diff --git a/.claude/sweep-style-state.csv b/.claude/sweep-style-state.csv
index 598c4de92..0c9b0d070 100644
--- a/.claude/sweep-style-state.csv
+++ b/.claude/sweep-style-state.csv
@@ -4,6 +4,7 @@ classify,2026-06-20,3402,HIGH,3;4,"F401 unused not_implemented_func import; isor
 contour,2026-05-29,2698,HIGH,3,"F821 line 557: contours() return annotation ""gpd.GeoDataFrame"" referenced gpd not bound at module scope (only imported inside _to_geopandas). Fixed via TYPE_CHECKING-guarded import geopandas as gpd, matching polygonize.py. No runtime change; geopandas stays optional. isort clean. Cat 1/2/4/5 clean. 24 contour tests pass. PR open."
 cost_distance,2026-06-15,3339,HIGH,1;3;4;5,"Cat 3 F401: removed unused 'from functools import partial' (L32, refactor leftover, not re-exported). Cat 5 mutable default target_values: list = [] was found here too, but the parallel api-consistency PR #3348 fixed it first (same None-sentinel normalization, merged to main); on reconciliation this PR's duplicate source change and its two target_values tests were dropped, so PR #3350 is now lint-only. Cat 1: E302 L63 (1->2 blank lines before @ngjit _heap_push; placed blanks between comment divider and decorator to satisfy both E302 and isort) + E127 x5 (_cost_distance_dask_cupy L461-462, _cost_distance_dask_iterative L1005-1007 signature continuation over-indent). Cat 4 isort: dataset_support before utils, utils from-import reflowed to 100 cols. Cat 2 grep clean. flake8+isort clean on module after fix; cost_distance tests pass (CUDA available). Pre-existing test-file lint (F401 L314/L504, E201 L449, isort drift) left untouched - out of module scope. PR open."
 dasymetric,2026-06-20,3411,HIGH,1;3;4,"F401 is_dask_cupy unused (HIGH); E128 line 323 continuation indent (MED); isort utils import block (MED). Fixed + flake8/isort clean, 61 tests pass. PR #3411. Issue-create was denied by auto-mode classifier (judged LOW); PR went through."
+diffusion,2026-06-20,3421,HIGH,3;4,F401 unused not_implemented_func (HIGH); isort import-ordering drift (MEDIUM); no Cat1/2/5. Fixed in PR off issue 3421.
 fire,2026-06-19,3395,HIGH,3;4,Cat3 F401 math.log unused + F841 S_T unused local; Cat4 isort utils import reorder. No Cat1/2/5. Fixed in #3395 rockout.
 focal,2026-05-29,2731,HIGH,3;4;5,"F401 not_implemented_func (import line 36, unused, not re-exported). isort: stdlib reorder (import math before from-imports), dropped stray blank lines in import groups, alphabetised+rewrapped convolution/utils from-imports, moved dataset_support import into order. Cat 5: mutable default excludes=[np.nan] in mean() (line 238) -> None sentinel, resolved to [np.nan] in body; never mutated so behaviour preserved; regression test test_mean_default_excludes_does_not_leak added. Cat 1/2 clean. 115 focal tests pass. PR pending."
 geotiff,2026-06-14,3329,MEDIUM,4,"Re-sweep after #3259: flake8 baseline 0 across all 33 production files (flake8 7.3.0). Cat 4 only: isort (line_length=100) drift in _writers/eager.py (the from .._validation import block wrapped at ~80 cols, not 100); rewrapped, no behavioural change. Cat 1/2/3/5 clean: no E/W/F codes; grep hits for == False (comment), ': dict'/': list' annotations, and type=getaddrinfo kwarg are all false positives, not shadowed builtins or comparisons. isort+flake8 clean after fix; import smoke test ok. PR via #3329."
diff --git a/xrspatial/diffusion.py b/xrspatial/diffusion.py
index 2179153ac..e1e99eb67 100644
--- a/xrspatial/diffusion.py
+++ b/xrspatial/diffusion.py
@@ -15,9 +15,8 @@
 
 import numpy as np
 import xarray as xr
-from xarray import DataArray
-
 from numba import cuda, prange
+from xarray import DataArray
 
 try:
     import cupy
@@ -30,21 +29,9 @@ class cupy:
 except ImportError:
     da = None
 
-from xrspatial.utils import (
-    ArrayTypeFunctionMapping,
-    _boundary_to_dask,
-    _dask_task_name_kwargs,
-    _pad_array,
-    _validate_boundary,
-    _validate_raster,
-    _validate_scalar,
-    calc_cuda_dims,
-    has_cuda_and_cupy,
-    is_cupy_array,
-    ngjit,
-    not_implemented_func,
-)
-
+from xrspatial.utils import (ArrayTypeFunctionMapping, _boundary_to_dask, _dask_task_name_kwargs,
+                             _pad_array, _validate_boundary, _validate_raster, _validate_scalar,
+                             calc_cuda_dims, has_cuda_and_cupy, is_cupy_array, ngjit)
 
 # ---------------------------------------------------------------------------
 # Memory guard