Move 'import cupy' to an _import module.

src/ragged/common/__init__.py

@@ -14,6 +14,7 @@
     RegularArray,
 )
 
+from . import _import
 from ._typing import Device, Dtype, NestedSequence, Shape, SupportsDLPack
 
 
@@ -96,8 +97,7 @@ def __init__(
             if isinstance(self._impl, ak.Array) and device != ak.backend(self._impl):
                 self._impl = ak.to_backend(self._impl, device)
             elif isinstance(self._impl, np.ndarray) and device == "cuda":
-                import cupy as cp  # pylint: disable=C0415
-
+                cp = _import.cupy()
                 self._impl = cp.array(self._impl.item())
 
     def __str__(self) -> str:

src/ragged/common/_import.py

@@ -0,0 +1,22 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/ragged/blob/main/LICENSE
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def cupy() -> Any:
+    try:
+        import cupy as cp  # pylint: disable=C0415
+
+        return cp
+    except ModuleNotFoundError as err:
+        error_message = """to use the "cuda" backend, you must install cupy:
+
+    pip install cupy
+
+or
+
+    conda install -c conda-forge cupy
+"""
+        raise ModuleNotFoundError(error_message) from err