Tests and helpers

core/tracker.py

@@ -1,5 +1,6 @@
 from uuid import uuid4
 from core.transaction import Transaction
+from utils.helpers import validate_pin, parse_price
 
 
 class Tracker:
@@ -70,7 +71,7 @@ class Tracker:
         from uuid import uuid4
         from core.transaction import Transaction
 
-        transaction = Transaction(str(uuid4()), user_id, "adjustment", -amount)
+        transaction = Transaction(str(uuid4()), user_id, "Abbuchung", -amount)
         self.transactions.append(transaction)
         self.data_manager.save_transactions(self.transactions)
 
@@ -97,12 +98,15 @@ class Tracker:
     def set_user_pin(self, user_id, pin):
         if user_id not in self.users:
             return {"status": "error", "message": "User not found"}
+        if not validate_pin(pin):
+            return {"status": "error", "message": "Invalid PIN format"}
         self.users[user_id].pin = pin
         self.data_manager.save_users(self.users)
         return {"status": "success", "message": "PIN updated successfully"}
 
     def add_user(self, user_id, name):
         from core.user import User
+
         if user_id in self.users:
             return {"status": "error", "message": "User ID already exists"}
         self.users[user_id] = User(user_id, name, "4242")
@@ -123,6 +127,8 @@ class Tracker:
     def edit_user(self, user_id, name, pin=None):
         if user_id not in self.users:
             return {"status": "error", "message": "User not found"}
+        if pin is not None and not validate_pin(pin):
+            return {"status": "error", "message": "Invalid PIN format"}
         self.users[user_id].name = name
         if pin is not None:
             self.users[user_id].pin = pin
@@ -131,10 +137,11 @@ class Tracker:
 
     def add_item(self, item_id, name, price):
         from core.item import Item
+
         if item_id in self.items:
             return {"status": "error", "message": "Item ID already exists"}
         try:
-            price_val = float(price)
+            price_val = parse_price(str(price))
         except ValueError:
             return {"status": "error", "message": "Invalid price value"}
         self.items[item_id] = Item(item_id, name, price_val)
@@ -153,11 +160,10 @@ class Tracker:
         if item_id not in self.items:
             return {"status": "error", "message": "Item not found"}
         try:
-            price_val = float(price)
+            price_val = parse_price(str(price))
         except ValueError:
             return {"status": "error", "message": "Invalid price value"}
         self.items[item_id].name = name
         self.items[item_id].price = price_val
         self.data_manager.save_items(self.items)
         return {"status": "success", "message": f"Item {name} updated successfully"}
-

tests/test_helpers.py

@@ -0,0 +1,37 @@
+import unittest
+from utils.helpers import format_currency, format_date, validate_pin, parse_price
+
+class TestHelpers(unittest.TestCase):
+    def test_format_currency_german(self):
+        self.assertEqual(format_currency(2.50), "2,50 €")
+        self.assertEqual(format_currency(1234.567), "1234,57 €")
+        self.assertEqual(format_currency(0.0), "0,00 €")
+
+    def test_format_currency_non_german(self):
+        self.assertEqual(format_currency(2.50, symbol="$", is_german=False), "$2.50")
+        self.assertEqual(format_currency(0.99, symbol="£", is_german=False), "£0.99")
+
+    def test_format_date(self):
+        self.assertEqual(format_date("2026-06-02T15:00:00.000000"), "02.06.2026")
+        self.assertEqual(format_date("2026-12-25T18:30:00"), "25.12.2026")
+        self.assertEqual(format_date("invalid-date-format"), "invalid-da")
+
+    def test_validate_pin(self):
+        self.assertTrue(validate_pin("1234"))
+        self.assertTrue(validate_pin("4242"))
+        self.assertTrue(validate_pin("123456"))
+        self.assertFalse(validate_pin("123"))  # too short
+        self.assertFalse(validate_pin("123a")) # non-numeric
+        self.assertFalse(validate_pin(""))     # empty
+
+    def test_parse_price(self):
+        self.assertEqual(parse_price("2.50"), 2.50)
+        self.assertEqual(parse_price("2,50"), 2.50)
+        self.assertEqual(parse_price(" 0,99 "), 0.99)
+        with self.assertRaises(ValueError):
+            parse_price("-2.50") # negative
+        with self.assertRaises(ValueError):
+            parse_price("abc")   # non-numeric
+
+if __name__ == "__main__":
+    unittest.main()

tests/test_item.py

@@ -0,0 +1,37 @@
+import unittest
+from core.item import Item
+
+class TestItem(unittest.TestCase):
+    def test_item_initialization(self):
+        item = Item("coffee", "Coffee", 2.50)
+        self.assertEqual(item.id, "coffee")
+        self.assertEqual(item.name, "Coffee")
+        self.assertEqual(item.price, 2.50)
+
+    def test_item_price_conversion(self):
+        item = Item("tea", "Tea", "1.80")
+        self.assertEqual(item.price, 1.80)
+        self.assertIsInstance(item.price, float)
+
+    def test_to_json(self):
+        item = Item("sandwich", "Sandwich", 5.00)
+        expected = {
+            "id": "sandwich",
+            "name": "Sandwich",
+            "price": 5.00
+        }
+        self.assertEqual(item.to_json(), expected)
+
+    def test_from_json(self):
+        data = {
+            "id": "sandwich",
+            "name": "Sandwich",
+            "price": 5.00
+        }
+        item = Item.from_json(data)
+        self.assertEqual(item.id, "sandwich")
+        self.assertEqual(item.name, "Sandwich")
+        self.assertEqual(item.price, 5.00)
+
+if __name__ == "__main__":
+    unittest.main()

tests/test_tracker.py

@@ -6,7 +6,6 @@ from core.item import Item
 import shutil
 import os
 
-
 class TestTracker(unittest.TestCase):
     def setUp(self):
         self.test_dir = "test_data"
@@ -24,7 +23,6 @@ class TestTracker(unittest.TestCase):
             shutil.rmtree(self.test_dir)
 
     def test_adjust_debt_new_user(self):
-        # Verify adjust_debt does not raise KeyError on uninitialized user debt
         res = self.tracker.adjust_debt("test_user", 2.0, "admin123")
         self.assertEqual(res["status"], "success")
         self.assertEqual(self.tracker.get_user_debt("test_user"), -2.0)
@@ -35,15 +33,12 @@ class TestTracker(unittest.TestCase):
         self.assertEqual(self.tracker.get_user_debt("test_user"), 2.50)
         self.assertEqual(len(self.tracker.transactions), 1)
 
-    def test_default_pin(self):
-        user = User("new_user", "New User")
-        self.assertEqual(user.pin, "4242")
-
     def test_add_user(self):
         res = self.tracker.add_user("test_user_2", "Test Bob")
         self.assertEqual(res["status"], "success")
         self.assertEqual(self.tracker.users["test_user_2"].name, "Test Bob")
         self.assertEqual(self.tracker.users["test_user_2"].pin, "4242")
+
     def test_remove_user(self):
         res = self.tracker.remove_user("test_user")
         self.assertEqual(res["status"], "success")
@@ -56,12 +51,26 @@ class TestTracker(unittest.TestCase):
         self.assertEqual(self.tracker.users["test_user"].name, "Updated Name")
         self.assertEqual(self.tracker.users["test_user"].pin, "9999")
 
+    def test_set_user_pin_validation(self):
+        res = self.tracker.set_user_pin("test_user", "invalid_pin")
+        self.assertEqual(res["status"], "error")
+        self.assertEqual(self.tracker.users["test_user"].pin, "4242")  # Unchanged
+
+        res2 = self.tracker.set_user_pin("test_user", "1234")
+        self.assertEqual(res2["status"], "success")
+        self.assertEqual(self.tracker.users["test_user"].pin, "1234")
+
     def test_add_item(self):
         res = self.tracker.add_item("new_item", "Donut", 1.80)
         self.assertEqual(res["status"], "success")
         self.assertEqual(self.tracker.items["new_item"].name, "Donut")
         self.assertEqual(self.tracker.items["new_item"].price, 1.80)
 
+    def test_add_item_invalid_price(self):
+        res = self.tracker.add_item("invalid_item", "Cake", "-2.50")
+        self.assertEqual(res["status"], "error")
+        self.assertNotIn("invalid_item", self.tracker.items)
+
     def test_remove_item(self):
         res = self.tracker.remove_item("test_item")
         self.assertEqual(res["status"], "success")
@@ -73,7 +82,10 @@ class TestTracker(unittest.TestCase):
         self.assertEqual(self.tracker.items["test_item"].name, "Premium Coffee")
         self.assertEqual(self.tracker.items["test_item"].price, 3.20)
 
+    def test_edit_item_invalid_price(self):
+        res = self.tracker.edit_item("test_item", "Premium Coffee", "not_a_number")
+        self.assertEqual(res["status"], "error")
+        self.assertEqual(self.tracker.items["test_item"].price, 2.50)  # Unchanged
 
 if __name__ == "__main__":
     unittest.main()
-

tests/test_transaction.py

@@ -0,0 +1,51 @@
+import unittest
+from core.transaction import Transaction
+
+
+class TestTransaction(unittest.TestCase):
+    def test_transaction_initialization_with_timestamp(self):
+        timestamp = "2026-06-02T15:00:00.000000"
+        t = Transaction("tx_1", "user_1", "item_1", 2.50, timestamp)
+        self.assertEqual(t.id, "tx_1")
+        self.assertEqual(t.user_id, "user_1")
+        self.assertEqual(t.item_id, "item_1")
+        self.assertEqual(t.amount, 2.50)
+        self.assertEqual(t.timestamp, timestamp)
+
+    def test_transaction_auto_timestamp(self):
+        t = Transaction("tx_2", "user_2", "item_2", 1.50)
+        self.assertIsNotNone(t.timestamp)
+        # Check if timestamp format is valid ISO string (at least parses or contains dates)
+        self.assertIn("-", t.timestamp)
+        self.assertIn("T", t.timestamp)
+
+    def test_to_json(self):
+        timestamp = "2026-06-02T15:00:00"
+        t = Transaction("tx_1", "user_1", "item_1", 2.50, timestamp)
+        expected = {
+            "id": "tx_1",
+            "user_id": "user_1",
+            "item_id": "item_1",
+            "amount": 2.50,
+            "timestamp": timestamp,
+        }
+        self.assertEqual(t.to_json(), expected)
+
+    def test_from_json(self):
+        data = {
+            "id": "tx_1",
+            "user_id": "user_1",
+            "item_id": "item_1",
+            "amount": 2.50,
+            "timestamp": "2026-06-02T15:00:00",
+        }
+        t = Transaction.from_json(data)
+        self.assertEqual(t.id, "tx_1")
+        self.assertEqual(t.user_id, "user_1")
+        self.assertEqual(t.item_id, "item_1")
+        self.assertEqual(t.amount, 2.50)
+        self.assertEqual(t.timestamp, "2026-06-02T15:00:00")
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/test_user.py

@@ -0,0 +1,50 @@
+import unittest
+from core.user import User
+
+class TestUser(unittest.TestCase):
+    def test_default_values(self):
+        user = User("alice", "Alice")
+        self.assertEqual(user.id, "alice")
+        self.assertEqual(user.name, "Alice")
+        self.assertEqual(user.pin, "4242")
+        self.assertEqual(user.debt, 0.0)
+
+    def test_custom_pin(self):
+        user = User("bob", "Bob", "1234")
+        self.assertEqual(user.pin, "1234")
+
+    def test_to_json(self):
+        user = User("charlie", "Charlie", "9999")
+        user.debt = 12.50
+        expected = {
+            "id": "charlie",
+            "name": "Charlie",
+            "debt": 12.50,
+            "pin": "9999"
+        }
+        self.assertEqual(user.to_json(), expected)
+
+    def test_from_json(self):
+        data = {
+            "id": "charlie",
+            "name": "Charlie",
+            "debt": 12.50,
+            "pin": "9999"
+        }
+        user = User.from_json(data)
+        self.assertEqual(user.id, "charlie")
+        self.assertEqual(user.name, "Charlie")
+        self.assertEqual(user.debt, 12.50)
+        self.assertEqual(user.pin, "9999")
+
+    def test_from_json_missing_pin(self):
+        data = {
+            "id": "charlie",
+            "name": "Charlie",
+            "debt": 12.50
+        }
+        user = User.from_json(data)
+        self.assertEqual(user.pin, "4242")
+
+if __name__ == "__main__":
+    unittest.main()

ui/cli.py

@@ -364,7 +364,7 @@ class CLI:
             for t in sorted(transactions, key=lambda x: x.timestamp, reverse=True):
                 user = self.tracker.get_user_by_id(t.user_id)
                 item = self.tracker.get_items().get(t.item_id)
-                item_name = item.name if item else ("Adjustment" if t.item_id == "adjustment" else "Deleted Item")
+                item_name = item.name if item else ("Abbuchung" if t.item_id in ("adjustment", "Abbuchung") else "Deleted Item")
                 print(
                     f"{t.timestamp[:10]} | {user.name:.<20} {item_name:.<15} ${t.amount:.2f}"
                 )

ui/kivy.py

@@ -1,6 +1,7 @@
-# type: ignore
+## type: ignore
 import kivy
 from kivy.app import App
+from kivy.graphics import Color, RoundedRectangle
 from kivy.uix.screenmanager import ScreenManager, Screen, SlideTransition
 from kivy.uix.boxlayout import BoxLayout
 from kivy.uix.gridlayout import GridLayout
@@ -10,7 +11,7 @@ from kivy.uix.button import Button
 from kivy.uix.textinput import TextInput
 from kivy.uix.popup import Popup
 from kivy.core.window import Window
-from kivy.graphics import Color, RoundedRectangle
+from utils.helpers import format_currency, format_date, validate_pin, parse_price
 
 
 class RoundedButton(Button):
@@ -104,45 +105,57 @@ class ConfirmPopup(Popup):
 
 
 class NumpadPopup(Popup):
-    def __init__(self, title, is_password=False, is_float=False, on_submit=None, **kwargs):
+    def __init__(
+        self, title, is_password=False, is_float=False, on_submit=None, **kwargs
+    ):
         self.is_password = is_password
         self.is_float = is_float
         self.on_submit = on_submit
         self.entered_text = ""
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         self.display = Label(
             text="PIN eingeben:" if is_password else "Betrag eingeben:",
-            font_size='20sp',
+            font_size="20sp",
             bold=True,
             size_hint_y=0.15,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        self.display.bind(size=self.display.setter('text_size'))
+        self.display.bind(size=self.display.setter("text_size"))
         content.add_widget(self.display)
 
         grid = GridLayout(cols=3, spacing=10, size_hint_y=0.6)
 
         buttons = [
-            '1', '2', '3',
+            "1",
-            '4', '5', '6',
+            "2",
-            '7', '8', '9',
+            "3",
-            ',' if is_float else 'C', '0', '⌫'
+            "4",
+            "5",
+            "6",
+            "7",
+            "8",
+            "9",
+            "," if is_float else "C",
+            "0",
+            "⌫",
         ]
 
         for btn_text in buttons:
             btn = RoundedButton(
                 text=btn_text,
-                btn_color=[0.15, 0.15, 0.17, 1] if btn_text in ['C', '⌫', ','] else [0.0, 0.65, 0.57, 1]
+                btn_color=[0.15, 0.15, 0.17, 1]
+                if btn_text in ["C", "⌫", ","]
+                else [0.0, 0.65, 0.57, 1],
             )
             btn.bind(on_release=self.on_key_press)
             grid.add_widget(btn)
 
         content.add_widget(grid)
 
-        actions = BoxLayout(orientation='horizontal', spacing=10, size_hint_y=0.25)
+        actions = BoxLayout(orientation="horizontal", spacing=10, size_hint_y=0.25)
         ok_btn = RoundedButton(text="OK", btn_color=[0.0, 0.65, 0.57, 1])
         cancel_btn = RoundedButton(text="Abbrechen", btn_color=[0.25, 0.25, 0.28, 1])
 
@@ -154,10 +167,10 @@ class NumpadPopup(Popup):
             title=title,
             content=content,
             size_hint=(0.85, 0.75),
-            title_align='center',
+            title_align="center",
-            title_size='18sp',
+            title_size="18sp",
             background_color=[0.08, 0.08, 0.09, 0.95],
-            **kwargs
+            **kwargs,
         )
 
         ok_btn.bind(on_release=self.submit)
@@ -165,20 +178,22 @@ class NumpadPopup(Popup):
 
     def on_key_press(self, instance):
         key = instance.text
-        if key == '⌫':
+        if key == "⌫":
             self.entered_text = self.entered_text[:-1]
-        elif key == 'C':
+        elif key == "C":
             self.entered_text = ""
-        elif key == ',':
+        elif key == ",":
-            if ',' not in self.entered_text:
+            if "," not in self.entered_text:
-                self.entered_text += ','
+                self.entered_text += ","
         else:
             self.entered_text += key
         self.update_display()
 
     def update_display(self):
         if not self.entered_text:
-            self.display.text = "PIN eingeben:" if self.is_password else "Betrag eingeben:"
+            self.display.text = (
+                "PIN eingeben:" if self.is_password else "Betrag eingeben:"
+            )
         else:
             if self.is_password:
                 self.display.text = "*" * len(self.entered_text)
@@ -197,34 +212,36 @@ class KeyboardPopup(Popup):
         self.on_submit = on_submit
         self.entered_text = ""
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         self.display = Label(
             text="Eingeben:" if not is_password else "****",
-            font_size='20sp',
+            font_size="20sp",
             bold=True,
             size_hint_y=0.15,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        self.display.bind(size=self.display.setter('text_size'))
+        self.display.bind(size=self.display.setter("text_size"))
         content.add_widget(self.display)
 
-        kbd_layout = BoxLayout(orientation='vertical', spacing=5, size_hint_y=0.6)
+        kbd_layout = BoxLayout(orientation="vertical", spacing=5, size_hint_y=0.6)
 
         rows = [
-            ['1', '2', '3', '4', '5', '6', '7', '8', '9', '0'],
+            ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"],
-            ['q', 'w', 'e', 'r', 't', 'z', 'u', 'i', 'o', 'p'],
+            ["q", "w", "e", "r", "t", "z", "u", "i", "o", "p"],
-            ['a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l'],
+            ["a", "s", "d", "f", "g", "h", "j", "k", "l"],
-            ['y', 'x', 'c', 'v', 'b', 'n', 'm', '⌫', 'C']
+            ["y", "x", "c", "v", "b", "n", "m", "⌫", "C"],
         ]
 
         for row in rows:
-            row_box = BoxLayout(orientation='horizontal', spacing=5)
+            row_box = BoxLayout(orientation="horizontal", spacing=5)
             for char in row:
                 btn = RoundedButton(
                     text=char,
-                    btn_color=[0.15, 0.15, 0.17, 1] if char in ['C', '⌫'] else [0.0, 0.65, 0.57, 1]
+                    btn_color=[0.15, 0.15, 0.17, 1]
+                    if char in ["C", "⌫"]
+                    else [0.0, 0.65, 0.57, 1],
                 )
                 btn.bind(on_release=self.on_key_press)
                 row_box.add_widget(btn)
@@ -232,7 +249,7 @@ class KeyboardPopup(Popup):
 
         content.add_widget(kbd_layout)
 
-        actions = BoxLayout(orientation='horizontal', spacing=10, size_hint_y=0.25)
+        actions = BoxLayout(orientation="horizontal", spacing=10, size_hint_y=0.25)
         ok_btn = RoundedButton(text="OK", btn_color=[0.0, 0.65, 0.57, 1])
         cancel_btn = RoundedButton(text="Abbrechen", btn_color=[0.25, 0.25, 0.28, 1])
 
@@ -244,10 +261,10 @@ class KeyboardPopup(Popup):
             title=title,
             content=content,
             size_hint=(0.95, 0.8),
-            title_align='center',
+            title_align="center",
-            title_size='18sp',
+            title_size="18sp",
             background_color=[0.08, 0.08, 0.09, 0.95],
-            **kwargs
+            **kwargs,
         )
 
         ok_btn.bind(on_release=self.submit)
@@ -255,9 +272,9 @@ class KeyboardPopup(Popup):
 
     def on_key_press(self, instance):
         key = instance.text
-        if key == '⌫':
+        if key == "⌫":
             self.entered_text = self.entered_text[:-1]
-        elif key == 'C':
+        elif key == "C":
             self.entered_text = ""
         else:
             self.entered_text += key
@@ -338,7 +355,7 @@ class AdjustDebtPopup(Popup):
                 title="Betrag eingeben",
                 is_password=False,
                 is_float=True,
-                on_submit=self.on_amount_submitted
+                on_submit=self.on_amount_submitted,
             )
             popup.open()
 
@@ -362,16 +379,16 @@ class CreateUserPopup(Popup):
     def __init__(self, on_create, **kwargs):
         self.on_create = on_create
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         info = Label(
             text="Name des neuen Benutzers eingeben:",
-            font_size='16sp',
+            font_size="16sp",
             size_hint_y=0.25,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        info.bind(size=info.setter('text_size'))
+        info.bind(size=info.setter("text_size"))
         content.add_widget(info)
 
         self.name_input = TextInput(
@@ -384,7 +401,7 @@ class CreateUserPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.3,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.name_input.bind(focus=self.trigger_keyboard)
         content.add_widget(self.name_input)
@@ -417,7 +434,7 @@ class CreateUserPopup(Popup):
             popup = KeyboardPopup(
                 title="Name eingeben",
                 is_password=False,
-                on_submit=self.on_name_submitted
+                on_submit=self.on_name_submitted,
             )
             popup.open()
 
@@ -435,21 +452,23 @@ class CreateUserPopup(Popup):
 
 
 class EditUserPopup(Popup):
-    def __init__(self, user_id, current_name, current_pin, on_save, on_delete, **kwargs):
+    def __init__(
+        self, user_id, current_name, current_pin, on_save, on_delete, **kwargs
+    ):
         self.user_id = user_id
         self.on_save = on_save
         self.on_delete = on_delete
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         info = Label(
             text="Benutzer bearbeiten:",
-            font_size='16sp',
+            font_size="16sp",
             size_hint_y=0.15,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        info.bind(size=info.setter('text_size'))
+        info.bind(size=info.setter("text_size"))
         content.add_widget(info)
 
         content.add_widget(Label(text="Name:", size_hint_y=0.08, halign="left"))
@@ -463,7 +482,7 @@ class EditUserPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.name_input.bind(focus=self.trigger_keyboard)
         content.add_widget(self.name_input)
@@ -479,7 +498,7 @@ class EditUserPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.pin_input.bind(focus=self.trigger_numpad)
         content.add_widget(self.pin_input)
@@ -515,7 +534,7 @@ class EditUserPopup(Popup):
             popup = KeyboardPopup(
                 title="Name eingeben",
                 is_password=False,
-                on_submit=self.on_name_submitted
+                on_submit=self.on_name_submitted,
             )
             popup.open()
 
@@ -526,9 +545,7 @@ class EditUserPopup(Popup):
         if value:
             instance.focus = False
             popup = NumpadPopup(
-                title="PIN eingeben",
+                title="PIN eingeben", is_password=False, on_submit=self.on_pin_submitted
-                is_password=False,
-                on_submit=self.on_pin_submitted
             )
             popup.open()
 
@@ -543,7 +560,9 @@ class EditUserPopup(Popup):
             err.open()
             return
         if len(pin) < 4:
-            err = MessagePopup(title="Fehler", message="Die PIN must mindestens 4 Ziffern lang sein!")
+            err = MessagePopup(
+                title="Fehler", message="Die PIN must mindestens 4 Ziffern lang sein!"
+            )
             err.open()
             return
         self.dismiss()
@@ -558,16 +577,16 @@ class CreateItemPopup(Popup):
     def __init__(self, on_create, **kwargs):
         self.on_create = on_create
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         info = Label(
             text="Neuen Artikel hinzufügen:",
-            font_size='16sp',
+            font_size="16sp",
             size_hint_y=0.15,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        info.bind(size=info.setter('text_size'))
+        info.bind(size=info.setter("text_size"))
         content.add_widget(info)
 
         content.add_widget(Label(text="Name:", size_hint_y=0.08, halign="left"))
@@ -581,7 +600,7 @@ class CreateItemPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.name_input.bind(focus=self.trigger_keyboard)
         content.add_widget(self.name_input)
@@ -597,7 +616,7 @@ class CreateItemPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.price_input.bind(focus=self.trigger_numpad)
         content.add_widget(self.price_input)
@@ -630,7 +649,7 @@ class CreateItemPopup(Popup):
             popup = KeyboardPopup(
                 title="Artikelname eingeben",
                 is_password=False,
-                on_submit=self.on_name_submitted
+                on_submit=self.on_name_submitted,
             )
             popup.open()
 
@@ -644,7 +663,7 @@ class CreateItemPopup(Popup):
                 title="Preis eingeben",
                 is_password=False,
                 is_float=True,
-                on_submit=self.on_price_submitted
+                on_submit=self.on_price_submitted,
             )
             popup.open()
 
@@ -672,21 +691,23 @@ class CreateItemPopup(Popup):
 
 
 class EditItemPopup(Popup):
-    def __init__(self, item_id, current_name, current_price, on_save, on_delete, **kwargs):
+    def __init__(
+        self, item_id, current_name, current_price, on_save, on_delete, **kwargs
+    ):
         self.item_id = item_id
         self.on_save = on_save
         self.on_delete = on_delete
 
-        content = BoxLayout(orientation='vertical', padding=15, spacing=15)
+        content = BoxLayout(orientation="vertical", padding=15, spacing=15)
 
         info = Label(
             text="Artikel bearbeiten:",
-            font_size='16sp',
+            font_size="16sp",
             size_hint_y=0.15,
-            halign='center',
+            halign="center",
-            valign='middle'
+            valign="middle",
         )
-        info.bind(size=info.setter('text_size'))
+        info.bind(size=info.setter("text_size"))
         content.add_widget(info)
 
         content.add_widget(Label(text="Name:", size_hint_y=0.08, halign="left"))
@@ -700,7 +721,7 @@ class EditItemPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.name_input.bind(focus=self.trigger_keyboard)
         content.add_widget(self.name_input)
@@ -716,7 +737,7 @@ class EditItemPopup(Popup):
             foreground_color=[1, 1, 1, 1],
             cursor_color=[1, 1, 1, 1],
             size_hint_y=0.15,
-            padding=[10, 10]
+            padding=[10, 10],
         )
         self.price_input.bind(focus=self.trigger_numpad)
         content.add_widget(self.price_input)
@@ -752,7 +773,7 @@ class EditItemPopup(Popup):
             popup = KeyboardPopup(
                 title="Artikelname eingeben",
                 is_password=False,
-                on_submit=self.on_name_submitted
+                on_submit=self.on_name_submitted,
             )
             popup.open()
 
@@ -766,7 +787,7 @@ class EditItemPopup(Popup):
                 title="Preis eingeben",
                 is_password=False,
                 is_float=True,
-                on_submit=self.on_price_submitted
+                on_submit=self.on_price_submitted,
             )
             popup.open()
 
@@ -847,7 +868,7 @@ class TransactionRow(BoxLayout):
         self.bind(pos=self.update_rect, size=self.update_rect)
 
         date_lbl = Label(
-            text=date[:10],
+            text=format_date(date),
             font_size="14sp",
             size_hint_x=0.25,
             halign="left",
@@ -1015,27 +1036,29 @@ class UserSelectionScreen(StyledScreen):
             popup = NumpadPopup(
                 title=f"PIN einrichten: {user.name}",
                 is_password=True,
-                on_submit=lambda pin: self.setup_pin_confirm(user_id, pin)
+                on_submit=lambda pin: self.setup_pin_confirm(user_id, pin),
             )
             popup.open()
         else:
             popup = NumpadPopup(
                 title=f"PIN eingeben: {user.name}",
                 is_password=True,
-                on_submit=lambda pin: self.verify_pin(user_id, pin)
+                on_submit=lambda pin: self.verify_pin(user_id, pin),
             )
             popup.open()
 
     def setup_pin_confirm(self, user_id, pin):
         if len(pin) < 4:
-            err = MessagePopup(title="Fehler", message="Die PIN muss mindestens 4 Ziffern lang sein!")
+            err = MessagePopup(
+                title="Fehler", message="Die PIN muss mindestens 4 Ziffern lang sein!"
+            )
             err.open()
             return
 
         popup = NumpadPopup(
             title="PIN bestätigen",
             is_password=True,
-            on_submit=lambda confirm_pin: self.save_new_pin(user_id, pin, confirm_pin)
+            on_submit=lambda confirm_pin: self.save_new_pin(user_id, pin, confirm_pin),
         )
         popup.open()
 
@@ -1048,7 +1071,7 @@ class UserSelectionScreen(StyledScreen):
         app = App.get_running_app()
         app.tracker.set_user_pin(user_id, pin)
         app.current_user = user_id
-        self.manager.current = 'user_menu'
+        self.manager.current = "user_menu"
 
     def verify_pin(self, user_id, pin):
         app = App.get_running_app()
@@ -1056,7 +1079,7 @@ class UserSelectionScreen(StyledScreen):
 
         if user.pin == pin:
             app.current_user = user_id
-            self.manager.current = 'user_menu'
+            self.manager.current = "user_menu"
         else:
             err = MessagePopup(title="Fehler", message="Falsche PIN!")
             err.open()
@@ -1125,7 +1148,7 @@ class UserMenuScreen(StyledScreen):
         popup = NumpadPopup(
             title="Aktuelle PIN eingeben",
             is_password=True,
-            on_submit=lambda pin: self.verify_current_pin(pin)
+            on_submit=lambda pin: self.verify_current_pin(pin),
         )
         popup.open()
 
@@ -1137,7 +1160,7 @@ class UserMenuScreen(StyledScreen):
             popup = NumpadPopup(
                 title="Neue PIN eingeben",
                 is_password=True,
-                on_submit=lambda new_pin: self.setup_new_pin(new_pin)
+                on_submit=lambda new_pin: self.setup_new_pin(new_pin),
             )
             popup.open()
         else:
@@ -1146,14 +1169,16 @@ class UserMenuScreen(StyledScreen):
 
     def setup_new_pin(self, new_pin):
         if len(new_pin) < 4:
-            err = MessagePopup(title="Fehler", message="Die PIN muss mindestens 4 Ziffern lang sein!")
+            err = MessagePopup(
+                title="Fehler", message="Die PIN muss mindestens 4 Ziffern lang sein!"
+            )
             err.open()
             return
 
         popup = NumpadPopup(
             title="Neue PIN bestätigen",
             is_password=True,
-            on_submit=lambda confirm_pin: self.save_changed_pin(new_pin, confirm_pin)
+            on_submit=lambda confirm_pin: self.save_changed_pin(new_pin, confirm_pin),
         )
         popup.open()
 
@@ -1374,7 +1399,7 @@ class AdminPasswordScreen(StyledScreen):
             popup = KeyboardPopup(
                 title="Admin-Passwort eingeben",
                 is_password=True,
-                on_submit=self.on_password_submitted
+                on_submit=self.on_password_submitted,
             )
             popup.open()
 
@@ -1494,7 +1519,6 @@ class AdminMenuScreen(StyledScreen):
         self.manager.current = "main_menu"
 
 
-
 class AdminAdjustDebtScreen(StyledScreen):
     def __init__(self, **kwargs):
         super(AdminAdjustDebtScreen, self).__init__(
@@ -1666,8 +1690,8 @@ class AdminHistoryScreen(StyledScreen):
             user = app.tracker.get_user_by_id(t.user_id)
             user_name = user.name if user else "Gelöschter Benutzer"
 
-            if t.item_id == "adjustment":
+            if t.item_id in ("adjustment", "Abbuchung"):
-                item_name = "Anpassung"
+                item_name = "Abbuchung"
             else:
                 item = app.tracker.get_items().get(t.item_id)
                 item_name = item.name if item else "Gelöschter Artikel"
@@ -1695,7 +1719,7 @@ class AdminManageUsersScreen(StyledScreen):
             size_hint=(0.7, None),
             height="50dp",
             pos_hint={"center_x": 0.5},
-            btn_color=[0.0, 0.65, 0.57, 1]
+            btn_color=[0.0, 0.65, 0.57, 1],
         )
         btn_add.bind(on_release=self.open_create_user_popup)
         self.content_layout.add_widget(btn_add)
@@ -1738,6 +1762,7 @@ class AdminManageUsersScreen(StyledScreen):
 
     def create_user(self, name):
         import uuid
+
         app = App.get_running_app()
         user_id = f"user_{uuid.uuid4().hex[:8]}"
         res = app.tracker.add_user(user_id, name)
@@ -1745,15 +1770,12 @@ class AdminManageUsersScreen(StyledScreen):
         if res["status"] == "success":
             msg = MessagePopup(
                 title="Erfolg",
-                message=f"Benutzer {name} erfolgreich erstellt!\n(Standard-PIN: 4242)"
+                message=f"Benutzer {name} erfolgreich erstellt!\n(Standard-PIN: 4242)",
             )
             msg.open()
             self.refresh_users()
         else:
-            msg = MessagePopup(
+            msg = MessagePopup(title="Fehler", message=f"Fehler: {res['message']}")
-                title="Fehler",
-                message=f"Fehler: {res['message']}"
-            )
             msg.open()
 
     def open_edit_user_popup(self, user_id, name, pin):
@@ -1762,7 +1784,7 @@ class AdminManageUsersScreen(StyledScreen):
             current_name=name,
             current_pin=pin,
             on_save=self.save_user,
-            on_delete=self.confirm_delete_user
+            on_delete=self.confirm_delete_user,
         )
         popup.open()
 
@@ -1785,7 +1807,7 @@ class AdminManageUsersScreen(StyledScreen):
         popup = ConfirmPopup(
             title="Benutzer löschen",
             message=f"Möchtest du den Benutzer {user.name} wirklich unwiderruflich löschen?",
-            on_confirm=lambda: self.delete_user(user_id)
+            on_confirm=lambda: self.delete_user(user_id),
         )
         popup.open()
 
@@ -1815,7 +1837,7 @@ class AdminManageItemsScreen(StyledScreen):
             size_hint=(0.7, None),
             height="50dp",
             pos_hint={"center_x": 0.5},
-            btn_color=[0.0, 0.65, 0.57, 1]
+            btn_color=[0.0, 0.65, 0.57, 1],
         )
         btn_add.bind(on_release=self.open_create_item_popup)
         self.content_layout.add_widget(btn_add)
@@ -1858,12 +1880,15 @@ class AdminManageItemsScreen(StyledScreen):
 
     def create_item(self, name, price):
         import uuid
+
         app = App.get_running_app()
         item_id = f"item_{uuid.uuid4().hex[:8]}"
         res = app.tracker.add_item(item_id, name, price)
 
         if res["status"] == "success":
-            msg = MessagePopup(title="Erfolg", message=f"Artikel {name} erfolgreich erstellt!")
+            msg = MessagePopup(
+                title="Erfolg", message=f"Artikel {name} erfolgreich erstellt!"
+            )
             msg.open()
             self.refresh_items()
         else:
@@ -1876,7 +1901,7 @@ class AdminManageItemsScreen(StyledScreen):
             current_name=name,
             current_price=price,
             on_save=self.save_item,
-            on_delete=self.confirm_delete_item
+            on_delete=self.confirm_delete_item,
         )
         popup.open()
 
@@ -1900,7 +1925,7 @@ class AdminManageItemsScreen(StyledScreen):
         popup = ConfirmPopup(
             title="Artikel löschen",
             message=f"Möchtest du den Artikel {item.name} wirklich unwiderruflich löschen?",
-            on_confirm=lambda: self.delete_item(item_id)
+            on_confirm=lambda: self.delete_item(item_id),
         )
         popup.open()
 
@@ -1946,7 +1971,6 @@ class KuehlschrankApp(App):
         return sm
 
 
-
 class KivyUI:
     def __init__(self, tracker):
         self.tracker = tracker

utils/helpers.py

@@ -0,0 +1,34 @@
+import re
+
+def format_currency(amount: float, symbol: str = "€", is_german: bool = True) -> str:
+    """Formats a numeric amount to a currency string representation."""
+    if is_german:
+        return f"{amount:.2f}".replace(".", ",") + f" {symbol}"
+    else:
+        return f"{symbol}{amount:.2f}"
+
+def format_date(iso_timestamp: str) -> str:
+    """Converts ISO timestamp to German date format DD.MM.YYYY."""
+    try:
+        date_part = iso_timestamp.split("T")[0]
+        parts = date_part.split("-")
+        if len(parts) == 3 and all(p.isdigit() for p in parts) and len(parts[0]) == 4:
+            return f"{parts[2]}.{parts[1]}.{parts[0]}"
+    except Exception:
+        pass
+    return iso_timestamp[:10]
+
+
+def validate_pin(pin: str) -> bool:
+    """Validates that a PIN consists of at least 4 numeric digits."""
+    return bool(re.match(r"^\d{4,}$", pin))
+
+def parse_price(price_str: str) -> float:
+    """Parses a string containing a price (handling comma and dot) to a float.
+    Raises ValueError if invalid.
+    """
+    clean_str = price_str.replace(",", ".").strip()
+    price = float(clean_str)
+    if price < 0:
+        raise ValueError("Price cannot be negative")
+    return price