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, 1 deletions

diff --git a/src/buildstream/_artifactelement.py b/src/buildstream/_artifactelement.py
index cfd3c29c8..e3bffaf72 100644
--- a/src/buildstream/_artifactelement.py
+++ b/src/buildstream/_artifactelement.py
@@ -16,12 +16,18 @@
 #
 #  Authors:
 #        James Ennis <james.ennis@codethink.co.uk>
+
+from typing import TYPE_CHECKING
+
 from . import Element
 from . import _cachekey
 from ._exceptions import ArtifactElementError
 from ._loader.metaelement import MetaElement
 from .types import Scope
 
+if TYPE_CHECKING:
+    from typing import Dict
+
 
 # ArtifactElement()
 #
@@ -33,7 +39,8 @@ from .types import Scope
 #
 class ArtifactElement(Element):
 
-    __instantiated_artifacts = {}  # A hash of ArtifactElement by ref
+    # A hash of ArtifactElement by ref
+    __instantiated_artifacts = {}   # type: Dict[str, ArtifactElement]
 
     def __init__(self, context, ref):
         _, element, key = verify_artifact_ref(ref)