refactor: expose event loop as a property rather than a field

event loops for Session and Nvim are final and can't be changed.

pynvim/api/nvim.py

@@ -1,4 +1,6 @@
 """Main Nvim interface."""
+
+import asyncio
 import os
 import sys
 import threading
@@ -140,7 +142,16 @@ def __init__(
             self._err_cb: Callable[[str], Any] = lambda _: None
         else:
             self._err_cb = err_cb
-        self.loop = self._session.loop._loop
+
+    @property
+    def loop(self) -> asyncio.AbstractEventLoop:
+        """Get the event loop (exposed to rplugins)."""  # noqa
+
+        # see #294: for python 3.4+, the only available and guaranteed
+        # implementation of msgpack_rpc BaseEventLoop is the AsyncioEventLoop.
+        # The underlying asyncio event loop is exposed to rplugins.
+        # pylint: disable=protected-access
+        return self._session.loop._loop  # type: ignore
 
     def _from_nvim(self, obj: Any, decode: Optional[TDecodeMode] = None) -> Any:
         if decode is None:

pynvim/msgpack_rpc/session.py

@@ -11,6 +11,7 @@
 
 from pynvim.compat import check_async
 from pynvim.msgpack_rpc.async_session import AsyncSession
+from pynvim.msgpack_rpc.event_loop.base import BaseEventLoop
 
 if sys.version_info < (3, 8):
     from typing_extensions import Literal
@@ -42,7 +43,7 @@ class Notification(NamedTuple):
 Message = Union[Request, Notification]
 
 
-class Session(object):
+class Session:
 
     """Msgpack-rpc session layer that uses coroutines for a synchronous API.
 
@@ -59,11 +60,15 @@ def __init__(self, async_session: AsyncSession):
         self._pending_messages: Deque[Message] = deque()
         self._is_running = False
         self._setup_exception: Optional[Exception] = None
-        self.loop = async_session.loop
         self._loop_thread: Optional[threading.Thread] = None
         self.error_wrapper: Callable[[Tuple[int, str]], Exception] = \
             lambda e: Exception(e[1])
 
+    @property
+    def loop(self) -> BaseEventLoop:
+        """Get the underlying msgpack EventLoop."""
+        return self._async_session.loop
+
     def threadsafe_call(
         self, fn: Callable[..., Any], *args: Any, **kwargs: Any
     ) -> None: