Upgrade sanity Docker image to debian:stretch

* Use latest pylint in Python 3.7 (they dropped support for PY2)
* Make latest pylint happy
* Forced to upgrade to shellcheck 0.4.4
* Make shellcheck 0.4.4 happy
* Adopt reviewers' advice to reduce global disabled rules

3 files changed, 8 insertions, 11 deletions

diff --git a/src/python/grpcio/grpc/_auth.py b/src/python/grpcio/grpc/_auth.py
index c17824563d..9b990f490d 100644
--- a/src/python/grpcio/grpc/_auth.py
+++ b/src/python/grpcio/grpc/_auth.py
@@ -46,7 +46,7 @@ class GoogleCallCredentials(grpc.AuthMetadataPlugin):
 
         # Hack to determine if these are JWT creds and we need to pass
         # additional_claims when getting a token
-        self._is_jwt = 'additional_claims' in inspect.getargspec(
+        self._is_jwt = 'additional_claims' in inspect.getargspec(  # pylint: disable=deprecated-method
             credentials.get_access_token).args
 
     def __call__(self, context, callback):
diff --git a/src/python/grpcio/grpc/_channel.py b/src/python/grpcio/grpc/_channel.py
index 35fa82d56b..951c6f33ff 100644
--- a/src/python/grpcio/grpc/_channel.py
+++ b/src/python/grpcio/grpc/_channel.py
@@ -525,7 +525,7 @@ class _UnaryUnaryMultiCallable(grpc.UnaryUnaryMultiCallable):
         state, operations, deadline, rendezvous = self._prepare(
             request, timeout, metadata, wait_for_ready)
         if state is None:
-            raise rendezvous
+            raise rendezvous  # pylint: disable-msg=raising-bad-type
         else:
             call = self._channel.segregated_call(
                 0, self._method, None, deadline, metadata, None
@@ -535,7 +535,7 @@ class _UnaryUnaryMultiCallable(grpc.UnaryUnaryMultiCallable):
                 ),))
             event = call.next_event()
             _handle_event(event, state, self._response_deserializer)
-            return state, call,
+            return state, call
 
     def __call__(self,
                  request,
@@ -566,7 +566,7 @@ class _UnaryUnaryMultiCallable(grpc.UnaryUnaryMultiCallable):
         state, operations, deadline, rendezvous = self._prepare(
             request, timeout, metadata, wait_for_ready)
         if state is None:
-            raise rendezvous
+            raise rendezvous  # pylint: disable-msg=raising-bad-type
         else:
             event_handler = _event_handler(state, self._response_deserializer)
             call = self._managed_call(
@@ -599,7 +599,7 @@ class _UnaryStreamMultiCallable(grpc.UnaryStreamMultiCallable):
         initial_metadata_flags = _InitialMetadataFlags().with_wait_for_ready(
             wait_for_ready)
         if serialized_request is None:
-            raise rendezvous
+            raise rendezvous  # pylint: disable-msg=raising-bad-type
         else:
             state = _RPCState(_UNARY_STREAM_INITIAL_DUE, None, None, None, None)
             operationses = (
@@ -653,7 +653,7 @@ class _StreamUnaryMultiCallable(grpc.StreamUnaryMultiCallable):
                 state.condition.notify_all()
                 if not state.due:
                     break
-        return state, call,
+        return state, call
 
     def __call__(self,
                  request_iterator,
@@ -745,10 +745,10 @@ class _InitialMetadataFlags(int):
     def with_wait_for_ready(self, wait_for_ready):
         if wait_for_ready is not None:
             if wait_for_ready:
-                self = self.__class__(self | cygrpc.InitialMetadataFlags.wait_for_ready | \
+                return self.__class__(self | cygrpc.InitialMetadataFlags.wait_for_ready | \
                     cygrpc.InitialMetadataFlags.wait_for_ready_explicitly_set)
             elif not wait_for_ready:
-                self = self.__class__(self & ~cygrpc.InitialMetadataFlags.wait_for_ready | \
+                return self.__class__(self & ~cygrpc.InitialMetadataFlags.wait_for_ready | \
                     cygrpc.InitialMetadataFlags.wait_for_ready_explicitly_set)
         return self
 
diff --git a/src/python/grpcio/grpc/_utilities.py b/src/python/grpcio/grpc/_utilities.py
index d90b34bcbd..2938a38b44 100644
--- a/src/python/grpcio/grpc/_utilities.py
+++ b/src/python/grpcio/grpc/_utilities.py
@@ -132,15 +132,12 @@ class _ChannelReadyFuture(grpc.Future):
 
     def result(self, timeout=None):
         self._block(timeout)
-        return None
 
     def exception(self, timeout=None):
         self._block(timeout)
-        return None
 
     def traceback(self, timeout=None):
         self._block(timeout)
-        return None
 
     def add_done_callback(self, fn):
         with self._condition: