Reduce the impact on every request

HISTORY.rst

@@ -7,6 +7,7 @@ Pending release
 .. Insert new release notes below this line
 
 * Fix interpretation of '*' by not automatically adding quotes.
+* Optimize header generation to reduce impact on every request.
 
 2.1.0 (2019-04-28)
 ------------------

django_feature_policy.py

@@ -1,10 +1,13 @@
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
+from django.core.signals import setting_changed
+from django.dispatch import receiver
+from django.utils.functional import cached_property
 
 __version__ = '2.1.0'
 
 # Retrieved from Chrome document.featurePolicy.allowedFeatures()
-FEATURE_NAMES = [
+FEATURE_NAMES = {
     'accelerometer',
     'ambient-light-sensor',
     'autoplay',
@@ -34,22 +37,24 @@
     'vertical-scroll',
     'vr',
     'wake-lock',
-]
+}
 
 
 class FeaturePolicyMiddleware:
     def __init__(self, get_response):
         self.get_response = get_response
-        self.get_header_value()  # to check
+        self.header_value  # Access at setup so ImproperlyConfigured can be raised
+        receiver(setting_changed)(self.clear_header_value)
 
     def __call__(self, request):
         response = self.get_response(request)
-        value = self.get_header_value()
+        value = self.header_value
         if value:
             response['Feature-Policy'] = value
         return response
 
-    def get_header_value(self):
+    @cached_property
+    def header_value(self):
         setting = getattr(settings, 'FEATURE_POLICY', {})
         pieces = []
         for feature, values in setting.items():
@@ -68,3 +73,10 @@ def get_header_value(self):
                     item.append(value)
             pieces.append(' '.join(item))
         return '; '.join(pieces)
+
+    def clear_header_value(self, setting, **kwargs):
+        if setting == 'FEATURE_POLICY':
+            try:
+                del self.header_value
+            except AttributeError:
+                pass

tests/test_django_feature_policy.py

@@ -81,3 +81,23 @@ def test_unknown_feature(client, settings):
 
     with pytest.raises(ImproperlyConfigured):
         client.get('/')
+
+
+def test_setting_changing(client, settings):
+    settings.FEATURE_POLICY = {}
+    client.get('/')  # Forces middleware instantiation
+    settings.FEATURE_POLICY = {'geolocation': 'self'}
+
+    resp = client.get('/')
+
+    assert resp['Feature-Policy'] == "geolocation 'self'"
+
+
+def test_other_setting_changing(client, settings):
+    settings.FEATURE_POLICY = {'geolocation': 'self'}
+    client.get('/')  # Forces middleware instantiation
+    settings.SECRET_KEY = 'foobar'
+
+    resp = client.get('/')
+
+    assert resp['Feature-Policy'] == "geolocation 'self'"