C:\> python
Python 2.7.3 (default, Apr 10 2012, 23:24:47) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pymongo
>>> c = pymongo.Connection(port=27999)
>>> db = c.test
>>> db.system_js.foo = "function(){return 5;}"
>>> db.system_js.foo()
5.0

>>> db2.system_js.bar = "return 5;"
>>> db2.system_js.bar()
5.0

>>> db.system_js.bar = "return 5;"
>>> db.system_js.bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\site-packages\pymongo\database.py", line 776, in <lambda>
    scope={'name': name}), *args)
  File "C:\Python27\lib\site-packages\pymongo\database.py", line 716, in eval
    result = self.command("$eval", code, args=args)
  File "C:\Python27\lib\site-packages\pymongo\database.py", line 355, in command
    msg, allowable_errors)
  File "C:\Python27\lib\site-packages\pymongo\helpers.py", line 129, in _check_command_response
    raise OperationFailure(msg % response["errmsg"])
pymongo.errors.OperationFailure: command SON([('$eval', Code('function() { return this[name].apply(this, arguments); }', {'name': 'bar'})), ('args', ())]) failed: invoke failed: error in invoke: _funcs1:1 TypeError: Cannot call method 'apply' of null
_funcs1 = function() { return this[name].apply(this, arguments); }
                                         ^

<unknown>:33: Uncaught SyntaxError: Unexpected token return
Mon Nov 19 10:32:38.381 [conn1] Could not compile function: ____MongoToV8_newFunction_temp = return 5;
Mon Nov 19 10:32:38.396 [conn1] error in invoke: _funcs1:1 TypeError: Cannot call method 'apply' of null
_funcs1 = function() { return this[name].apply(this, arguments); }
                                         ^