fix: use default_host even with custom endpoints

Following guidance in https://docs.google.com/document/d/1FoJPYQu8ZHdGBFY9TBRZCDrWJJnxw3fAz5CG5mdZR9E/edit

Author: busunkim96

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

@@ -67,7 +67,7 @@ class {{ service.name }}Transport(abc.ABC):
             scopes: Optional[Sequence[str]] = None,
             quota_project_id: Optional[str] = None,
             client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
-            **kwargs,       
+            **kwargs,
             ) -> None:
         """Instantiate the transport.
 
@@ -85,10 +85,10 @@ class {{ service.name }}Transport(abc.ABC):
             scopes (Optional[Sequence[str]]): A list of scopes.
             quota_project_id (Optional[str]): An optional project to use for billing
                 and quota.
-            client_info (google.api_core.gapic_v1.client_info.ClientInfo):	
-                The client info used to send a user-agent string along with	
-                API requests. If ``None``, then default info will be used.	
-                Generally, you only need to set this if you're developing	
+            client_info (google.api_core.gapic_v1.client_info.ClientInfo):
+                The client info used to send a user-agent string along with
+                API requests. If ``None``, then default info will be used.
+                Generally, you only need to set this if you're developing
                 your own client library.
         """
         # Save the hostname. Default to port 443 (HTTPS) if none is specified.
@@ -99,13 +99,13 @@ class {{ service.name }}Transport(abc.ABC):
         scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
 
         # Save the scopes.
-        self._scopes = scopes_kwargs["scopes"]
+        self._scopes = scopes or self.AUTH_SCOPES
 
         # If no credentials are provided, then determine the appropriate
         # defaults.
         if credentials and credentials_file:
             raise exceptions.DuplicateCredentialArgs("'credentials_file' and 'credentials' are mutually exclusive")
-        
+
         if credentials_file is not None:
             credentials, _ = auth.load_credentials_from_file(
                                 credentials_file,
@@ -120,19 +120,7 @@ class {{ service.name }}Transport(abc.ABC):
         self._credentials = credentials
 
 
-    @classmethod
-    def _get_user_scopes(cls, host: str, scopes: Optional[Sequence[str]]) -> Optional[Sequence[str]]:
-        if scopes is None:
-            # If a custom API endpoint is set, set user scopes to ensure the auth
-            # library does not used the self-signed JWT flow for service
-            # accounts.
-            if host.split(":")[0] != cls.DEFAULT_HOST.split(":")[0]:
-                scopes = cls.AUTH_SCOPES
-        
-        return scopes
-
-
-    # TODO(busunkim): These two class methods are in the base transport 
+    # TODO(busunkim): These two class methods are in the base transport
     # to avoid duplicating code across the transport classes. These functions
     # should be deleted once the minimum required versions of google-api-core
     # and google-auth are increased.
@@ -142,7 +130,6 @@ class {{ service.name }}Transport(abc.ABC):
     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 = cls._get_user_scopes(host, scopes)
         scopes_kwargs = {}
 
         if _GOOGLE_AUTH_VERSION and (
@@ -152,17 +139,16 @@ class {{ service.name }}Transport(abc.ABC):
             scopes_kwargs = {"scopes": scopes, "default_scopes": cls.AUTH_SCOPES}
         else:
             scopes_kwargs = {"scopes": scopes or cls.AUTH_SCOPES}
-        
+
         return scopes_kwargs
 
     # TODO: Remove this function once google-api-core >= 1.26.0 is required
     @classmethod
     def _get_self_signed_jwt_kwargs(cls, host: str, scopes: Optional[Sequence[str]]) -> Dict[str, Union[Optional[Sequence[str]], str]]:
         """Returns kwargs to pass to grpc_helpers.create_channel depending on the google-api-core version"""
-        scopes = cls._get_user_scopes(host, scopes)
 
         self_signed_jwt_kwargs: Dict[str, Union[Optional[Sequence[str]], str]] = {}
-        
+
         if _API_CORE_VERSION and (
             packaging.version.parse(_API_CORE_VERSION)
             >= packaging.version.parse("1.26.0")
@@ -172,7 +158,7 @@ class {{ service.name }}Transport(abc.ABC):
             self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
         else:
             self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
-        
+
         return self_signed_jwt_kwargs
 
 
@@ -251,7 +237,7 @@ class {{ service.name }}Transport(abc.ABC):
         ],
     ]:
         raise NotImplementedError()
-    {% endif %} 
+    {% endif %}
 
 __all__ = (
     '{{ service.name }}Transport',

gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -1505,32 +1505,6 @@ def test_{{ service.name|snake_case }}_base_transport_with_credentials_file_old_
         )
 
 
-@requires_google_auth_gte_1_25_0
-def test_{{ service.name|snake_case }}_base_transport_custom_host():
-    # Test the default credentials are used if credentials and credentials_file are None.
-    with mock.patch.object(auth, 'load_credentials_from_file', autospec=True) as load_creds, mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.transports.{{ service.name }}Transport._prep_wrapped_messages') as Transport:
-        Transport.return_value = None
-        load_creds.return_value = (credentials.AnonymousCredentials(), None)
-        transport = transports.{{ service.name }}Transport(
-            host="squid.clam.whelk",
-            credentials_file="credentials.json",
-            quota_project_id="octopus",
-        )
-        load_creds.assert_called_once_with("credentials.json",
-            # scopes should be set since custom endpoint was passed
-            scopes=(
-            {%- for scope in service.oauth_scopes %}
-            '{{ scope }}',
-            {%- endfor %}
-            ),
-            default_scopes=(
-            {%- for scope in service.oauth_scopes %}
-            '{{ scope }}',
-            {%- endfor %}
-            ),
-            quota_project_id="octopus",
-        )
-
 def test_{{ service.name|snake_case }}_base_transport_with_adc():
     # Test the default credentials are used if credentials and credentials_file are None.
     with mock.patch.object(auth, 'default', autospec=True) as adc, mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.transports.{{ service.name }}Transport._prep_wrapped_messages') as Transport:
@@ -1596,34 +1570,6 @@ def test_{{ service.name|snake_case }}_transport_auth_adc(transport_class):
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class",
-    [
-        transports.{{ service.name }}GrpcTransport,
-        transports.{{ service.name }}GrpcAsyncIOTransport,
-    ],
-)
-@requires_google_auth_gte_1_25_0
-def test_{{ service.name|snake_case }}_transport_auth_adc_custom_host(transport_class):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(auth, 'default', autospec=True) as adc:
-        adc.return_value = (credentials.AnonymousCredentials(), None)
-        transport_class(host="squid.clam.whelk", quota_project_id="octopus")
-        adc.assert_called_once_with(
-            # scopes should be set since a custom endpoint (host) was set
-            scopes=(
-                {%- for scope in service.oauth_scopes %}
-                '{{ scope }}',
-                {%- endfor %}),
-            default_scopes=(
-                {%- for scope in service.oauth_scopes %}
-                '{{ scope }}',
-                {%- endfor %}),
-            quota_project_id="octopus",
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [