diff --git a/recipes.py b/recipes.py
index b2bb808..6994dd5 100644
--- a/recipes.py
+++ b/recipes.py
@@ -16,11 +16,13 @@ ISSUE_UNKNOWN_INGREDIENT = "unknown-ingredient"
 ISSUE_DUPLICATE_UNITS = "duplicate-units"
 ISSUE_KNOWN_PRICE_UNKNOWN_CONVERSION = "known-price-unknown-conversion"
 
+
 class Issue:
     def __init__(self, id: str, msg: str) -> None:
         self.id = id
         self.msg = msg
 
+
 class Issues:
     def __init__(self) -> None:
         self.errors: List[Issue] = []
@@ -44,6 +46,7 @@ class Issues:
         self.warnings.clear()
         return retcode
 
+
 class Context:
     def __init__(self) -> None:
         self.units = Units(self)
@@ -141,8 +144,11 @@ class Units:
         for unit in self.units:
             unitnames.append(unit["name"])
         for unitname, num in collections.Counter(unitnames).items():
-            if num>1:
-                self.ctx.issues.error(ISSUE_DUPLICATE_UNITS, f"units.yaml: {unitname} should only have one entry, found {num}")
+            if num > 1:
+                self.ctx.issues.error(
+                    ISSUE_DUPLICATE_UNITS,
+                    f"units.yaml: {unitname} should only have one entry, found {num}",
+                )
 
 
 class Ingredient(Element):
@@ -174,14 +180,12 @@ class Ingredient(Element):
         for nextunit in conversions[startname].keys():
             if nextunit in visited:
                 continue
-            result = self.dfs(conversions, nextunit, endname, visited+[startname])
+            result = self.dfs(conversions, nextunit, endname, visited + [startname])
             if result is not None:
                 return result
         return None
 
-    def convert(
-        self, amount: float, unitfrom: Unit, unitto: Unit
-    ) -> Optional[float]:
+    def convert(self, amount: float, unitfrom: Unit, unitto: Unit) -> Optional[float]:
         conversions: Dict[str, Dict[str, float]] = defaultdict(dict)
         # construct node tree
         convs = self["conversions"]
@@ -196,8 +200,9 @@ class Ingredient(Element):
         # find path between conversions
         path = self.dfs(conversions, unitfrom["name"], unitto["name"])
         if path is None:
-            self.ctx.issues.warn(ISSUE_KNOWN_PRICE_UNKNOWN_CONVERSION,
-                f'{self["name"]} has a known price, but conversion {unitfrom["name"]} -> {unitto["name"]} not known'
+            self.ctx.issues.warn(
+                ISSUE_KNOWN_PRICE_UNKNOWN_CONVERSION,
+                f'{self["name"]} has a known price, but conversion {unitfrom["name"]} -> {unitto["name"]} not known',
             )
             return None
         assert len(path) != 0
@@ -278,7 +283,9 @@ class IngredientInstance(Element):
     def load(self, dct: Dict[str, Any]) -> None:
         ingredient = self.ctx.ingredients.get(dct["name"])
         if ingredient is None:
-            self.ctx.issues.error(ISSUE_UNKNOWN_INGREDIENT, f"unknown ingredient {dct['name']}")
+            self.ctx.issues.error(
+                ISSUE_UNKNOWN_INGREDIENT, f"unknown ingredient {dct['name']}"
+            )
         self["ingredient"] = ingredient
 
         if "amount" not in dct: