mypy_synapse_plugin.py

# -*- 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.nodes import ARG_NAMED_OPT
from mypy.plugin import MethodSigContext, Plugin
from mypy.typeops import bind_self
from mypy.types import CallableType, NoneType


class SynapsePlugin(Plugin):
    def get_method_signature_hook(
        self, fullname: str
    ) -> Optional[Callable[[MethodSigContext], CallableType]]:
        if fullname.startswith(
            "synapse.util.caches.descriptors._CachedFunction.__call__"
        ) or fullname.startswith(
            "synapse.util.caches.descriptors._LruCachedFunction.__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";
        2. any `cache_context` argument should be removed;
        3. an optional keyword argument `on_invalidated` should be added.
    """

    # 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

    arg_types = list(signature.arg_types)
    arg_names = list(signature.arg_names)
    arg_kinds = list(signature.arg_kinds)

    if context_arg_index:
        arg_types.pop(context_arg_index)
        arg_names.pop(context_arg_index)
        arg_kinds.pop(context_arg_index)

    # Third, we add an optional "on_invalidate" argument.
    #
    # This is a callable which accepts no input and returns nothing.
    calltyp = CallableType(
        arg_types=[],
        arg_kinds=[],
        arg_names=[],
        ret_type=NoneType(),
        fallback=ctx.api.named_generic_type("builtins.function", []),
    )

    arg_types.append(calltyp)
    arg_names.append("on_invalidate")
    arg_kinds.append(ARG_NAMED_OPT)  # Arg is an optional kwarg.

    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