diff --git a/test_ncattrs.py b/test_ncattrs.py
index 434ad284c5..6bcf105e17 100644
--- a/test_ncattrs.py
+++ b/test_ncattrs.py
@@ -93,8 +93,10 @@ def local_attr(request):
 def test_support():
     raise ValueError("iris.FUTURE = " + str(iris.FUTURE))
 
+
 _SKIP_WARNCHECK = "_no_warnings_check"
 
+
 def check_captured_warnings(
     expected_keys: List[str], captured_warnings: List[warnings.WarningMessage]
 ):
@@ -483,9 +485,14 @@ def fetch_results(
 _MATRIX_ATTRNAMES = _LOCAL_TEST_ATTRS + list(_GLOBAL_TEST_ATTRS) + ["user"]
 # remove special-cases, for now
 _SPECIAL_ATTRS = [
-    'Conventions', "ukmo__process_flags", "missing_value", "standard_error_multiplier"
+    "Conventions",
+    "ukmo__process_flags",
+    "missing_value",
+    "standard_error_multiplier",
+]
+_MATRIX_ATTRNAMES = [
+    attr for attr in _MATRIX_ATTRNAMES if not attr in _SPECIAL_ATTRS
 ]
-_MATRIX_ATTRNAMES = [attr for attr in _MATRIX_ATTRNAMES if not attr in _SPECIAL_ATTRS]
 
 
 #
@@ -493,6 +500,8 @@ def fetch_results(
 # Possible ones are "globalstyle", "localstyle", "userstyle".
 #
 _ATTR_STYLES = ["localstyle", "globalstyle", "userstyle"]
+
+
 def deduce_attr_style(attrname: str) -> str:
     # Extract the attribute "style type" from an attr_param name
     if attrname in _LOCAL_TEST_ATTRS:
@@ -504,31 +513,30 @@ def deduce_attr_style(attrname: str) -> str:
         style = "userstyle"
     return style
 
+
 #
 # Decode a matrix "input spec" to codes for global + local values.
 #
 
+
 def decode_matrix_input(input_spec):
     def decode_specstring(spec: str) -> List[Union[str, None]]:
         # Decode an input spec-string to input/output attribute values
         assert spec[0] == "G" and spec[2] == "L"
         global_val = spec[1]
         allvals = spec[1] + spec[3:]
-        result = [
-            None if valchar == '-' else valchar
-            for valchar  in allvals
-        ]
+        result = [None if valchar == "-" else valchar for valchar in allvals]
         return result
 
     if isinstance(input_spec, str):
         # Single-source spec (one cube or one file)
         gA, vA = decode_specstring(input_spec)
-        result = [[gA, vA ]]
+        result = [[gA, vA]]
     else:
         # Dual-source spec (two files, or sets of cubes with common global)
         gA, vA = decode_specstring(input_spec[0])
         gB, vB = decode_specstring(input_spec[1])
-        result = [[gA, vA ], [gB, vB]]
+        result = [[gA, vA], [gB, vB]]
 
     return result
 
@@ -539,17 +547,14 @@ def encode_matrix_result(results: List[List[str]]):
     if isinstance(results[0], str):
         results = [results]
     assert all(
-        all(
-            val is None or len(val) == 1
-            for val in vals
-        )
-        for vals in results
+        all(val is None or len(val) == 1 for val in vals) for vals in results
     )
+
     def valrep(val):
         return "-" if val is None else val
+
     return list(
-        ''.join(['G', valrep(vals[0]), 'L']
-                + list(map(valrep, vals[1:])))
+        "".join(["G", valrep(vals[0]), "L"] + list(map(valrep, vals[1:])))
         for vals in results
     )
 
@@ -559,6 +564,7 @@ def valrep(val):
 # We have the technology to save the found results also.
 #
 
+
 @pytest.fixture(autouse=True, scope="session")
 def matrix_results():
     matrix_filepath = Path(__file__).parent / "_testattrs_matrix_results.json"
@@ -570,11 +576,11 @@ def matrix_results():
     else:
         # Initialise empty matrix results content
         matrix_results = {}
-        for testtype in ('load', 'save', 'roundtrip'):
+        for testtype in ("load", "save", "roundtrip"):
             test_specs = matrix_results.setdefault(testtype, {})
             for testcase in _MATRIX_TESTCASES:
                 test_case_spec = test_specs.setdefault(testcase, {})
-                test_case_spec['input'] = _MATRIX_TESTCASE_INPUTS[testcase]
+                test_case_spec["input"] = _MATRIX_TESTCASE_INPUTS[testcase]
                 for attrstyle in _ATTR_STYLES:
                     test_case_spec[attrstyle] = None  # empty
 
@@ -585,7 +591,6 @@ def matrix_results():
         json.dump(matrix_results, matrix_filepath)
 
 
-
 class TestRoundtrip(MixinAttrsTesting):
     """
     Test handling of attributes in roundtrip netcdf-iris-netcdf.
@@ -963,8 +968,8 @@ def test_16_localstyle(self, local_attr, origin_style, do_split):
     @pytest.mark.parametrize("attrname", _MATRIX_ATTRNAMES)
     def test_matrix(self, testcase, attrname, matrix_results):
         do_saves, matrix_results = matrix_results
-        test_spec = matrix_results['roundtrip'][testcase]
-        input_spec = test_spec['input']
+        test_spec = matrix_results["roundtrip"][testcase]
+        input_spec = test_spec["input"]
         values = decode_matrix_input(input_spec)
 
         self.run_roundtrip_testcase(attrname, values)