10 files changed, 21 insertions, 11 deletions
diff --git a/changelog.d/9742.misc b/changelog.d/9742.misc
new file mode 100644
index 0000000000..681ab04df8
--- /dev/null
+++ b/changelog.d/9742.misc
@@ -0,0 +1 @@
+Start linting mypy with `no_implicit_optional`.
\ No newline at end of file
diff --git a/mypy.ini b/mypy.ini
index 3ae5d45787..32e6197409 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -8,6 +8,7 @@ show_traceback = True
 mypy_path = stubs
 warn_unreachable = True
 local_partial_types = True
+no_implicit_optional = True
 
 # To find all folders that pass mypy you run:
 #
diff --git a/synapse/handlers/account_validity.py b/synapse/handlers/account_validity.py
index d781bb251d..bee1447c2e 100644
--- a/synapse/handlers/account_validity.py
+++ b/synapse/handlers/account_validity.py
@@ -18,7 +18,7 @@ import email.utils
 import logging
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
-from typing import TYPE_CHECKING, List
+from typing import TYPE_CHECKING, List, Optional
 
 from synapse.api.errors import StoreError, SynapseError
 from synapse.logging.context import make_deferred_yieldable
@@ -241,7 +241,10 @@ class AccountValidityHandler:
         return True
 
     async def renew_account_for_user(
-        self, user_id: str, expiration_ts: int = None, email_sent: bool = False
+        self,
+        user_id: str,
+        expiration_ts: Optional[int] = None,
+        email_sent: bool = False,
     ) -> int:
         """Renews the account attached to a given user by pushing back the
         expiration date by the current validity period in the server's
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 2ad9b6d930..739653a3fa 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1008,7 +1008,7 @@ class E2eKeysHandler:
         return signature_list, failures
 
     async def _get_e2e_cross_signing_verify_key(
-        self, user_id: str, key_type: str, from_user_id: str = None
+        self, user_id: str, key_type: str, from_user_id: Optional[str] = None
     ) -> Tuple[JsonDict, str, VerifyKey]:
         """Fetch locally or remotely query for a cross-signing public key.
 
diff --git a/synapse/http/client.py b/synapse/http/client.py
index a0caba84e4..e691ba6d88 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -590,7 +590,7 @@ class SimpleHttpClient:
         uri: str,
         json_body: Any,
         args: Optional[QueryParams] = None,
-        headers: RawHeaders = None,
+        headers: Optional[RawHeaders] = None,
     ) -> Any:
         """Puts some json to the given URI.
 
diff --git a/synapse/notifier.py b/synapse/notifier.py
index d35c1f3f02..c178db57e3 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -548,7 +548,7 @@ class Notifier:
         pagination_config: PaginationConfig,
         timeout: int,
         is_guest: bool = False,
-        explicit_room_id: str = None,
+        explicit_room_id: Optional[str] = None,
     ) -> EventStreamResult:
         """For the given user and rooms, return any new events for them. If
         there are no new events wait for up to `timeout` milliseconds for any
diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 2f4d407f94..98bdeb0ec6 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -60,7 +60,7 @@ class ConstantProperty(Generic[T, V]):
 
     constant = attr.ib()  # type: V
 
-    def __get__(self, obj: Optional[T], objtype: Type[T] = None) -> V:
+    def __get__(self, obj: Optional[T], objtype: Optional[Type[T]] = None) -> V:
         return self.constant
 
     def __set__(self, obj: Optional[T], value: V):
diff --git a/synapse/storage/databases/main/group_server.py b/synapse/storage/databases/main/group_server.py
index ac07e0197b..8f462dfc31 100644
--- a/synapse/storage/databases/main/group_server.py
+++ b/synapse/storage/databases/main/group_server.py
@@ -1027,8 +1027,8 @@ class GroupServerStore(GroupServerWorkerStore):
         user_id: str,
         is_admin: bool = False,
         is_public: bool = True,
-        local_attestation: dict = None,
-        remote_attestation: dict = None,
+        local_attestation: Optional[dict] = None,
+        remote_attestation: Optional[dict] = None,
     ) -> None:
         """Add a user to the group server.
 
diff --git a/synapse/util/caches/deferred_cache.py b/synapse/util/caches/deferred_cache.py
index 1adc92eb90..dd392cf694 100644
--- a/synapse/util/caches/deferred_cache.py
+++ b/synapse/util/caches/deferred_cache.py
@@ -283,7 +283,9 @@ class DeferredCache(Generic[KT, VT]):
         # we return a new Deferred which will be called before any subsequent observers.
         return observable.observe()
 
-    def prefill(self, key: KT, value: VT, callback: Callable[[], None] = None):
+    def prefill(
+        self, key: KT, value: VT, callback: Optional[Callable[[], None]] = None
+    ):
         callbacks = [callback] if callback else []
         self.cache.set(key, value, callbacks=callbacks)
 
diff --git a/tests/rest/client/v2_alpha/test_auth.py b/tests/rest/client/v2_alpha/test_auth.py
index 9734a2159a..ed433d9333 100644
--- a/tests/rest/client/v2_alpha/test_auth.py
+++ b/tests/rest/client/v2_alpha/test_auth.py
@@ -13,7 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import Union
+from typing import Optional, Union
 
 from twisted.internet.defer import succeed
 
@@ -74,7 +74,10 @@ class FallbackAuthTests(unittest.HomeserverTestCase):
         return channel
 
     def recaptcha(
-        self, session: str, expected_post_response: int, post_session: str = None
+        self,
+        session: str,
+        expected_post_response: int,
+        post_session: Optional[str] = None,
     ) -> None:
         """Get and respond to a fallback recaptcha. Returns the second request."""
         if post_session is None: