Added pillar.flask_extra.vary_xhr() decorator

This produces a 'Vary: X-Requested-With' header on the response of
decorated view functions, which indicates to the browser (or intermediate
proxy servers) that the response may/will will be different for XHR and
non-XHR requests.

pillar/flask_extra.py

@@ -0,0 +1,29 @@
+import functools
+import flask
+
+
+def add_response_headers(headers: dict):
+    """This decorator adds the headers passed in to the response"""
+
+    def decorator(f):
+        @functools.wraps(f)
+        def decorated_function(*args, **kwargs):
+            resp = flask.make_response(f(*args, **kwargs))
+            h = resp.headers
+            for header, value in headers.items():
+                h[header] = value
+            return resp
+
+        return decorated_function
+
+    return decorator
+
+
+def vary_xhr():
+    """View function decorator; adds HTTP header "Vary: X-Requested-With" to the response"""
+
+    def decorator(f):
+        header_adder = add_response_headers({'Vary': 'X-Requested-With'})
+        return header_adder(f)
+
+    return decorator

tests/test_flask_extra.py

@@ -0,0 +1,35 @@
+import unittest
+
+import flask
+
+
+class FlaskExtraTest(unittest.TestCase):
+    def test_vary_xhr(self):
+        import pillar.flask_extra
+
+        class TestApp(flask.Flask):
+            def __init__(self, *args, **kwargs):
+                super().__init__(*args, **kwargs)
+
+                self.add_url_rule('/must-vary', 'must-vary', self.must_vary)
+                self.add_url_rule('/no-vary', 'no-vary', self.no_vary)
+
+            @pillar.flask_extra.vary_xhr()
+            def must_vary(self):
+                return 'yay'
+
+            def no_vary(self):
+                return 'nah', 201
+
+        app = TestApp(__name__)
+        client = app.test_client()
+
+        resp = client.get('/must-vary')
+        self.assertEqual(200, resp.status_code)
+        self.assertEqual('X-Requested-With', resp.headers['Vary'])
+        self.assertEqual('yay', resp.data.decode())
+
+        resp = client.get('/no-vary')
+        self.assertEqual(201, resp.status_code)
+        self.assertNotIn('Vary', resp.headers)
+        self.assertEqual('nah', resp.data.decode())