test: fixup log for cuDF (#2565)

fixup log for cuDF

Author: MarcoGorelli

narwhals/_pandas_like/series.py

@@ -1023,9 +1023,12 @@ def log(self, base: float) -> Self:
         dtype_backend = get_dtype_backend(native.dtype, implementation=implementation)
 
         if implementation.is_cudf():
-            import cupy as cp
+            import cupy as cp  # ignore-banned-import  # cuDF dependency.
 
-            return self._with_native(cp.log(native) / cp.log(base))
+            native = self.native
+            log_arr = cp.log(native) / cp.log(base)
+            result_native = type(native)(log_arr, index=native.index, name=native.name)
+            return self._with_native(result_native)
 
         if dtype_backend == "pyarrow":
             import pyarrow.compute as pc

utils/import_check.py

@@ -5,6 +5,7 @@
 
 BANNED_IMPORTS = {
     "cudf",
+    "cupy",
     "dask",
     "dask.dataframe",
     "dask_expr",