diff --git a/health_check/management/commands/health_check.py b/health_check/management/commands/health_check.py
index eb36ac88..191d50d1 100644
--- a/health_check/management/commands/health_check.py
+++ b/health_check/management/commands/health_check.py
@@ -10,7 +10,7 @@ class Command(CheckMixin, BaseCommand):
     help = "Run health checks and exit 0 if everything went well."
 
     def add_arguments(self, parser):
-        parser.add_argument("-s", '--subset', type=str, nargs=1)
+        parser.add_argument("-s", "--subset", type=str, nargs=1)
 
     def handle(self, *args, **options):
         # perform all checks
diff --git a/health_check/mixins.py b/health_check/mixins.py
index e32be97e..dd04180c 100644
--- a/health_check/mixins.py
+++ b/health_check/mixins.py
@@ -44,7 +44,7 @@ def filter_plugins(self, subset=None):
         if subset is None:
             return self.plugins
 
-        health_check_subsets = HEALTH_CHECK['SUBSETS']
+        health_check_subsets = HEALTH_CHECK["SUBSETS"]
         if subset not in health_check_subsets or not self.plugins:
             raise Http404(f"Specify subset: '{subset}' does not exists.")
 
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 7c7aaf15..da2a7aec 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -38,30 +38,25 @@ def test_command(self):
         )
 
     def test_command_with_subset(self):
-        SUBSET_NAME_1 = 'subset-1'
-        SUBSET_NAME_2 = 'subset-2'
-        HEALTH_CHECK['SUBSETS'] = {
+        SUBSET_NAME_1 = "subset-1"
+        SUBSET_NAME_2 = "subset-2"
+        HEALTH_CHECK["SUBSETS"] = {
             SUBSET_NAME_1: ["OkPlugin"],
-            SUBSET_NAME_2: ["OkPlugin", "FailPlugin"]
+            SUBSET_NAME_2: ["OkPlugin", "FailPlugin"],
         }
 
         stdout = StringIO()
-        call_command(f"health_check", f"--subset={SUBSET_NAME_1}", stdout=stdout)
+        call_command("health_check", f"--subset={SUBSET_NAME_1}", stdout=stdout)
         stdout.seek(0)
-        assert stdout.read() == (
-            "OkPlugin                 ... working\n"
-        )
-
+        assert stdout.read() == ("OkPlugin                 ... working\n")
 
     def test_command_with_failed_check_subset(self):
-        SUBSET_NAME = 'subset-2'
-        HEALTH_CHECK['SUBSETS'] = {
-            SUBSET_NAME: ["OkPlugin", "FailPlugin"]
-        }
+        SUBSET_NAME = "subset-2"
+        HEALTH_CHECK["SUBSETS"] = {SUBSET_NAME: ["OkPlugin", "FailPlugin"]}
 
         stdout = StringIO()
         with pytest.raises(SystemExit):
-            call_command(f"health_check", f"--subset={SUBSET_NAME}", stdout=stdout)
+            call_command("health_check", f"--subset={SUBSET_NAME}", stdout=stdout)
         stdout.seek(0)
         assert stdout.read() == (
             "FailPlugin               ... unknown error: Oops\n"
@@ -69,15 +64,15 @@ def test_command_with_failed_check_subset(self):
         )
 
     def test_command_with_non_existence_subset(self):
-        SUBSET_NAME = 'subset-2'
+        SUBSET_NAME = "subset-2"
         NON_EXISTENCE_SUBSET_NAME = "abcdef12"
-        HEALTH_CHECK['SUBSETS'] = {
-            SUBSET_NAME: ["OkPlugin"]
-        }
+        HEALTH_CHECK["SUBSETS"] = {SUBSET_NAME: ["OkPlugin"]}
 
         stdout = StringIO()
         with pytest.raises(SystemExit):
-            call_command(f"health_check", f"--subset={NON_EXISTENCE_SUBSET_NAME}", stdout=stdout)
+            call_command(
+                "health_check", f"--subset={NON_EXISTENCE_SUBSET_NAME}", stdout=stdout
+            )
         stdout.seek(0)
         assert stdout.read() == (
             f"Specify subset: '{NON_EXISTENCE_SUBSET_NAME}' does not exists.\n"
diff --git a/tests/test_views.py b/tests/test_views.py
index 61047a29..d9161081 100644
--- a/tests/test_views.py
+++ b/tests/test_views.py
@@ -2,7 +2,6 @@
 
 import pytest
 
-from health_check import conf
 from health_check.backends import BaseHealthCheckBackend
 from health_check.conf import HEALTH_CHECK
 from health_check.exceptions import ServiceWarning
@@ -292,7 +291,7 @@ def run_check(self, subset=None):
         plugin_dir.register(SuccessOneBackend)
         plugin_dir.register(SuccessTwoBackend)
 
-        HEALTH_CHECK['SUBSETS'] = {
+        HEALTH_CHECK["SUBSETS"] = {
             "startup-probe": ["SuccessOneBackend", "SuccessTwoBackend"],
             "liveness-probe": ["SuccessTwoBackend"],
         }