diff --git a/backend/app/alembic/versions/c8e9f3a2d1b4_add_user_locale_preference_column.py b/backend/app/alembic/versions/c8e9f3a2d1b4_add_user_locale_preference_column.py
index 1b83358..c9d94e4 100644
--- a/backend/app/alembic/versions/c8e9f3a2d1b4_add_user_locale_preference_column.py
+++ b/backend/app/alembic/versions/c8e9f3a2d1b4_add_user_locale_preference_column.py
@@ -23,10 +23,7 @@ def upgrade() -> None:
     # VARCHAR(10) supports BCP 47 format (e.g., "en", "it", "en-US", "it-IT")
     # Nullable: NULL means "not set yet", will use Accept-Language header fallback
     # Indexed: For analytics queries and filtering by locale
-    op.add_column(
-        "users",
-        sa.Column("locale", sa.String(length=10), nullable=True)
-    )
+    op.add_column("users", sa.Column("locale", sa.String(length=10), nullable=True))
 
     # Create index on locale column for performance
     op.create_index(
diff --git a/backend/app/api/dependencies/locale.py b/backend/app/api/dependencies/locale.py
index 7c3d25e..72b691f 100644
--- a/backend/app/api/dependencies/locale.py
+++ b/backend/app/api/dependencies/locale.py
@@ -117,8 +117,9 @@ async def get_locale(
     if current_user and current_user.locale:
         # Validate that saved locale is still supported
         # (in case SUPPORTED_LOCALES changed after user set preference)
-        if current_user.locale in SUPPORTED_LOCALES:
-            return current_user.locale
+        locale_value = str(current_user.locale)
+        if locale_value in SUPPORTED_LOCALES:
+            return locale_value
 
     # Priority 2: Accept-Language header
     accept_language = request.headers.get("accept-language", "")
diff --git a/backend/app/schemas/users.py b/backend/app/schemas/users.py
index 729c22d..1229954 100755
--- a/backend/app/schemas/users.py
+++ b/backend/app/schemas/users.py
@@ -40,9 +40,9 @@ class UserUpdate(BaseModel):
     locale: str | None = Field(
         None,
         max_length=10,
-        pattern=r'^[a-z]{2}(-[A-Z]{2})?$',
+        pattern=r"^[a-z]{2}(-[A-Z]{2})?$",
         description="User's preferred locale (BCP 47 format: en, it, en-US, it-IT)",
-        examples=["en", "it", "en-US", "it-IT"]
+        examples=["en", "it", "en-US", "it-IT"],
     )
     is_active: bool | None = (
         None  # Changed default from True to None to avoid unintended updates
@@ -70,12 +70,12 @@ class UserUpdate(BaseModel):
             return v
         # Only support English and Italian for template showcase
         # Note: Locales stored in lowercase for case-insensitive matching
-        SUPPORTED_LOCALES = {"en", "it", "en-us", "en-gb", "it-it"}
+        supported_locales = {"en", "it", "en-us", "en-gb", "it-it"}
         # Normalize to lowercase for comparison and storage
         v_lower = v.lower()
-        if v_lower not in SUPPORTED_LOCALES:
+        if v_lower not in supported_locales:
             raise ValueError(
-                f"Unsupported locale '{v}'. Supported locales: {sorted(SUPPORTED_LOCALES)}"
+                f"Unsupported locale '{v}'. Supported locales: {sorted(supported_locales)}"
             )
         # Return normalized lowercase version for consistency
         return v_lower
diff --git a/backend/tests/api/dependencies/test_locale_dependencies.py b/backend/tests/api/dependencies/test_locale_dependencies.py
index fa8da4c..d10704b 100644
--- a/backend/tests/api/dependencies/test_locale_dependencies.py
+++ b/backend/tests/api/dependencies/test_locale_dependencies.py
@@ -67,9 +67,7 @@ class TestParseAcceptLanguage:
 
     def test_parse_complex_header(self):
         """Test complex Accept-Language header with multiple locales"""
-        result = parse_accept_language(
-            "it-IT,it;q=0.9,en-US;q=0.8,en;q=0.7,fr;q=0.6"
-        )
+        result = parse_accept_language("it-IT,it;q=0.9,en-US;q=0.8,en;q=0.7,fr;q=0.6")
         assert result == "it-it"
 
     def test_parse_whitespace_handling(self):
@@ -199,9 +197,7 @@ class TestGetLocale:
         assert result == "en"
 
     @pytest.mark.asyncio
-    async def test_locale_from_accept_language_header(
-        self, async_user_without_locale
-    ):
+    async def test_locale_from_accept_language_header(self, async_user_without_locale):
         """Test locale detection from Accept-Language header when user has no preference"""
         # Mock request with Italian Accept-Language (it-IT has highest priority)
         mock_request = MagicMock()
diff --git a/backend/tests/schemas/test_user_schemas.py b/backend/tests/schemas/test_user_schemas.py
index a04dd33..16c5647 100755
--- a/backend/tests/schemas/test_user_schemas.py
+++ b/backend/tests/schemas/test_user_schemas.py
@@ -334,11 +334,7 @@ class TestLocaleValidation:
     def test_locale_in_user_update_with_other_fields(self):
         """Test locale validation works when combined with other fields"""
         # Valid locale with other fields
-        user = UserUpdate(
-            first_name="Mario",
-            last_name="Rossi",
-            locale="it"
-        )
+        user = UserUpdate(first_name="Mario", last_name="Rossi", locale="it")
         assert user.locale == "it"
         assert user.first_name == "Mario"
 
@@ -347,7 +343,7 @@ class TestLocaleValidation:
             UserUpdate(
                 first_name="Pierre",
                 last_name="Dupont",
-                locale="fr"  # Unsupported
+                locale="fr",  # Unsupported
             )
 
     def test_supported_locales_list(self):
@@ -357,7 +353,9 @@ class TestLocaleValidation:
         # Expected output (normalized to lowercase)
         expected_outputs = ["en", "it", "en-us", "en-gb", "it-it"]
 
-        for input_locale, expected_output in zip(input_locales, expected_outputs):
+        for input_locale, expected_output in zip(
+            input_locales, expected_outputs, strict=True
+        ):
             user = UserUpdate(locale=input_locale)
             assert user.locale == expected_output