Add helpful error messages when importing optional dependencies

google/auth/transport/grpc.py

@@ -16,7 +16,12 @@
 
 from __future__ import absolute_import
 
-import grpc
+try:
+    import grpc
+except ImportError:  # pragma: NO COVER
+    raise ImportError(
+        'gRPC is not installed, please install grpcio to use the gRPC '
+        'transport.')
 import six
 
 

google/auth/transport/requests.py

@@ -18,8 +18,12 @@
 
 import logging
 
-
-import requests
+try:
+    import requests
+except ImportError:  # pragma: NO COVER
+    raise ImportError(
+        'The requests library is not installed, please install requests to '
+        'use the requests transport.')
 import requests.exceptions
 
 from google.auth import exceptions

google/auth/transport/urllib3.py

@@ -30,7 +30,12 @@
 except ImportError:  # pragma: NO COVER
     certifi = None
 
-import urllib3
+try:
+    import urllib3
+except ImportError:  # pragma: NO COVER
+    raise ImportError(
+        'The urllib3 library is not installed, please install urllib3 to '
+        'use the urllib3 transport.')
 import urllib3.exceptions
 
 from google.auth import exceptions

google/oauth2/oauthlib.py

@@ -27,7 +27,12 @@
 
 import json
 
-import requests_oauthlib
+try:
+    import requests_oauthlib
+except ImportError:  # pragma: NO COVER
+    raise ImportError(
+        'The requests-oauthlib library is not installed, please install '
+        'requests-oauthlib to use google.oauth2.oauthlib.')
 
 import google.oauth2.credentials