[feat] Make get_transport_class more convenient. (#87)

This adds the `get_transport_class` method to clients, which require
significantly less plumbing to import.

Author: lukesneeringer

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/services/$service/client.py.j2

@@ -12,7 +12,7 @@ from google.auth import credentials
 {{ import_ }}
 {% endfor %}
 from ...{{ '.' * api.subpackage_view|length }}utils import dispatch
-from .transports import get_transport_class
+from .transports import _transport_registry
 from .transports import {{ service.name }}Transport
 
 
@@ -46,7 +46,7 @@ class {{ service.name }}:
                                  'provide its credentials directly.')
             self._transport = transport
         else:
-            Transport = get_transport_class(transport)
+            Transport = self.get_transport_class(transport)
             self._transport = Transport(credentials=credentials, host=host)
 
     {% for method in service.methods.values() -%}
@@ -156,6 +156,26 @@ class {{ service.name }}:
     {% endfor %}
     {% endfor -%}
 
+    @classmethod
+    def get_transport_class(cls,
+            label: str = None) -> {{ service.name }}Transport:
+        """Return an appropriate transport class.
+
+        Args:
+            label (str): The name of the desired transport. If none is
+                provided, then the first transport in the registry is used.
+
+        Returns:
+            Type[{{ service.name }}Transport]: The transport class to use.
+        """
+        # If a specific transport is requested, return that one.
+        if label:
+            return _transport_registry[label]
+
+        # No transport is requested; return the default (that is, the first one
+        # in the dictionary).
+        return next(iter(_transport_registry.values()))
+
     @property
     def client_info(self) -> gapic_v1.client_info.ClientInfo:
         """Return information about this client (for metrics).

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/services/$service/transports/__init__.py.j2

@@ -8,45 +8,22 @@ from .base import {{ service.name }}Transport
 from .http import {{ service.name }}HttpTransport
 
 # Compile a registry of transports.
-_registry = collections.OrderedDict()
+_transport_registry = collections.OrderedDict()
 
 # gRPC is not guaranteed to be available, because `grpcio` may or may not
 # be installed. If it is available, however, it should be the "first in"
 # (default).
 try:
     from .grpc import {{ service.name }}GrpcTransport
-    _registry['grpc'] = {{ service.name }}GrpcTransport
+    _transport_registry['grpc'] = {{ service.name }}GrpcTransport
 except ImportError:
     pass
 
 # Always provide an HTTP/1.1 transport.
-_registry['http'] = {{ service.name }}HttpTransport
-
-
-# Provide a function for the client to get the appropriate transport.
-def get_transport_class(
-        label: str = None,
-        ) -> typing.Type[{{ service.name }}Transport]:
-    """Return an appropriate transport class.
-
-    Args:
-        label (str): The name of the desired transport. If none is provided,
-            then the first transport in the registry is used.
-
-    Returns:
-        Type[{{ service.name }}Transport]: The transport class to use.
-    """
-    # If a specific transport is requested, return that one.
-    if label:
-        return _registry[label]
-
-    # No transport is requested; return the default (that is, the first one
-    # in the dictionary).
-    return next(iter(_registry.values()))
+_transport_registry['http'] = {{ service.name }}HttpTransport
 
 
 __all__ = (
     '{{ service.name }}Transport',
-    'get_transport_class',
 )
 {% endblock %}