fix: add mypy checking + 'py.typed' file

google/__init__.py

@@ -21,4 +21,4 @@
 except ImportError:
     import pkgutil
 
-    __path__ = pkgutil.extend_path(__path__, __name__)
+    __path__ = pkgutil.extend_path(__path__, __name__)  # type: ignore

google/api_core/client_info.py

@@ -19,6 +19,7 @@
 """
 
 import platform
+from typing import Union
 
 import pkg_resources
 
@@ -27,6 +28,8 @@
 _PY_VERSION = platform.python_version()
 _API_CORE_VERSION = api_core_version.__version__
 
+_GRPC_VERSION: Union[str, None]
+
 try:
     _GRPC_VERSION = pkg_resources.get_distribution("grpcio").version
 except pkg_resources.DistributionNotFound:  # pragma: NO COVER

google/api_core/exceptions.py

@@ -22,17 +22,18 @@
 from __future__ import unicode_literals
 
 import http.client
+from typing import Dict
+from typing import Union
 
 try:
     import grpc
-
 except ImportError:  # pragma: NO COVER
     grpc = None
 
 # Lookup tables for mapping exceptions from HTTP and gRPC transports.
 # Populated by _GoogleAPICallErrorMeta
-_HTTP_CODE_TO_EXCEPTION = {}
-_GRPC_CODE_TO_EXCEPTION = {}
+_HTTP_CODE_TO_EXCEPTION: Dict[int, Exception] = {}
+_GRPC_CODE_TO_EXCEPTION: Dict[int, Exception] = {}
 
 # Additional lookup table to map integer status codes to grpc status code
 # grpc does not currently support initializing enums from ints
@@ -100,7 +101,7 @@ class GoogleAPICallError(GoogleAPIError, metaclass=_GoogleAPICallErrorMeta):
             gRPC call metadata.
     """
 
-    code = None
+    code: Union[int, None] = None
     """Optional[int]: The HTTP status code associated with this error.
 
     This may be ``None`` if the exception does not have a direct mapping

google/api_core/py.typed

@@ -0,0 +1,2 @@
+# Marker file for PEP 561.
+# The google-api-core package uses inline types.

mypy.ini

@@ -0,0 +1,7 @@
+[mypy]
+python_version = 3.6
+namespace_packages = True
+ignore_missing_imports = True
+
+[mypy-google.protobuf]
+ignore_missing_imports = True

noxfile.py

@@ -35,6 +35,7 @@
     "unit_grpc_gcp",
     "cover",
     "pytype",
+    "mypy",
     "lint",
     "lint_setup_py",
     "blacken",
@@ -155,6 +156,14 @@ def pytype(session):
     session.run("pytype")
 
 
+@nox.session(python=DEFAULT_PYTHON_VERSION)
+def mypy(session):
+    """Run type-checking."""
+    session.install(".[grpc, grpcgcp]", "mypy")
+    session.install("types-setuptools", "types-requests", "types-mock")
+    session.run("mypy", "google", "tests")
+
+
 @nox.session(python="3.6")
 def cover(session):
     """Run the final coverage report.