move url matching after opening session

CHANGES.rst

@@ -71,6 +71,10 @@ Unreleased
 -   The ``flask run`` command no longer fails if Python is not built
     with SSL support. Using the ``--cert`` option will show an
     appropriate error message. :issue:`3211`
+-   URL matching now occurs after the request context is pushed, rather
+    than when it's created. This allows custom URL converters to access
+    the app and request contexts, such as to query a database for an id.
+    :issue:`3088`
 
 .. _#2935: https://github.com/pallets/flask/issues/2935
 .. _#2957: https://github.com/pallets/flask/issues/2957

src/flask/ctx.py

@@ -347,8 +347,8 @@ class RequestContext(object):
         of the request.
         """
         try:
-            url_rule, self.request.view_args = self.url_adapter.match(return_rule=True)
-            self.request.url_rule = url_rule
+            result = self.url_adapter.match(return_rule=True)
+            self.request.url_rule, self.request.view_args = result
         except HTTPException as e:
             self.request.routing_exception = e
 
@@ -381,9 +381,6 @@ class RequestContext(object):
 
         _request_ctx_stack.push(self)
 
-        if self.url_adapter is not None:
-            self.match_request()
-
         # Open the session at the moment that the request context is available.
         # This allows a custom open_session method to use the request context.
         # Only open a new session if this is the first time the request was
@@ -395,6 +392,9 @@ class RequestContext(object):
             if self.session is None:
                 self.session = session_interface.make_null_session(self.app)
 
+        if self.url_adapter is not None:
+            self.match_request()
+
     def pop(self, exc=_sentinel):
         """Pops the request context and unbinds it by doing that.  This will
         also trigger the execution of functions registered by the

tests/test_converters.py

@@ -1,9 +1,10 @@
-from flask.globals import _app_ctx_stack
+from werkzeug.routing import BaseConverter
+
+from flask import has_request_context
+from flask import url_for
 
 
 def test_custom_converters(app, client):
-    from werkzeug.routing import BaseConverter
-
     class ListConverter(BaseConverter):
         def to_python(self, value):
             return value.split(",")
@@ -20,19 +21,20 @@ def test_custom_converters(app, client):
 
     assert client.get("/1,2,3").data == b"1|2|3"
 
+    with app.test_request_context():
+        assert url_for("index", args=[4, 5, 6]) == "/4,5,6"
 
-def test_model_converters(app, client):
-    from werkzeug.routing import BaseConverter
 
-    class ModelConverterTester(BaseConverter):
+def test_context_available(app, client):
+    class ContextConverter(BaseConverter):
         def to_python(self, value):
-            assert _app_ctx_stack.top is not None
+            assert has_request_context()
             return value
 
-    app.url_map.converters["model"] = ModelConverterTester
+    app.url_map.converters["ctx"] = ContextConverter
 
-    @app.route("/<model:user_name>")
-    def index(user_name):
-        return user_name, 200
+    @app.route("/<ctx:name>")
+    def index(name):
+        return name
 
-    client.get("/admin").data
+    assert client.get("/admin").data == b"admin"

tests/test_testing.py

@@ -154,6 +154,9 @@ def test_blueprint_with_subdomain():
     ctx = app.test_request_context("/", subdomain="xxx")
     assert ctx.request.url == "http://xxx.example.com:1234/foo/"
 
+    with ctx:
+        assert ctx.request.blueprint == bp.name
+
     rv = client.get("/", subdomain="xxx")
     assert rv.data == b"http://xxx.example.com:1234/foo/"