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+"""[Protocols] defining conversion methods between representations.
+
+These come in 3 flavors and are [generic] to promote reuse.
+
+The following examples use the placeholder types `Narwhal` and `Other`:
+- `Narwhal`: some class written in `narwhals`.
+- `Other`: any other class, could be native, compliant, or a builtin.
+
+## `To<Other>`
+When we want to convert or unwrap a `Narwhal` into an `Other`,
+we provide an **instance** method:
+
+    ToOtherT_co = TypeVar("ToOtherT_co", covariant=True)
+
+    class ToOther(Protocol[ToOtherT_co]):
+        def to_other(self, *args: Any, **kwds: Any) -> ToOtherT_co: ...
+
+- `*args`, `**kwds` are defined to be *permissive* and allow a wider set of signatures when implementing.
+  - In most cases, they are unused.
+  - But come in handy when adapting an [upstream signature].
+- We use a  **covariant** `TypeVar`.
+
+## `From<Other>`
+But what if we have `Other` and want to do the reverse?
+
+Our `Narwhal` will need to provide a `@classmethod`:
+
+    FromOtherT_contra = TypeVar("FromOtherT_contra", contravariant=True)
+
+    class FromOther(Protocol[FromOtherT_contra]):
+        @classmethod
+        def from_other(cls, data: FromOtherT_contra, *args: Any, **kwds: Any) -> Self: ...
+
+- `*args`, `**kwds` serve a similar purpose as before, but are much more frequently used.
+- We've added a **required** [positional-only] parameter `data` which will always be passed `Other`.
+  - This removes the name from the contract of the protocol.
+  - Implementations are free to use something more descriptive for documentation purposes.
+- We use a  **contravariant** `TypeVar`.
+
+## `<Other>Convertible`
+Combining our `to_` and `from_` methods allows us to convert in both directions `Narwhal` <-> `Other`:
+
+    class OtherConvertible(
+        ToOther[ToOtherT_co],
+        FromOther[FromOtherT_contra],
+        Protocol[ToOtherT_co, FromOtherT_contra],
+    ): ...
+
+## See Also
+Variance of `TypeVar`(s) can be tricky to wrap your head around.
+
+To learn more see [moist], [dry], or [even drier] - depending on how deep you wanna go.
+
+[Protocols]: https://typing.python.org/en/latest/spec/protocol.html
+[generic]: https://typing.python.org/en/latest/spec/generics.html
+[upstream signature]: https://numpy.org/doc/stable/user/basics.interoperability.html#the-array-method
+[positional-only]: https://peps.python.org/pep-0570/
+[moist]: https://mypy.readthedocs.io/en/stable/generics.html#variance-of-generic-types
+[dry]: https://typing.python.org/en/latest/spec/generics.html#variance
+[even drier]: https://en.wikipedia.org/wiki/Covariance_and_contravariance_%28computer_science%29
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any, Iterable, Mapping, Protocol
+
+from narwhals._typing_compat import TypeVar
+
+if TYPE_CHECKING:
+    import pyarrow as pa
+    from typing_extensions import Self, TypeAlias, TypeIs
+
+
+class ArrowStreamExportable(Protocol):
+    def __arrow_c_stream__(self, requested_schema: object | None = None) -> object: ...
+
+
+ToNumpyT_co = TypeVar("ToNumpyT_co", covariant=True)
+FromNumpyDT_contra = TypeVar(
+    "FromNumpyDT_contra", contravariant=True, default=ToNumpyT_co
+)
+FromNumpyT_contra = TypeVar("FromNumpyT_contra", contravariant=True)
+
+
+class ToNumpy(Protocol[ToNumpyT_co]):
+    def to_numpy(self, *args: Any, **kwds: Any) -> ToNumpyT_co: ...
+
+
+class FromNumpy(Protocol[FromNumpyT_contra]):
+    @classmethod
+    def from_numpy(cls, data: FromNumpyT_contra, *args: Any, **kwds: Any) -> Self: ...
+
+
+class NumpyConvertible(
+    ToNumpy[ToNumpyT_co],
+    FromNumpy[FromNumpyDT_contra],
+    Protocol[ToNumpyT_co, FromNumpyDT_contra],
+):
+    def to_numpy(self, dtype: Any, *, copy: bool | None) -> ToNumpyT_co: ...
+
+
+FromIterableT_contra = TypeVar("FromIterableT_contra", contravariant=True, default=Any)
+
+
+class FromIterable(Protocol[FromIterableT_contra]):
+    @classmethod
+    def from_iterable(
+        cls, data: Iterable[FromIterableT_contra], *args: Any, **kwds: Any
+    ) -> Self: ...
+
+
+ToDictDT_co = TypeVar(
+    "ToDictDT_co", bound=Mapping[str, Any], covariant=True, default="dict[str, Any]"
+)
+FromDictDT_contra = TypeVar(
+    "FromDictDT_contra",
+    bound=Mapping[str, Any],
+    contravariant=True,
+    default=Mapping[str, Any],
+)
+
+
+class ToDict(Protocol[ToDictDT_co]):
+    def to_dict(self, *args: Any, **kwds: Any) -> ToDictDT_co: ...
+
+
+class FromDict(Protocol[FromDictDT_contra]):
+    @classmethod
+    def from_dict(cls, data: FromDictDT_contra, *args: Any, **kwds: Any) -> Self: ...
+
+
+class DictConvertible(
+    ToDict[ToDictDT_co],
+    FromDict[FromDictDT_contra],
+    Protocol[ToDictDT_co, FromDictDT_contra],
+): ...
+
+
+IntoArrowTable: TypeAlias = "ArrowStreamExportable | pa.Table"
+"""An object supporting the [Arrow PyCapsule Interface], or a native [`pyarrow.Table`].
+
+[Arrow PyCapsule Interface]: https://arrow.apache.org/docs/format/CDataInterface/PyCapsuleInterface.html#arrowstream-export
+[`pyarrow.Table`]: https://arrow.apache.org/docs/python/generated/pyarrow.Table.html
+"""
+ToArrowT_co = TypeVar("ToArrowT_co", covariant=True)
+FromArrowDT_contra = TypeVar(
+    "FromArrowDT_contra", contravariant=True, default=IntoArrowTable
+)
+
+
+class ToArrow(Protocol[ToArrowT_co]):
+    def to_arrow(self, *args: Any, **kwds: Any) -> ToArrowT_co: ...
+
+
+class FromArrow(Protocol[FromArrowDT_contra]):
+    @classmethod
+    def from_arrow(cls, data: FromArrowDT_contra, *args: Any, **kwds: Any) -> Self: ...
+
+
+class ArrowConvertible(
+    ToArrow[ToArrowT_co],
+    FromArrow[FromArrowDT_contra],
+    Protocol[ToArrowT_co, FromArrowDT_contra],
+): ...
+
+
+FromNativeT = TypeVar("FromNativeT")
+
+
+class FromNative(Protocol[FromNativeT]):
+    @classmethod
+    def from_native(cls, data: FromNativeT, *args: Any, **kwds: Any) -> Self: ...
+    @staticmethod
+    def _is_native(obj: FromNativeT | Any, /) -> TypeIs[FromNativeT]:
+        """Return `True` if `obj` can be passed to `from_native`."""
+        ...
+
+
+ToNarwhalsT_co = TypeVar("ToNarwhalsT_co", covariant=True)
+
+
+class ToNarwhals(Protocol[ToNarwhalsT_co]):
+    def to_narwhals(self) -> ToNarwhalsT_co:
+        """Convert into public representation."""
+        ...