feat: add credentials_file and scopes support

Author: busunkim96

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -196,15 +196,22 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         # instance provides an extensibility point for unusual situations.
         if isinstance(transport, {{ service.name }}Transport):
             # transport is a {{ service.name }}Transport instance.
-            if credentials:
+            if credentials or client_options.credentials_file:
                 raise ValueError('When providing a transport instance, '
                                  'provide its credentials directly.')
+            if client_options.scopes:
+                raise ValueError(
+                    "When providing a transport instance, "
+                    "provide its scopes directly."
+                )
             self._transport = transport
         else:
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
                 credentials=credentials,
-                host=client_options.api_endpoint,
+                credentials_file=client_options.credentials_file,
+                host=self.DEFAULT_ENDPOINT,
+                scopes=client_options.scopes,
                 api_mtls_endpoint=client_options.api_endpoint,
                 client_cert_source=client_options.client_cert_source,
             )

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/base.py.j2

@@ -30,6 +30,8 @@ class {{ service.name }}Transport(abc.ABC):
             self, *,
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,
+            credentials_file: str = None,
+            scopes: Sequence[str] = self.AUTH_SCOPES,
             **kwargs,
             ) -> None:
         """Instantiate the transport.
@@ -42,6 +44,10 @@ class {{ service.name }}Transport(abc.ABC):
                 credentials identify the application to the service; if none
                 are specified, the client will attempt to ascertain the
                 credentials from the environment.
+            credentials_file (Optional[str]): A file with credentials that can
+                be loaded with :func:`google.auth.load_credentials_from_file`.
+                This argument is mutually exclusive with credentials.
+            scope (Optional[Sequence[str]]): A list of scopes.
         """
         # Save the hostname. Default to port 443 (HTTPS) if none is specified.
         if ':' not in host:
@@ -50,8 +56,13 @@ class {{ service.name }}Transport(abc.ABC):
 
         # If no credentials are provided, then determine the appropriate
         # defaults.
-        if credentials is None:
-            credentials, _ = auth.default(scopes=self.AUTH_SCOPES)
+        if credentials and credentials_file:
+            raise ValueError("'credentials_file' and 'credentials' are mutually exclusive")
+        
+        if credentials_file is not None:
+            credentials, _ = auth.load_credentials_from_file(credentials_file, scopes=scopes)
+        elif credentials is None:
+            credentials, _ = auth.default(scopes=scopes)
 
         # Save the credentials.
         self._credentials = credentials

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2

@@ -40,6 +40,8 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
     def __init__(self, *,
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,
+            credentials_file: str = None,
+            scopes: Sequence[str] = None,
             channel: grpc.Channel = None,
             api_mtls_endpoint: str = None,
             client_cert_source: Callable[[], Tuple[bytes, bytes]] = None) -> None:
@@ -54,6 +56,11 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
                 are specified, the client will attempt to ascertain the
                 credentials from the environment.
                 This argument is ignored if ``channel`` is provided.
+            credentials_file (Optional[str]): A file with credentials that can
+                be loaded with :func:`google.auth.load_credentials_from_file`.
+                This argument is ignored if ``channel`` is provided.
+            scopes (Optional(Sequence[str])): A list of scopes. This argument is
+                ignored if ``channel`` is provided.
             channel (Optional[grpc.Channel]): A ``Channel`` instance through
                 which to make calls.
             api_mtls_endpoint (Optional[str]): The mutual TLS endpoint. If
@@ -66,8 +73,9 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
                 is None.
 
         Raises:
-            google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
-                creation failed for any reason.
+          google.auth.exceptions.MutualTLSChannelError: If mutual TLS transport
+              creation failed for any reason.
+          ValueError: If both ``credentials`` and ``credentials_file`` are passed.
         """
         if channel:
             # Sanity check: Ensure that channel and credentials are not both
@@ -96,18 +104,20 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
             self._grpc_channel = type(self).create_channel(
                 host,
                 credentials=credentials,
+                credentials_file=credentials_file,
                 ssl_credentials=ssl_credentials,
-                scopes=self.AUTH_SCOPES,
+                scopes=scopes or self.AUTH_SCOPES,
             )
 
         # Run the base constructor.
-        super().__init__(host=host, credentials=credentials)
+        super().__init__(host=host, credentials=credentials, credentials_file=credentials_file, scopes=scopes)
         self._stubs = {}  # type: Dict[str, Callable]
 
     @classmethod
     def create_channel(cls,
                        host: str{% if service.host %} = '{{ service.host }}'{% endif %},
                        credentials: credentials.Credentials = None,
+                       credentials_file: str = None,
                        scopes: Optional[Sequence[str]] = None,
                        **kwargs) -> grpc.Channel:
         """Create and return a gRPC channel object.
@@ -118,18 +128,25 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
                 credentials identify this application to the service. If
                 none are specified, the client will attempt to ascertain
                 the credentials from the environment.
+            credentials_file (Optional[str]): A file with credentials that can
+                be loaded with :func:`google.auth.load_credentials_from_file`.
+                This argument is mutually exclusive with credentials.
             scopes (Optional[Sequence[str]]): A optional list of scopes needed for this
                 service. These are only used when credentials are not specified and
                 are passed to :func:`google.auth.default`.
             kwargs (Optional[dict]): Keyword arguments, which are passed to the
                 channel creation.
         Returns:
             grpc.Channel: A gRPC channel object.
+
+        Raises:
+            ValueError: If both ``credentials`` and ``credentials_file`` are passed.
         """
         scopes = scopes or cls.AUTH_SCOPES
         return grpc_helpers.create_channel(
             host,
             credentials=credentials,
+            credentials_file=credentials_file,
             scopes=scopes,
             **kwargs
         )