Document password_command behavior in keyring.rst

docs/keyring.rst

@@ -15,6 +15,10 @@ Custom command
 A custom command/binary can be specified to retrieve the password for a
 username/hostname combination. See :ref:`general_config`.
 
+.. versionchanged:: 0.6.0
+
+    Setting a custom command now disables all other methods.
+
 netrc
 =====
 

vdirsyncer/utils/password.py

@@ -118,7 +118,7 @@ def _password_from_command(username, host):
     try:
         stdout = subprocess.check_output(command + [username, host],
                                          universal_newlines=True)
-        return stdout.strip('\n') or None
+        return stdout.strip('\n')
     except OSError as e:
         raise exceptions.UserError('Failed to execute command: {}\n{}'.
                                    format(' '.join(command), str(e)))