Add initial mypy configuration and types

As a first step, add type hints to variables whose type `mypy` cannot
infer automatically. This is the minimal set of type hints that allow
running `mypy` without any arguments, and having it not fail.

We currently ignore C extensions that mypy can't process directly.
Later, we can look into generating stubs for such modules (potentially
automatically).

1 files changed, 8 insertions, 3 deletions

diff --git a/src/buildstream/_scheduler/queues/queue.py b/src/buildstream/_scheduler/queues/queue.py
index 8c81ce21d..745b59417 100644
--- a/src/buildstream/_scheduler/queues/queue.py
+++ b/src/buildstream/_scheduler/queues/queue.py
@@ -23,6 +23,7 @@ import os
 from collections import deque
 import heapq
 import traceback
+from typing import TYPE_CHECKING
 
 # Local imports
 from ..jobs import ElementJob, JobStatus
@@ -33,6 +34,9 @@ from ..._exceptions import BstError, ImplError, set_last_task_error
 from ..._message import Message, MessageType
 from ...types import FastEnum
 
+if TYPE_CHECKING:
+    from typing import List, Optional
+
 
 # Queue status for a given element
 #
@@ -56,9 +60,10 @@ class QueueStatus(FastEnum):
 class Queue():
 
     # These should be overridden on class data of of concrete Queue implementations
-    action_name = None
-    complete_name = None
-    resources = []                     # Resources this queues' jobs want
+    action_name = None      # type: Optional[str]
+    complete_name = None    # type: Optional[str]
+    # Resources this queues' jobs want
+    resources = []  # type: List[int]
 
     def __init__(self, scheduler):