Display missing route details.

starlette/routing.py

@@ -31,6 +31,11 @@ class NoMatchFound(Exception):
     if no matching route exists.
     """
 
+    def __init__(self, name: str, path_params: typing.Dict[str, typing.Any]) -> None:
+        super().__init__(
+            f'No route exists for name "{name}" and params "{path_params}".'
+        )
+
 
 class Match(Enum):
     NONE = 0
@@ -240,7 +245,7 @@ def url_path_for(self, name: str, **path_params: typing.Any) -> URLPath:
         expected_params = set(self.param_convertors.keys())
 
         if name != self.name or seen_params != expected_params:
-            raise NoMatchFound()
+            raise NoMatchFound(name, path_params)
 
         path, remaining_params = replace_params(
             self.path_format, self.param_convertors, path_params
@@ -306,7 +311,7 @@ def url_path_for(self, name: str, **path_params: typing.Any) -> URLPath:
         expected_params = set(self.param_convertors.keys())
 
         if name != self.name or seen_params != expected_params:
-            raise NoMatchFound()
+            raise NoMatchFound(name, path_params)
 
         path, remaining_params = replace_params(
             self.path_format, self.param_convertors, path_params
@@ -405,7 +410,7 @@ def url_path_for(self, name: str, **path_params: typing.Any) -> URLPath:
                     )
                 except NoMatchFound:
                     pass
-        raise NoMatchFound()
+        raise NoMatchFound(name, path_params)
 
     async def handle(self, scope: Scope, receive: Receive, send: Send) -> None:
         await self.app(scope, receive, send)
@@ -469,7 +474,7 @@ def url_path_for(self, name: str, **path_params: typing.Any) -> URLPath:
                     return URLPath(path=str(url), protocol=url.protocol, host=host)
                 except NoMatchFound:
                     pass
-        raise NoMatchFound()
+        raise NoMatchFound(name, path_params)
 
     async def handle(self, scope: Scope, receive: Receive, send: Send) -> None:
         await self.app(scope, receive, send)
@@ -590,7 +595,7 @@ def url_path_for(self, name: str, **path_params: typing.Any) -> URLPath:
                 return route.url_path_for(name, **path_params)
             except NoMatchFound:
                 pass
-        raise NoMatchFound()
+        raise NoMatchFound(name, path_params)
 
     async def startup(self) -> None:
         """

tests/test_routing.py

@@ -238,7 +238,9 @@ def test_url_path_for():
     assert app.url_path_for("homepage") == "/"
     assert app.url_path_for("user", username="tomchristie") == "/users/tomchristie"
     assert app.url_path_for("websocket_endpoint") == "/ws"
-    with pytest.raises(NoMatchFound):
+    with pytest.raises(
+        NoMatchFound, match='No route exists for name "broken" and params "{}".'
+    ):
         assert app.url_path_for("broken")
     with pytest.raises(AssertionError):
         app.url_path_for("user", username="tom/christie")