refactor: Refactor query parameter handling and pagination

- Added PromoListQuerySerializer to validate query parameters.
- CompanyPromoListView: moved query parameter checking to PromoListQuerySerializer.
- CustomLimitOffsetPagination: removed unreachable max_limit check, dropped try/except in get_limit since serializer now guarantees numeric input.

Author: RandomProgramm3r

promo_code/business/pagination.py

@@ -9,19 +9,15 @@ class CustomLimitOffsetPagination(
     max_limit = 100
 
     def get_limit(self, request):
-        param_limit = request.query_params.get(self.limit_query_param)
-        if param_limit is not None:
-            limit = int(param_limit)
+        raw_limit = request.query_params.get(self.limit_query_param)
 
-            if limit == 0:
-                return 0
+        if raw_limit is None:
+            return self.default_limit
 
-            if self.max_limit:
-                return min(limit, self.max_limit)
+        limit = int(raw_limit)
 
-            return limit
-
-        return self.default_limit
+        # Allow 0, otherwise cut by max_limit
+        return 0 if limit == 0 else min(limit, self.max_limit)
 
     def get_paginated_response(self, data):
         response = rest_framework.response.Response(data)

promo_code/business/serializers.py

@@ -282,6 +282,123 @@ def to_representation(self, instance):
         return data
 
 
+class PromoListQuerySerializer(rest_framework.serializers.Serializer):
+    """
+    Serializer for validating query parameters of promo list requests.
+    """
+
+    limit = rest_framework.serializers.CharField(
+        required=False,
+        allow_blank=True,
+    )
+    offset = rest_framework.serializers.CharField(
+        required=False,
+        allow_blank=True,
+    )
+    sort_by = rest_framework.serializers.ChoiceField(
+        choices=['active_from', 'active_until'],
+        required=False,
+    )
+    country = rest_framework.serializers.CharField(
+        required=False,
+        allow_blank=True,
+    )
+
+    _allowed_params = None
+
+    def get_allowed_params(self):
+        if self._allowed_params is None:
+            self._allowed_params = set(self.fields.keys())
+        return self._allowed_params
+
+    def validate(self, attrs):
+        query_params = self.initial_data
+        allowed_params = self.get_allowed_params()
+
+        unexpected_params = set(query_params.keys()) - allowed_params
+        if unexpected_params:
+            raise rest_framework.exceptions.ValidationError('Invalid params.')
+
+        field_errors = {}
+
+        attrs = self._validate_int_field('limit', attrs, field_errors)
+        attrs = self._validate_int_field('offset', attrs, field_errors)
+
+        self._validate_country(query_params, attrs, field_errors)
+
+        if field_errors:
+            raise rest_framework.exceptions.ValidationError(field_errors)
+
+        return attrs
+
+    def _validate_int_field(self, field_name, attrs, field_errors):
+        value_str = self.initial_data.get(field_name)
+        if value_str is None:
+            return attrs
+
+        if value_str == '':
+            raise rest_framework.exceptions.ValidationError(
+                f'Invalid {field_name} format.',
+            )
+
+        try:
+            value_int = int(value_str)
+            if value_int < 0:
+                raise rest_framework.exceptions.ValidationError(
+                    f'{field_name.capitalize()} cannot be negative.',
+                )
+            attrs[field_name] = value_int
+        except (ValueError, TypeError):
+            raise rest_framework.exceptions.ValidationError(
+                f'Invalid {field_name} format.',
+            )
+
+        return attrs
+
+    def _validate_country(self, query_params, attrs, field_errors):
+        countries_raw = query_params.getlist('country', [])
+
+        if '' in countries_raw:
+            raise rest_framework.exceptions.ValidationError(
+                'Invalid country format.',
+            )
+
+        country_codes = []
+        invalid_codes = []
+
+        for country_group in countries_raw:
+            if not country_group.strip():
+                continue
+
+            parts = [part.strip() for part in country_group.split(',')]
+
+            if '' in parts:
+                raise rest_framework.exceptions.ValidationError(
+                    'Invalid country format.',
+                )
+
+            country_codes.extend(parts)
+
+        country_codes_upper = [c.upper() for c in country_codes]
+
+        for code in country_codes_upper:
+            if len(code) != 2:
+                invalid_codes.append(code)
+                continue
+            try:
+                pycountry.countries.lookup(code)
+            except LookupError:
+                invalid_codes.append(code)
+
+        if invalid_codes:
+            field_errors['country'] = (
+                f'Invalid country codes: {", ".join(invalid_codes)}'
+            )
+
+        attrs['countries'] = country_codes
+        attrs.pop('country', None)
+
+
 class PromoReadOnlySerializer(rest_framework.serializers.ModelSerializer):
     promo_id = rest_framework.serializers.UUIDField(
         source='id',

promo_code/business/views.py

@@ -1,14 +1,11 @@
 import re
 
 import django.db.models
-import pycountry
-import rest_framework.exceptions
 import rest_framework.generics
 import rest_framework.permissions
 import rest_framework.response
 import rest_framework.serializers
 import rest_framework.status
-import rest_framework.views
 import rest_framework_simplejwt.exceptions
 import rest_framework_simplejwt.tokens
 import rest_framework_simplejwt.views
@@ -155,14 +152,21 @@ class CompanyPromoListView(rest_framework.generics.ListAPIView):
     serializer_class = business.serializers.PromoReadOnlySerializer
     pagination_class = business.pagination.CustomLimitOffsetPagination
 
+    def initial(self, request, *args, **kwargs):
+        super().initial(request, *args, **kwargs)
+
+        serializer = business.serializers.PromoListQuerySerializer(
+            data=request.query_params,
+        )
+        serializer.is_valid(raise_exception=True)
+        request.validated_query_params = serializer.validated_data
+
     def get_queryset(self):
+        params = self.request.validated_query_params
+        countries = [c.upper() for c in params.get('countries', [])]
+        sort_by = params.get('sort_by')
+
         queryset = business.models.Promo.objects.for_company(self.request.user)
-        countries = [
-            country.strip()
-            for group in self.request.query_params.getlist('country', [])
-            for country in group.split(',')
-            if country.strip()
-        ]
 
         if countries:
             regex_pattern = r'(' + '|'.join(map(re.escape, countries)) + ')'
@@ -171,115 +175,9 @@ def get_queryset(self):
                 | django.db.models.Q(target__country__isnull=True),
             )
 
-        sort_by = self.request.query_params.get('sort_by')
-        if sort_by in ['active_from', 'active_until']:
-            queryset = queryset.order_by(f'-{sort_by}')
-        else:
-            queryset = queryset.order_by('-created_at')  # noqa: R504
-
-        return queryset  # noqa: R504
-
-    def list(self, request, *args, **kwargs):
-        try:
-            self.validate_query_params()
-        except rest_framework.exceptions.ValidationError as e:
-            return rest_framework.response.Response(
-                e.detail,
-                status=rest_framework.status.HTTP_400_BAD_REQUEST,
-            )
-
-        return super().list(request, *args, **kwargs)
-
-    def validate_query_params(self):
-        self._validate_allowed_params()
-        errors = {}
-        self._validate_countries(errors)
-        self._validate_sort_by(errors)
-        self._validate_offset()
-        self._validate_limit()
-        if errors:
-            raise rest_framework.exceptions.ValidationError(errors)
-
-    def _validate_allowed_params(self):
-        allowed_params = {'country', 'limit', 'offset', 'sort_by'}
-        unexpected_params = (
-            set(self.request.query_params.keys()) - allowed_params
-        )
-
-        if unexpected_params:
-            raise rest_framework.exceptions.ValidationError('Invalid params.')
-
-    def _validate_countries(self, errors):
-        countries = self.request.query_params.getlist('country', [])
-        country_list = []
-
-        for country_group in countries:
-            parts = [part.strip() for part in country_group.split(',')]
-
-            if any(part == '' for part in parts):
-                raise rest_framework.exceptions.ValidationError(
-                    'Invalid country format.',
-                )
-
-            country_list.extend(parts)
-
-        country_list = [c.strip().upper() for c in country_list if c.strip()]
-
-        invalid_countries = []
-
-        for code in country_list:
-            if len(code) != 2:
-                invalid_countries.append(code)
-                continue
-
-            try:
-                pycountry.countries.lookup(code)
-            except LookupError:
-                invalid_countries.append(code)
-
-        if invalid_countries:
-            errors['country'] = (
-                f'Invalid country codes: {", ".join(invalid_countries)}'
-            )
-
-    def _validate_sort_by(self, errors):
-        sort_by = self.request.query_params.get('sort_by')
-        if sort_by and sort_by not in ['active_from', 'active_until']:
-            errors['sort_by'] = (
-                'Invalid sort_by parameter. '
-                'Available values: active_from, active_until'
-            )
+        ordering = f'-{sort_by}' if sort_by else '-created_at'
 
-    def _validate_offset(self):
-        offset = self.request.query_params.get('offset')
-        if offset is not None:
-            try:
-                offset = int(offset)
-            except (TypeError, ValueError):
-                raise rest_framework.exceptions.ValidationError(
-                    'Invalid offset format.',
-                )
-
-            if offset < 0:
-                raise rest_framework.exceptions.ValidationError(
-                    'Offset cannot be negative.',
-                )
-
-    def _validate_limit(self):
-        limit = self.request.query_params.get('limit')
-
-        if limit is not None:
-            try:
-                limit = int(limit)
-            except (TypeError, ValueError):
-                raise rest_framework.exceptions.ValidationError(
-                    'Invalid limit format.',
-                )
-
-            if limit < 0:
-                raise rest_framework.exceptions.ValidationError(
-                    'Limit cannot be negative.',
-                )
+        return queryset.order_by(ordering)
 
 
 class CompanyPromoDetailView(rest_framework.generics.RetrieveUpdateAPIView):