Fix mypy errors

src/optimagic/optimization/history.py

@@ -316,8 +316,8 @@ def __getitem__(self, key: str) -> Any:
 
 
 def _get_flat_params(params: list[PyTree]) -> list[list[float]]:
-    if len(params) > 0 and _is_1d_array(params[0]):
-        # fast path
+    fast_path = len(params) > 0 and _is_1d_array(params[0])
+    if fast_path:
         flatten = lambda x: x.tolist()
     else:
         registry = get_registry(extended=True)
@@ -327,9 +327,11 @@ def _get_flat_params(params: list[PyTree]) -> list[list[float]]:
 
 
 def _get_flat_param_names(param: PyTree) -> list[str]:
-    if _is_1d_array(param):
-        # fast path
-        return np.arange(param.size).astype(str).tolist()
+    fast_path = _is_1d_array(param)
+    if fast_path:
+        # Mypy raises an error here because .tolist() returns a str for zero-dimensional
+        # arrays, but the fast path is only taken for 1d arrays, so it can be ignored.
+        return np.arange(param.size).astype(str).tolist()  # type: ignore[return-value]
 
     registry = get_registry(extended=True)
     return leaf_names(param, registry=registry)
@@ -409,7 +411,7 @@ def _apply_to_batch(
     batch_starts = _get_batch_start(batch_ids)
     batch_stops = [*batch_starts[1:], len(data)]
 
-    batch_results = []
+    batch_results: list[float] = []
     for start, stop in zip(batch_starts, batch_stops, strict=True):
         batch_data = data[start:stop]
         batch_id = batch_ids[start]
@@ -432,7 +434,7 @@ def _apply_to_batch(
             )
             raise ValueError(msg)
 
-        batch_results.append(reduced)
+        batch_results.append(reduced)  # type: ignore[arg-type]
 
     out = np.zeros_like(data)
     out[batch_starts] = batch_results
@@ -447,4 +449,5 @@ def _get_batch_start(batch_ids: list[int]) -> list[int]:
     """
     ids_arr = np.array(batch_ids, dtype=np.int64)
     indices = np.where(ids_arr[:-1] != ids_arr[1:])[0] + 1
-    return np.insert(indices, 0, 0).tolist()
+    list_indices: list[int] = indices.tolist()  # type: ignore[assignment]
+    return [0, *list_indices]