fix: better error message for mismatched views (#794)

* fix: better error message for mismatched views

Signed-off-by: Henry Schreiner <[email protected]>

* fix: slightly nicer still err output

Signed-off-by: Henry Schreiner <[email protected]>

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

src/boost_histogram/_internal/view.py

@@ -39,16 +39,22 @@ def __setitem__(self, ind: StrIndex, value: ArrayLike) -> None:
             super().__setitem__(ind, value)  # type: ignore[no-untyped-call]
             return
 
+        current_ndim = super().__getitem__(ind).ndim
+
         array: "np.typing.NDArray[Any]" = np.asarray(value)
-        if (
-            array.ndim == super().__getitem__(ind).ndim + 1
-            and len(self._FIELDS) == array.shape[-1]
-        ):
-            self.__setitem__(ind, self._PARENT._array(*np.moveaxis(array, -1, 0)))  # type: ignore[attr-defined]
-        elif self.dtype == array.dtype:
+        msg = "Needs matching ndarray or n+1 dim array"
+        if array.ndim == current_ndim + 1:
+            if len(self._FIELDS) == array.shape[-1]:
+                self.__setitem__(ind, self._PARENT._array(*np.moveaxis(array, -1, 0)))  # type: ignore[attr-defined]
+                return
+            msg += f", final dimension should be {len(self._FIELDS)} for this storage, got {array.shape[-1]} instead"
+            raise ValueError(msg)
+        if self.dtype == array.dtype:
             super().__setitem__(ind, array)  # type: ignore[no-untyped-call]
-        else:
-            raise ValueError("Needs matching ndarray or n+1 dim array")
+            return
+
+        msg += f", {current_ndim}D {self.dtype} or {current_ndim+1}D required, got {array.ndim}D {array.dtype}"
+        raise ValueError(msg)
 
 
 def make_getitem_property(name: str) -> property: