Massive changes for rest transport

* Rest transport methods now return callable class instances to
facilitate patching in generated unit tests
* Inject into generated unit tests via parameters
* Add to showcase tests and genericize showcase test file names
* Minor wording cleanups
* Minor bug cleanups

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/transports/base.py.j2

@@ -3,25 +3,29 @@
 {% block content %}
 
 import abc
-import typing
-import packaging.version
+from typing import Awaitable, Callable, Dict, Optional, Sequence, Union
 import pkg_resources
-from typing import Dict, Optional, {% if service.any_server_streaming %}Iterable, {% endif %}{% if service.any_client_streaming %}Iterator, {% endif %}Sequence, Tuple, Type, Union
 
 import google.auth  # type: ignore
-from google.api_core import gapic_v1    # type: ignore
+import google.api_core  # type: ignore
 from google.api_core import exceptions as core_exceptions  # type: ignore
+from google.api_core import gapic_v1    # type: ignore
 from google.api_core import retry as retries  # type: ignore
 {% if service.has_lro %}
 from google.api_core import operations_v1
 {% endif %}
 from google.auth import credentials as ga_credentials  # type: ignore
+from google.oauth2 import service_account # type: ignore
 
 {% filter sort_lines %}
 {% for method in service.methods.values() %}
 {{ method.input.ident.python_import }}
 {{ method.output.ident.python_import }}
 {% endfor %}
+{% if opts.add_iam_methods %}
+from google.iam.v1 import iam_policy_pb2  # type: ignore
+from google.iam.v1 import policy_pb2  # type: ignore
+{% endif %}
 {% endfilter %}
 
 try:
@@ -33,17 +37,8 @@ try:
 except pkg_resources.DistributionNotFound:
     DEFAULT_CLIENT_INFO = gapic_v1.client_info.ClientInfo()
 
-try:
-    # google.auth.__version__ was added in 1.26.0
-    _GOOGLE_AUTH_VERSION = google.auth.__version__
-except AttributeError:
-    try:  # try pkg_resources if it is available
-        _GOOGLE_AUTH_VERSION = pkg_resources.get_distribution("google-auth").version
-    except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-        _GOOGLE_AUTH_VERSION = None
 
-
-class {{ service.name }}Transport(metaclass=abc.ABCMeta):
+class {{ service.name }}Transport(abc.ABC):
     """Abstract transport class for {{ service.name }}."""
 
     AUTH_SCOPES = (
@@ -56,13 +51,14 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
 
     def __init__(
             self, *,
-            host: str{% if service.host %} = '{{ service.host }}'{% endif %},
+            host: str = DEFAULT_HOST,
             credentials: ga_credentials.Credentials = None,
             credentials_file: Optional[str] = None,
             scopes: Optional[Sequence[str]] = None,
             quota_project_id: Optional[str] = None,
             client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
             always_use_jwt_access: Optional[bool] = False,
+            **kwargs,
             ) -> None:
         """Instantiate the transport.
 
@@ -93,7 +89,7 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
             host += ':443'
         self._host = host
 
-        scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
+        scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
 
         # Save the scopes.
         self._scopes = scopes
@@ -112,40 +108,16 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
         elif credentials is None:
             credentials, _ = google.auth.default(**scopes_kwargs, quota_project_id=quota_project_id)
 
+        # If the credentials are service account credentials, then always try to use self signed JWT.
         if always_use_jwt_access and isinstance(credentials, service_account.Credentials) and hasattr(service_account.Credentials, "with_always_use_jwt_access"):
             credentials = credentials.with_always_use_jwt_access(True)
 
-        # If no credentials are provided, then determine the appropriate
-        # defaults.
-        if credentials is None:
-            credentials, _ = google.auth.default(scopes=self.AUTH_SCOPES)
-
         # Save the credentials.
         self._credentials = credentials
 
-        # Lifted into its own function so it can be stubbed out during tests.
-        self._prep_wrapped_messages(client_info)
-
-
-    # TODO: Remove this function once google-auth >= 1.25.0 is required
-    @classmethod
-    def _get_scopes_kwargs(cls, host: str, scopes: Optional[Sequence[str]]) -> Dict[str, Optional[Sequence[str]]]:
-        """Returns scopes kwargs to pass to google-auth methods depending on the google-auth version"""
-
-        scopes_kwargs = {}
-
-        if _GOOGLE_AUTH_VERSION and (
-            packaging.version.parse(_GOOGLE_AUTH_VERSION)
-            >= packaging.version.parse("1.25.0")
-        ):
-            scopes_kwargs = {"scopes": scopes, "default_scopes": cls.AUTH_SCOPES}
-        else:
-            scopes_kwargs = {"scopes": scopes or cls.AUTH_SCOPES}
-
-        return scopes_kwargs
 
     def _prep_wrapped_messages(self, client_info):
-        # Precomputed wrapped methods
+        # Precompute the wrapped methods.
         self._wrapped_methods = {
             {% for method in service.methods.values() %}
             self.{{ method.name|snake_case }}: gapic_v1.method.wrap_method(
@@ -172,7 +144,7 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
     def close(self):
         """Closes resources associated with the transport.
 
-        .. warning::
+       .. warning::
             Only call this method if the transport is NOT shared
             with other clients - this may cause errors in other clients!
         """
@@ -183,18 +155,53 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
     @property
     def operations_client(self):
         """Return the client designed to process long-running operations."""
-        raise NotImplementedError
+        raise NotImplementedError()
     {% endif %}
     {% for method in service.methods.values() %}
 
     @property
-    def {{ method.name|snake_case }}(self) -> typing.Callable[
+    def {{ method.name|snake_case }}(self) -> Callable[
             [{{ method.input.ident }}],
-            {{ method.output.ident }}]:
-        raise NotImplementedError
+            Union[
+                {{ method.output.ident }},
+                Awaitable[{{ method.output.ident }}]
+            ]]:
+        raise NotImplementedError()
     {% endfor %}
 
 
+    {% if opts.add_iam_methods %}
+    @property
+    def set_iam_policy(
+        self,
+    ) -> Callable[
+        [iam_policy_pb2.SetIamPolicyRequest],
+        Union[policy_pb2.Policy, Awaitable[policy_pb2.Policy]],
+    ]:
+        raise NotImplementedError()
+
+    @property
+    def get_iam_policy(
+        self,
+    ) -> Callable[
+        [iam_policy_pb2.GetIamPolicyRequest],
+        Union[policy_pb2.Policy, Awaitable[policy_pb2.Policy]],
+    ]:
+        raise NotImplementedError()
+
+    @property
+    def test_iam_permissions(
+        self,
+    ) -> Callable[
+        [iam_policy_pb2.TestIamPermissionsRequest],
+        Union[
+            iam_policy_pb2.TestIamPermissionsResponse,
+            Awaitable[iam_policy_pb2.TestIamPermissionsResponse],
+        ],
+    ]:
+        raise NotImplementedError()
+    {% endif %}
+
 __all__ = (
     '{{ service.name }}Transport',
 )