Adding more user-friendly error when filename missing for user creds.

Author: dhermes

gcloud/credentials.py

@@ -137,9 +137,11 @@ def get_credentials_from_user_flow(scope, client_secrets_file=None):
 
     :rtype: :class:`oauth2client.client.OAuth2Credentials`
     :returns: A new credentials instance.
-    :raises: ``EnvironmentError`` if stdout is not a TTY or
+    :raises: ``EnvironmentError`` if stdout is not a TTY,
+             ``ValueError`` if ``client_secrets_file`` is not passed in as an
+             argument or set as an environment variable, or
              ``ValueError`` if the client secrets file is not for an installed
-             application
+             application.
     """
     if not sys.stdout.isatty():
         raise EnvironmentError('Cannot initiate user flow unless user can '
@@ -148,6 +150,9 @@ def get_credentials_from_user_flow(scope, client_secrets_file=None):
     if client_secrets_file is None:
         client_secrets_file = os.getenv('GCLOUD_CLIENT_SECRETS')
 
+    if client_secrets_file is None:
+        raise ValueError('Client secrets file not specified.')
+
     client_type, client_info = client.clientsecrets.loadfile(
         client_secrets_file)
     if client_type != client.clientsecrets.TYPE_INSTALLED:

gcloud/test_credentials.py

@@ -147,6 +147,17 @@ def test_no_tty(self):
             with self.assertRaises(EnvironmentError):
                 self._callFUT(None)
 
+    def test_no_filename(self):
+        import sys
+
+        from gcloud._testing import _Monkey
+        from oauth2client import client
+
+        STDOUT = _MockStdout(isatty=True)
+        with _Monkey(sys, stdout=STDOUT):
+            with self.assertRaises(ValueError):
+                self._callFUT(None)
+
     def test_filename_from_environ(self):
         import os
         import sys