diff --git a/lib/iris/_concatenate.py b/lib/iris/_concatenate.py
index 7330928fe3..0053a555f7 100644
--- a/lib/iris/_concatenate.py
+++ b/lib/iris/_concatenate.py
@@ -5,7 +5,6 @@
 """Automatic concatenation of multiple cubes over one or more existing dimensions."""
 
 from collections import defaultdict, namedtuple
-from iris.exceptions import warn_once_at_level
 
 import dask.array as da
 import numpy as np
@@ -13,6 +12,7 @@
 import iris.coords
 import iris.cube
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 from iris.util import array_equal, guess_coord_axis
 
 #
diff --git a/lib/iris/analysis/_regrid.py b/lib/iris/analysis/_regrid.py
index 637e380500..982a2de0ff 100644
--- a/lib/iris/analysis/_regrid.py
+++ b/lib/iris/analysis/_regrid.py
@@ -5,7 +5,6 @@
 
 import copy
 import functools
-from iris.exceptions import warn_once_at_level
 
 import numpy as np
 import numpy.ma as ma
@@ -19,7 +18,7 @@
     snapshot_grid,
 )
 from iris.analysis._scipy_interpolate import _RegularGridInterpolator
-from iris.exceptions import IrisImpossibleUpdateWarning
+from iris.exceptions import IrisImpossibleUpdateWarning, warn_once_at_level
 from iris.util import _meshgrid, guess_coord_axis
 
 
diff --git a/lib/iris/analysis/calculus.py b/lib/iris/analysis/calculus.py
index 4e04737c3a..992b776d56 100644
--- a/lib/iris/analysis/calculus.py
+++ b/lib/iris/analysis/calculus.py
@@ -9,7 +9,6 @@
 """
 
 import re
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import numpy as np
@@ -22,7 +21,7 @@
 import iris.analysis.maths
 import iris.coord_systems
 import iris.coords
-from iris.exceptions import IrisUserWarning
+from iris.exceptions import IrisUserWarning, warn_once_at_level
 from iris.util import delta
 
 __all__ = ["DIRECTIONAL_NAMES", "cube_delta", "curl", "differentiate"]
diff --git a/lib/iris/analysis/cartography.py b/lib/iris/analysis/cartography.py
index 3f20c0b891..b69d1e57d1 100644
--- a/lib/iris/analysis/cartography.py
+++ b/lib/iris/analysis/cartography.py
@@ -6,7 +6,6 @@
 
 from collections import namedtuple
 import copy
-from iris.exceptions import warn_once_at_level
 
 import cartopy.crs as ccrs
 import cartopy.img_transform
@@ -18,6 +17,7 @@
 import iris.coord_systems
 import iris.coords
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 from iris.util import _meshgrid
 
 from ._grid_angles import gridcell_angles, rotate_grid_vectors
diff --git a/lib/iris/analysis/geometry.py b/lib/iris/analysis/geometry.py
index be446c3b3d..2be351ea50 100644
--- a/lib/iris/analysis/geometry.py
+++ b/lib/iris/analysis/geometry.py
@@ -9,12 +9,11 @@
 
 """
 
-from iris.exceptions import warn_once_at_level
-
 import numpy as np
 from shapely.geometry import Polygon
 
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 
 
 def _extract_relevant_cube_slice(cube, geometry):
diff --git a/lib/iris/analysis/maths.py b/lib/iris/analysis/maths.py
index a6999c5c3d..2cb6ac8980 100644
--- a/lib/iris/analysis/maths.py
+++ b/lib/iris/analysis/maths.py
@@ -8,7 +8,6 @@
 import inspect
 import math
 import operator
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import dask.array as da
@@ -22,6 +21,7 @@
 from iris.config import get_logger
 import iris.coords
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.util
 
 # Configure the logger.
diff --git a/lib/iris/aux_factory.py b/lib/iris/aux_factory.py
index f81fe9a52d..e0a5e51a1d 100644
--- a/lib/iris/aux_factory.py
+++ b/lib/iris/aux_factory.py
@@ -5,7 +5,6 @@
 """Definitions of derived coordinates."""
 
 from abc import ABCMeta, abstractmethod
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import dask.array as da
@@ -13,7 +12,7 @@
 
 from iris.common import CFVariableMixin, CoordMetadata, metadata_manager_factory
 import iris.coords
-from iris.exceptions import IrisIgnoringBoundsWarning
+from iris.exceptions import IrisIgnoringBoundsWarning, warn_once_at_level
 
 
 class AuxCoordFactory(CFVariableMixin, metaclass=ABCMeta):
@@ -429,7 +428,9 @@ def _check_dependencies(pressure_at_top, sigma, surface_air_pressure):
                     f"Coordinate '{coord.name()}' has bounds. These will "
                     "be disregarded"
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         # Check units
         if sigma.units.is_unknown():
@@ -704,7 +705,9 @@ def update(self, old_coord, new_coord=None):
                     "Orography coordinate {!r} has bounds."
                     " These will be disregarded.".format(new_coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
             self.orography = new_coord
 
 
@@ -968,7 +971,9 @@ def _check_dependencies(sigma, eta, depth, depth_c, nsigma, zlev):
                     "The {} coordinate {!r} has bounds. "
                     "These are being disregarded.".format(term, coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         for coord, term in ((depth_c, "depth_c"), (nsigma, "nsigma")):
             if coord is not None and coord.shape != (1,):
@@ -1125,7 +1130,9 @@ def make_coord(self, coord_dims_func):
                         "The {} coordinate {!r} has bounds. "
                         "These are being disregarded.".format(key, name)
                     )
-                    warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                    warn_once_at_level(
+                        msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                    )
                     # Swap bounds with points.
                     bds_shape = list(nd_points_by_key[key].shape) + [1]
                     bounds = nd_points_by_key[key].reshape(bds_shape)
@@ -1205,7 +1212,9 @@ def _check_dependencies(sigma, eta, depth):
                     "The {} coordinate {!r} has bounds. "
                     "These are being disregarded.".format(term, coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         # Check units.
         if sigma is not None and sigma.units.is_unknown():
@@ -1281,7 +1290,9 @@ def make_coord(self, coord_dims_func):
                         "The {} coordinate {!r} has bounds. "
                         "These are being disregarded.".format(key, name)
                     )
-                    warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                    warn_once_at_level(
+                        msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                    )
                     # Swap bounds with points.
                     bds_shape = list(nd_points_by_key[key].shape) + [1]
                     bounds = nd_points_by_key[key].reshape(bds_shape)
@@ -1372,7 +1383,9 @@ def _check_dependencies(s, c, eta, depth, depth_c):
                     "The {} coordinate {!r} has bounds. "
                     "These are being disregarded.".format(term, coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         if depth_c is not None and depth_c.shape != (1,):
             msg = "Expected scalar {} coordinate {!r}: got shape {!r}.".format(
@@ -1466,7 +1479,9 @@ def make_coord(self, coord_dims_func):
                         "The {} coordinate {!r} has bounds. "
                         "These are being disregarded.".format(key, name)
                     )
-                    warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                    warn_once_at_level(
+                        msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                    )
                     # Swap bounds with points.
                     bds_shape = list(nd_points_by_key[key].shape) + [1]
                     bounds = nd_points_by_key[key].reshape(bds_shape)
@@ -1561,7 +1576,9 @@ def _check_dependencies(s, eta, depth, a, b, depth_c):
                     "The {} coordinate {!r} has bounds. "
                     "These are being disregarded.".format(term, coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         coords = ((a, "a"), (b, "b"), (depth_c, "depth_c"))
         for coord, term in coords:
@@ -1658,7 +1675,9 @@ def make_coord(self, coord_dims_func):
                         "The {} coordinate {!r} has bounds. "
                         "These are being disregarded.".format(key, name)
                     )
-                    warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                    warn_once_at_level(
+                        msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                    )
                     # Swap bounds with points.
                     bds_shape = list(nd_points_by_key[key].shape) + [1]
                     bounds = nd_points_by_key[key].reshape(bds_shape)
@@ -1753,7 +1772,9 @@ def _check_dependencies(s, c, eta, depth, depth_c):
                     "The {} coordinate {!r} has bounds. "
                     "These are being disregarded.".format(term, coord.name())
                 )
-                warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                warn_once_at_level(
+                    msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                )
 
         if depth_c is not None and depth_c.shape != (1,):
             msg = "Expected scalar depth_c coordinate {!r}: got shape {!r}.".format(
@@ -1847,7 +1868,9 @@ def make_coord(self, coord_dims_func):
                         "The {} coordinate {!r} has bounds. "
                         "These are being disregarded.".format(key, name)
                     )
-                    warn_once_at_level(msg, category=IrisIgnoringBoundsWarning, stacklevel=2)
+                    warn_once_at_level(
+                        msg, category=IrisIgnoringBoundsWarning, stacklevel=2
+                    )
                     # Swap bounds with points.
                     bds_shape = list(nd_points_by_key[key].shape) + [1]
                     bounds = nd_points_by_key[key].reshape(bds_shape)
diff --git a/lib/iris/config.py b/lib/iris/config.py
index e5822f3002..fd85d70ebb 100644
--- a/lib/iris/config.py
+++ b/lib/iris/config.py
@@ -29,9 +29,9 @@
 import contextlib
 import logging
 import os.path
-from iris.exceptions import warn_once_at_level
 
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 
 
 def get_logger(name, datefmt=None, fmt=None, level=None, propagate=None, handler=True):
diff --git a/lib/iris/coord_systems.py b/lib/iris/coord_systems.py
index 91610d3377..6014e12b53 100644
--- a/lib/iris/coord_systems.py
+++ b/lib/iris/coord_systems.py
@@ -7,13 +7,13 @@
 from abc import ABCMeta, abstractmethod
 from functools import cached_property
 import re
-from iris.exceptions import warn_once_at_level
 
 import cartopy.crs as ccrs
 import numpy as np
 
 from iris._deprecation import warn_deprecated
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 
 
 def _arg_default(value, default, cast_as=float):
diff --git a/lib/iris/coords.py b/lib/iris/coords.py
index 3f61e67e93..fe426932e2 100644
--- a/lib/iris/coords.py
+++ b/lib/iris/coords.py
@@ -11,7 +11,6 @@
 from functools import lru_cache
 from itertools import zip_longest
 import operator
-from iris.exceptions import warn_once_at_level
 import zlib
 
 import dask.array as da
@@ -30,6 +29,7 @@
     metadata_manager_factory,
 )
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.time
 import iris.util
 
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 8ac0bff2f0..04173b8375 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -20,7 +20,6 @@
     Optional,
     Union,
 )
-from iris.exceptions import warn_once_at_level
 from xml.dom.minidom import Document
 import zlib
 
@@ -44,6 +43,7 @@
 import iris.coord_systems
 import iris.coords
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.util
 
 __all__ = ["Cube", "CubeAttrsDict", "CubeList"]
diff --git a/lib/iris/exceptions.py b/lib/iris/exceptions.py
index ad7e6b06cc..4637627a4d 100644
--- a/lib/iris/exceptions.py
+++ b/lib/iris/exceptions.py
@@ -356,8 +356,7 @@ def warn_once(msg, type, stacklevel, frame, **kwargs):
 
 
 def warn_once_at_level(msg, type=None, stacklevel=0, **kwargs):
-    """
-    Raise a warning only if a similar one hasn't been raised from the same line
+    """Raise a warning only if a similar one hasn't been raised from the same line
     for a given stack level.
     """
     stacklevel += 1
diff --git a/lib/iris/experimental/regrid.py b/lib/iris/experimental/regrid.py
index 4f56a1b6e8..5a523f5349 100644
--- a/lib/iris/experimental/regrid.py
+++ b/lib/iris/experimental/regrid.py
@@ -18,7 +18,6 @@
 """
 import copy
 import functools
-from iris.exceptions import warn_once_at_level
 
 import cartopy.crs as ccrs
 import numpy as np
@@ -37,7 +36,7 @@
 import iris.analysis.cartography
 import iris.coord_systems
 import iris.cube
-from iris.exceptions import IrisImpossibleUpdateWarning
+from iris.exceptions import IrisImpossibleUpdateWarning, warn_once_at_level
 from iris.util import _meshgrid
 
 wmsg = (
diff --git a/lib/iris/experimental/ugrid/cf.py b/lib/iris/experimental/ugrid/cf.py
index b958b9afcd..688791abab 100644
--- a/lib/iris/experimental/ugrid/cf.py
+++ b/lib/iris/experimental/ugrid/cf.py
@@ -65,7 +65,9 @@ def identify(cls, variables, ignore=None, target=None, warn=True):
                                 f"{nc_var_name}"
                             )
                             if warn:
-                                warn_once_at_level(message, category=IrisCfMissingVarWarning)
+                                warn_once_at_level(
+                                    message, category=IrisCfMissingVarWarning
+                                )
                         else:
                             # Restrict to non-string type i.e. not a
                             # CFLabelVariable.
@@ -217,7 +219,9 @@ def identify(cls, variables, ignore=None, target=None, warn=True):
                             f"referenced by netCDF variable {nc_var_name}"
                         )
                         if warn:
-                            warn_once_at_level(message, category=IrisCfMissingVarWarning)
+                            warn_once_at_level(
+                                message, category=IrisCfMissingVarWarning
+                            )
                     else:
                         # Restrict to non-string type i.e. not a
                         # CFLabelVariable.
@@ -230,7 +234,9 @@ def identify(cls, variables, ignore=None, target=None, warn=True):
                                 f"variable."
                             )
                             if warn:
-                                warn_once_at_level(message, category=IrisCfLabelVarWarning)
+                                warn_once_at_level(
+                                    message, category=IrisCfLabelVarWarning
+                                )
 
         return result
 
diff --git a/lib/iris/experimental/ugrid/load.py b/lib/iris/experimental/ugrid/load.py
index 7df8018a67..acd4d24a58 100644
--- a/lib/iris/experimental/ugrid/load.py
+++ b/lib/iris/experimental/ugrid/load.py
@@ -15,6 +15,7 @@
 from itertools import groupby
 from pathlib import Path
 import threading
+
 from iris.exceptions import warn_once_at_level
 
 from ...config import get_logger
diff --git a/lib/iris/fileformats/_ff.py b/lib/iris/fileformats/_ff.py
index 4e9721a585..0e4c3e3d33 100644
--- a/lib/iris/fileformats/_ff.py
+++ b/lib/iris/fileformats/_ff.py
@@ -5,7 +5,6 @@
 """Provides UK Met Office Fields File (FF) format specific capabilities."""
 
 import os
-from iris.exceptions import warn_once_at_level
 
 import numpy as np
 
@@ -13,6 +12,7 @@
     IrisDefaultingWarning,
     IrisLoadWarning,
     NotYetImplementedError,
+    warn_once_at_level,
 )
 from iris.fileformats._ff_cross_references import STASH_TRANS
 
@@ -799,7 +799,9 @@ def _extract_field(self):
                         "Input field skipped as PPField creation failed :"
                         " error = {!r}"
                     )
-                    warn_once_at_level(msg.format(str(valerr)), category=IrisLoadWarning)
+                    warn_once_at_level(
+                        msg.format(str(valerr)), category=IrisLoadWarning
+                    )
 
     def __iter__(self):
         return pp._interpret_fields(self._extract_field())
diff --git a/lib/iris/fileformats/_nc_load_rules/actions.py b/lib/iris/fileformats/_nc_load_rules/actions.py
index a9b5cdd2f0..ccfa0c6668 100644
--- a/lib/iris/fileformats/_nc_load_rules/actions.py
+++ b/lib/iris/fileformats/_nc_load_rules/actions.py
@@ -40,10 +40,10 @@
 """
 
 from functools import wraps
-from iris.exceptions import warn_once_at_level
 
 from iris.config import get_logger
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.fileformats.cf
 import iris.fileformats.pp as pp
 
diff --git a/lib/iris/fileformats/_nc_load_rules/helpers.py b/lib/iris/fileformats/_nc_load_rules/helpers.py
index 01ee90593c..903d54623b 100644
--- a/lib/iris/fileformats/_nc_load_rules/helpers.py
+++ b/lib/iris/fileformats/_nc_load_rules/helpers.py
@@ -321,7 +321,9 @@ def _split_cell_methods(nc_cell_methods: str) -> List[re.Match]:
         nc_cell_method_match = _CM_PARSE.match(nc_cell_method_str.strip())
         if not nc_cell_method_match:
             msg = f"Failed to fully parse cell method string: {nc_cell_methods}"
-            warn_once_at_level(msg, category=iris.exceptions.IrisCfLoadWarning, stacklevel=2)
+            warn_once_at_level(
+                msg, category=iris.exceptions.IrisCfLoadWarning, stacklevel=2
+            )
             continue
         nc_cell_methods_matches.append(nc_cell_method_match)
 
@@ -521,7 +523,9 @@ def _get_ellipsoid(cf_grid_var):
             "applied. To apply the datum when loading, use the "
             "iris.FUTURE.datum_support flag."
         )
-        iris.exceptions.warn_once_at_level(wmsg, FutureWarning, category=FutureWarning, stacklevel=14)
+        iris.exceptions.warn_once_at_level(
+            wmsg, FutureWarning, category=FutureWarning, stacklevel=14
+        )
         datum = None
 
     if datum is not None:
diff --git a/lib/iris/fileformats/cf.py b/lib/iris/fileformats/cf.py
index 1c99181333..ceabac62f8 100644
--- a/lib/iris/fileformats/cf.py
+++ b/lib/iris/fileformats/cf.py
@@ -18,12 +18,12 @@
 from collections.abc import Iterable, MutableMapping
 import os
 import re
-from iris.exceptions import warn_once_at_level
 
 import numpy as np
 import numpy.ma as ma
 
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 from iris.fileformats.netcdf import _thread_safe_nc
 import iris.util
 
diff --git a/lib/iris/fileformats/name_loaders.py b/lib/iris/fileformats/name_loaders.py
index 2c11d01138..f6d1f414c3 100644
--- a/lib/iris/fileformats/name_loaders.py
+++ b/lib/iris/fileformats/name_loaders.py
@@ -8,7 +8,6 @@
 import datetime
 from operator import itemgetter
 import re
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import numpy as np
@@ -16,7 +15,7 @@
 import iris.coord_systems
 from iris.coords import AuxCoord, CellMethod, DimCoord
 import iris.cube
-from iris.exceptions import IrisLoadWarning, TranslationError
+from iris.exceptions import IrisLoadWarning, TranslationError, warn_once_at_level
 import iris.util
 
 EARTH_RADIUS = 6371229.0
@@ -265,7 +264,9 @@ def _parse_units(units):
     try:
         units = cf_units.Unit(units)
     except ValueError:
-        warn_once_at_level("Unknown units: {!r}".format(units), category=IrisLoadWarning)
+        warn_once_at_level(
+            "Unknown units: {!r}".format(units), category=IrisLoadWarning
+        )
         units = cf_units.Unit(None)
 
     return units
diff --git a/lib/iris/fileformats/netcdf/loader.py b/lib/iris/fileformats/netcdf/loader.py
index b9cc4870d1..24c240eaa0 100644
--- a/lib/iris/fileformats/netcdf/loader.py
+++ b/lib/iris/fileformats/netcdf/loader.py
@@ -16,7 +16,6 @@
 from enum import Enum, auto
 import threading
 from typing import Union
-from iris.exceptions import warn_once_at_level
 
 import numpy as np
 
@@ -35,6 +34,7 @@
 import iris.coord_systems
 import iris.coords
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.fileformats.cf
 from iris.fileformats.netcdf import _thread_safe_nc
 from iris.fileformats.netcdf.saver import _CF_ATTRS
diff --git a/lib/iris/fileformats/netcdf/saver.py b/lib/iris/fileformats/netcdf/saver.py
index c2f043ebac..77febe7440 100644
--- a/lib/iris/fileformats/netcdf/saver.py
+++ b/lib/iris/fileformats/netcdf/saver.py
@@ -20,7 +20,6 @@
 import re
 import string
 from typing import List
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import dask
@@ -45,6 +44,7 @@
 import iris.coords
 from iris.coords import AncillaryVariable, AuxCoord, CellMeasure, DimCoord
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.fileformats.cf
 from iris.fileformats.netcdf import _dask_locks, _thread_safe_nc
 import iris.io
@@ -2565,7 +2565,9 @@ def complete(self, issue_warnings=True) -> List[Warning]:
         if issue_warnings:
             # Issue any delayed warnings from the compute.
             for delayed_warning in result_warnings:
-                warn_once_at_level(delayed_warning, category=iris.exceptions.IrisSaveWarning)
+                warn_once_at_level(
+                    delayed_warning, category=iris.exceptions.IrisSaveWarning
+                )
 
         return result_warnings
 
diff --git a/lib/iris/fileformats/nimrod_load_rules.py b/lib/iris/fileformats/nimrod_load_rules.py
index 0df1404d8c..521dbf2400 100644
--- a/lib/iris/fileformats/nimrod_load_rules.py
+++ b/lib/iris/fileformats/nimrod_load_rules.py
@@ -6,7 +6,6 @@
 
 import re
 import string
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import cftime
@@ -19,6 +18,7 @@
     CoordinateNotFoundError,
     IrisNimrodTranslationWarning,
     TranslationError,
+    warn_once_at_level,
 )
 
 __all__ = ["run"]
diff --git a/lib/iris/fileformats/pp.py b/lib/iris/fileformats/pp.py
index 33140dc382..868ea09ef4 100644
--- a/lib/iris/fileformats/pp.py
+++ b/lib/iris/fileformats/pp.py
@@ -11,7 +11,6 @@
 import os
 import re
 import struct
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 import cftime
@@ -24,6 +23,7 @@
 import iris.config
 import iris.coord_systems
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 
 # NOTE: this is for backwards-compatitibility *ONLY*
 # We could simply remove it for v2.0 ?
diff --git a/lib/iris/fileformats/pp_save_rules.py b/lib/iris/fileformats/pp_save_rules.py
index 501be5e5b4..04aedd18bb 100644
--- a/lib/iris/fileformats/pp_save_rules.py
+++ b/lib/iris/fileformats/pp_save_rules.py
@@ -5,13 +5,11 @@
 
 """PP Save Rules."""
 
-from iris.exceptions import warn_once_at_level
-
 import cftime
 
 import iris
 from iris.aux_factory import HybridHeightFactory, HybridPressureFactory
-from iris.exceptions import IrisPpClimModifiedWarning
+from iris.exceptions import IrisPpClimModifiedWarning, warn_once_at_level
 from iris.fileformats._ff_cross_references import STASH_TRANS
 from iris.fileformats._pp_lbproc_pairs import LBPROC_MAP
 from iris.fileformats.rules import (
diff --git a/lib/iris/fileformats/rules.py b/lib/iris/fileformats/rules.py
index 6b835baaa0..d167fb2804 100644
--- a/lib/iris/fileformats/rules.py
+++ b/lib/iris/fileformats/rules.py
@@ -5,13 +5,13 @@
 """Generalised mechanisms for metadata translation and cube construction."""
 
 import collections
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 
 from iris.analysis import Linear
 import iris.cube
 import iris.exceptions
+from iris.exceptions import warn_once_at_level
 import iris.fileformats.um_cf_map
 
 Factory = collections.namedtuple("Factory", ["factory_class", "args"])
diff --git a/lib/iris/iterate.py b/lib/iris/iterate.py
index 8982abcac3..f726fac74a 100644
--- a/lib/iris/iterate.py
+++ b/lib/iris/iterate.py
@@ -6,11 +6,10 @@
 
 from collections.abc import Iterator
 import itertools
-from iris.exceptions import warn_once_at_level
 
 import numpy as np
 
-from iris.exceptions import IrisUserWarning
+from iris.exceptions import IrisUserWarning, warn_once_at_level
 
 __all__ = ["izip"]
 
diff --git a/lib/iris/pandas.py b/lib/iris/pandas.py
index 8f0d631b13..b6ad373aa1 100644
--- a/lib/iris/pandas.py
+++ b/lib/iris/pandas.py
@@ -9,7 +9,6 @@
 """
 import datetime
 from itertools import chain, combinations
-from iris.exceptions import warn_once_at_level
 
 import cf_units
 from cf_units import Unit
@@ -18,6 +17,8 @@
 import numpy.ma as ma
 import pandas as pd
 
+from iris.exceptions import warn_once_at_level
+
 try:
     from pandas.core.indexes.datetimes import DatetimeIndex  # pandas >=0.20
 except ImportError:
diff --git a/lib/iris/plot.py b/lib/iris/plot.py
index 6a6c1bd2dc..811bf5c099 100644
--- a/lib/iris/plot.py
+++ b/lib/iris/plot.py
@@ -10,7 +10,6 @@
 
 import collections
 import datetime
-from iris.exceptions import warn_once_at_level
 
 import cartopy.crs as ccrs
 from cartopy.geodesic import Geodesic
@@ -30,7 +29,11 @@
 import iris.coord_systems
 import iris.coords
 import iris.cube
-from iris.exceptions import IrisError, IrisUnsupportedPlottingWarning
+from iris.exceptions import (
+    IrisError,
+    IrisUnsupportedPlottingWarning,
+    warn_once_at_level,
+)
 
 # Importing iris.palette to register the brewer palettes.
 import iris.palette