fix: address code review comments for lazy imports

Author: hebaalazzeh

packages/google-auth/google/auth/transport/grpc.py

@@ -42,6 +42,8 @@
 __lazy_modules__: Set[str] = {"grpc"}
 
 
+# Inheriting from object instead of grpc.AuthMetadataPlugin is intentional.
+# This prevents eagerly loading grpc at module import time, preserving lazy loading support.
 class AuthMetadataPlugin(object):
     """A `gRPC AuthMetadataPlugin`_ that inserts the credentials into each
     request.

packages/google-auth/google/auth/transport/requests.py

@@ -23,12 +23,14 @@
 import time
 from typing import Optional, Set
 
-try:
-    import requests
-except ImportError as caught_exc:  # pragma: NO COVER
+import importlib.util
+
+if importlib.util.find_spec("requests") is None:
     raise ImportError(
-        "The requests library is not installed from please install the requests package to use the requests transport."
-    ) from caught_exc
+        "The requests library is not installed, please install the requests package to use the requests transport."
+    )
+
+import requests
 import requests.adapters  # pylint: disable=ungrouped-imports
 import requests.exceptions  # pylint: disable=ungrouped-imports
 from requests.packages.urllib3.util.ssl_ import (  # type: ignore

packages/google-auth/google/auth/transport/urllib3.py

@@ -299,18 +299,26 @@ def my_cert_callback():
     """
 
     _encode_url_methods = {"DELETE", "GET", "HEAD", "OPTIONS"}
+    _request_methods_class = None
 
     def __getattr__(self, name):
         if name in ("request", "request_encode_url", "request_encode_body"):
             import types
 
-            if version.parse(urllib3.__version__) >= version.parse("2.0.0"):
-                rm = urllib3._request_methods.RequestMethods
-            else:
-                rm = urllib3.request.RequestMethods
+            if AuthorizedHttp._request_methods_class is None:
+                if version.parse(urllib3.__version__) >= version.parse("2.0.0"):
+                    AuthorizedHttp._request_methods_class = (
+                        urllib3._request_methods.RequestMethods
+                    )
+                else:
+                    AuthorizedHttp._request_methods_class = (
+                        urllib3.request.RequestMethods
+                    )
 
-            method = getattr(rm, name)
-            return types.MethodType(method, self)
+            method = getattr(AuthorizedHttp._request_methods_class, name)
+            bound_method = types.MethodType(method, self)
+            setattr(self, name, bound_method)
+            return bound_method
 
         raise AttributeError(
             f"'{self.__class__.__name__}' object has no attribute '{name}'"