 5, 6]}
+    compare_dicts(result, expected)
+
+
+def test_from_pycapsule_invalid() -> None:
+    with pytest.raises(TypeError, match="PyCapsule"):
+        nw.from_pycapsule({"a": [1]}, native_namespace=pa)  # type: ignore[arg-type]