diff --git a/scripts-dev/mypy_synapse_plugin.py b/scripts-dev/mypy_synapse_plugin.py
new file mode 100644
index 0000000000..a5b88731f1
--- /dev/null
+++ b/scripts-dev/mypy_synapse_plugin.py
@@ -0,0 +1,85 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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.
+
+"""This is a mypy plugin for Synpase to deal with some of the funky typing that
+can crop up, e.g the cache descriptors.
+"""
+
+from typing import Callable, Optional
+
+from mypy.plugin import MethodSigContext, Plugin
+from mypy.typeops import bind_self
+from mypy.types import CallableType
+
+
+class SynapsePlugin(Plugin):
+ def get_method_signature_hook(
+ self, fullname: str
+ ) -> Optional[Callable[[MethodSigContext], CallableType]]:
+ if fullname.startswith(
+ "synapse.util.caches.descriptors._CachedFunction.__call__"
+ ):
+ return cached_function_method_signature
+ return None
+
+
+def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
+ """Fixes the `_CachedFunction.__call__` signature to be correct.
+
+ It already has *almost* the correct signature, except:
+
+ 1. the `self` argument needs to be marked as "bound"; and
+ 2. any `cache_context` argument should be removed.
+ """
+
+ # First we mark this as a bound function signature.
+ signature = bind_self(ctx.default_signature)
+
+ # Secondly, we remove any "cache_context" args.
+ #
+ # Note: We should be only doing this if `cache_context=True` is set, but if
+ # it isn't then the code will raise an exception when its called anyway, so
+ # its not the end of the world.
+ context_arg_index = None
+ for idx, name in enumerate(signature.arg_names):
+ if name == "cache_context":
+ context_arg_index = idx
+ break
+
+ if context_arg_index:
+ arg_types = list(signature.arg_types)
+ arg_types.pop(context_arg_index)
+
+ arg_names = list(signature.arg_names)
+ arg_names.pop(context_arg_index)
+
+ arg_kinds = list(signature.arg_kinds)
+ arg_kinds.pop(context_arg_index)
+
+ signature = signature.copy_modified(
+ arg_types=arg_types, arg_names=arg_names, arg_kinds=arg_kinds,
+ )
+
+ return signature
+
+
+def plugin(version: str):
+ # This is the entry point of the plugin, and let's us deal with the fact
+ # that the mypy plugin interface is *not* stable by looking at the version
+ # string.
+ #
+ # However, since we pin the version of mypy Synapse uses in CI, we don't
+ # really care.
+ return SynapsePlugin
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