added a more helpful message when using -m/-mongo_db and the pymongo …

…package is missing

HISTORY.rst

@@ -1,7 +1,7 @@
 Release History
 ---------------
 
-0.6.8 (2016-??-??)
+0.6.8 (2016-01-14)
 ++++++++++++++++++
 * Feature: Added automatic conversion of ``pandas`` datastructures in the
            custom info dict to json-format in the MongoObserver.
@@ -19,6 +19,7 @@ Release History
 * Bugfix: Fixed a problem with numpy datatypes in the configuration
 * Bugfix: More helpful error messages when using ``return`` or ``yield`` in a
           config scope
+* Bugfix: Be more helpful when using -m/--mongo_db and pymongo is not installed
 
 0.6.7 (2015-09-11)
 ++++++++++++++++++

sacred/observers/__init__.py

@@ -2,6 +2,8 @@
 # coding=utf-8
 from __future__ import division, print_function, unicode_literals
 
+from sacred.commandline_options import CommandLineOption
+
 from sacred.observers.base import RunObserver
 import sacred.optional as opt
 
@@ -10,4 +12,15 @@
 else:
     MongoObserver = opt.MissingDependencyMock('pymongo')
 
+    class MongoDbOption(CommandLineOption):
+        """To use the MongoObserver you need to install pymongo first."""
+
+        arg = 'DB'
+
+        @classmethod
+        def apply(cls, args, run):
+            raise ImportError('cannot use -m/--mongo_db flag: '
+                              'missing pymongo dependency')
+
+
 __all__ = ('RunObserver', 'MongoObserver')