Replace mutable "allowed_hosts" argument of TrustedHostMiddleware with None (#448)

* Replace mutable "allowed_hosts" argument of starlette.middleware.trustedhost.TrustedHostMiddleware with None.

* Add unit test to cover case when TrustedHostMiddleware.allowed_hosts is None

* Fix style issues

* Code style fixes

* Fix unit tests for TrustedHostMiddleware

starlette/middleware/trustedhost.py

@@ -11,9 +11,12 @@ class TrustedHostMiddleware:
     def __init__(
         self,
         app: ASGIApp,
-        allowed_hosts: typing.Sequence[str] = ["*"],
+        allowed_hosts: typing.Sequence[str] = None,
         www_redirect: bool = True,
     ) -> None:
+        if allowed_hosts is None:
+            allowed_hosts = ["*"]
+
         for pattern in allowed_hosts:
             assert "*" not in pattern[1:], ENFORCE_DOMAIN_WILDCARD
             if pattern.startswith("*") and pattern != "*":

tests/middleware/test_trusted_host.py

@@ -28,6 +28,12 @@ def test_trusted_host_middleware():
     assert response.status_code == 400
 
 
+def test_default_allowed_hosts():
+    app = Starlette()
+    middleware = TrustedHostMiddleware(app)
+    assert middleware.allowed_hosts == ["*"]
+
+
 def test_www_redirect():
     app = Starlette()