Generated test and impl for client factory function (#246)

Author: software-dov

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

@@ -9,7 +9,8 @@ from google.api_core import exceptions        # type: ignore
 from google.api_core import gapic_v1          # type: ignore
 from google.api_core import retry as retries  # type: ignore
 from google.auth import credentials           # type: ignore
-
+from google.oauth2 import service_account     # type: ignore
+  
 {% filter sort_lines -%}
 {% for method in service.methods.values() -%}
 {% for ref_type in method.ref_types_legacy -%}
@@ -54,6 +55,29 @@ class {{ service.client_name }}Meta(type):
 
 class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
     """{{ service.meta.doc|rst(width=72, indent=4) }}"""
+
+    @classmethod
+    def from_service_account_file(cls, filename: str, *args, **kwargs):
+        """Creates an instance of this client using the provided credentials
+        file.
+
+        Args:
+            filename (str): The path to the service account private key json
+                file.
+            args: Additional arguments to pass to the constructor.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            {@api.name}: The constructed client.
+        """
+        credentials = service_account.Credentials.from_service_account_file(
+            filename)
+        kwargs['credentials'] = credentials
+        return cls(*args, **kwargs)
+
+    from_service_account_json = from_service_account_file
+
+
     def __init__(self, *,
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,

packages/gapic-generator/gapic/templates/tests/unit/$name_$version/$sub/test_$service.py.j2

@@ -11,6 +11,7 @@ import pytest
 {% filter sort_lines -%}
 from google import auth
 from google.auth import credentials
+from google.oauth2 import service_account
 from {{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }} import {{ service.client_name }}
 from {{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }} import transports
 {% if service.has_lro -%}
@@ -26,6 +27,17 @@ from google.longrunning import operations_pb2
 {% endfilter %}
 
 
+def test_{{ service.client_name|snake_case }}_from_service_account_file():
+    creds = credentials.AnonymousCredentials()
+    with mock.patch.object(service_account.Credentials, 'from_service_account_file') as factory:
+        factory.return_value = creds
+        client = {{ service.client_name }}.from_service_account_file("dummy/file/path.json")
+        assert client._transport._credentials == creds
+
+        client = {{ service.client_name }}.from_service_account_json("dummy/file/path.json")
+        assert client._transport._credentials == creds
+
+
 {% for method in service.methods.values() -%}
 def test_{{ method.name|snake_case }}(transport: str = 'grpc'):
     client = {{ service.client_name }}(