Update _util.py

Author: cthoyt

src/ptwt/_util.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import functools
+import itertools as itt
 import typing
 import warnings
 from collections.abc import Callable, Sequence
@@ -854,7 +855,8 @@ def _group_for_symmetric(padding: tuple[int, ...]) -> list[tuple[int, int]]:
 
 def _get_padding_n(data, wavelet, n: int) -> tuple[int, ...]:
     wavelet_length = _get_len(wavelet)
-    rv = []
-    for i in range(1, n + 1):
-        rv.extend(_get_pad(data.shape[-i], wavelet_length))
-    return tuple(rv)
+    return tuple(
+        itt.chain.from_iterable(
+            _get_pad(data.shape[-i], wavelet_length) for i in range(1, n + 1)
+        )
+    )