diff --git a/mqtt/fyrtur.py b/mqtt/fyrtur.py
index 8f81de8..604ad1a 100644
--- a/mqtt/fyrtur.py
+++ b/mqtt/fyrtur.py
@@ -16,7 +16,6 @@ class Position(Enum):
 
 
 class Fyrtur:
-
     def __init__(self, topic, top, bottom):
         self.topic = topic
         self.top = top
@@ -55,7 +54,9 @@ class FyrturWatcher:
         elif position == Position.DOWN:
             fyrtur.wanted_position = fyrtur.bottom
         elif position == Position.HALF:
-            fyrtur.wanted_position = round((fyrtur.top - fyrtur.bottom) / 2 + fyrtur.bottom)
+            fyrtur.wanted_position = round(
+                (fyrtur.top - fyrtur.bottom) / 2 + fyrtur.bottom
+            )
 
     def publish(self, client):
         for fyrtur in self.fyrturs.values():
@@ -65,11 +66,13 @@ class FyrturWatcher:
                 time.sleep(2)
 
 
-watcher = FyrturWatcher({
-    "office1": Fyrtur(topic="zigbee2mqtt/office_fyrtur_1", top=100, bottom=16),
-    "office2": Fyrtur(topic="zigbee2mqtt/office_fyrtur_2", top=100, bottom=22),
-    "bedroom": Fyrtur(topic="zigbee2mqtt/bedroom_fyrtur_1", top=100, bottom=16),
-})
+watcher = FyrturWatcher(
+    {
+        "office1": Fyrtur(topic="zigbee2mqtt/office_fyrtur_1", top=100, bottom=16),
+        "office2": Fyrtur(topic="zigbee2mqtt/office_fyrtur_2", top=100, bottom=22),
+        "bedroom": Fyrtur(topic="zigbee2mqtt/bedroom_fyrtur_1", top=100, bottom=16),
+    }
+)
 
 
 # The callback for when the client receives a CONNACK response from the server.
@@ -90,8 +93,8 @@ def on_message(client, _userdata, msg):
     global scene
     (topic, payload) = parse_message(msg)
     if topic == "control/lights/set":
-        print("set scene %s -> %s" % (scene, payload['scene']))
-        scene = payload['scene']
+        print("set scene %s -> %s" % (scene, payload["scene"]))
+        scene = payload["scene"]
         update_scene(client)
     else:
         print("got %s" % topic)
diff --git a/mqtt/heater.py b/mqtt/heater.py
index e62219f..bbb21da 100644
--- a/mqtt/heater.py
+++ b/mqtt/heater.py
@@ -23,9 +23,14 @@ class Heater:
         if self.not_initialized_yet:
             self.not_initialized_yet = False
             self.wanted_temperature = heating_setpoint
-            print("%s: update wanted temperature %d" % (self.topic, self.actual_temperature))
+            print(
+                "%s: update wanted temperature %d"
+                % (self.topic, self.actual_temperature)
+            )
         self.actual_temperature = heating_setpoint
-        print("%s: update actual temperature %d" % (self.topic, self.actual_temperature))
+        print(
+            "%s: update actual temperature %d" % (self.topic, self.actual_temperature)
+        )
 
     def topic_and_payload_for_query(self):
         payload = {
@@ -33,24 +38,23 @@ class Heater:
             "occupied_heating_setpoint": "",
             "unoccupied_heating_setpoint": "",
             "local_temperature": "",
-            #"pi_heating_demand": "",
-            #"system_mode": "",
+            # "pi_heating_demand": "",
+            # "system_mode": "",
         }
         return ("%s/get" % self.topic), json.dumps(payload)
 
     def topic_and_payload_for_set(self):
         payload = {
             "system_mode": "auto",
-            #"current_heating_setpoint": str(self.wanted_temperature),
+            # "current_heating_setpoint": str(self.wanted_temperature),
             "occupied_heating_setpoint": str(self.wanted_temperature),
             "unoccupied_heating_setpoint": str(self.wanted_temperature),
-            "eurotronic_host_flags": {"window_open": True}
+            "eurotronic_host_flags": {"window_open": True},
         }
         return ("%s/set" % self.topic), json.dumps(payload)
 
 
 class Watcher:
-
     def __init__(self, heater: Dict[str, Heater]):
         self.heater = heater
 
@@ -82,12 +86,14 @@ class Watcher:
 
 scene = "default"
 
-watcher = Watcher({
-    "office1": Heater(topic="zigbee2mqtt/office_heater_1"),
-    "office2": Heater(topic="zigbee2mqtt/office_heater_2"),
-    "bedroom": Heater(topic="zigbee2mqtt/bedroom_heater_1"),
-    "storage": Heater(topic="zigbee2mqtt/storage_heater_1"),
-})
+watcher = Watcher(
+    {
+        "office1": Heater(topic="zigbee2mqtt/office_heater_1"),
+        "office2": Heater(topic="zigbee2mqtt/office_heater_2"),
+        "bedroom": Heater(topic="zigbee2mqtt/bedroom_heater_1"),
+        "storage": Heater(topic="zigbee2mqtt/storage_heater_1"),
+    }
+)
 
 
 # The callback for when the client receives a CONNACK response from the server.
@@ -109,8 +115,8 @@ def on_message(client, _userdata, msg):
     global scene
     (topic, payload) = parse_message(msg)
     if topic == "control/lights/set":
-        print("set scene %s -> %s" % (scene, payload['scene']))
-        scene = payload['scene']
+        print("set scene %s -> %s" % (scene, payload["scene"]))
+        scene = payload["scene"]
         update_scene(client)
     else:
         print("got %s" % topic)
diff --git a/mqtt/shell.nix b/mqtt/shell.nix
index 97f0099..34c82e6 100644
--- a/mqtt/shell.nix
+++ b/mqtt/shell.nix
@@ -8,8 +8,12 @@ let
     ${myPython}/bin/python ./heater.py
   '';
 
+  reformat = pkgs.writers.writeBashBin "reformat" ''
+    ${pkgs.black}/bin/black --exclude venv ${toString ./.}
+  '';
+
 in pkgs.mkShell {
 
-  buildInputs = with pkgs; [ myPython startServer ];
+  buildInputs = with pkgs; [ myPython startServer reformat ];
 
 }