Using the Python mock module, how can I patch a class so that it stubs only the methods I want to stub and leaves other properties and methods alone?

Here's a simple case that shows what I want (Python 2.7). (Note: in my actual use case, MyClass is a class that gets instantiated indirectly within some code I'm testing):

from mock import patch

class MyClass(object):
    def __init__(self):
        self.prop = 'prop'

    def foo(self):
        return 'foo'

    def bar(self):
        return 'bar'

patcher = patch('__main__.MyClass')
MockedClass = patcher.start()
instance = MockedClass.return_value
instance.foo.return_value = 'mocked foo!'

my_instance = MyClass()
assert my_instance.foo() == 'mocked foo!', my_instance.foo()

# These asserts will fail
assert my_instance.bar() == 'bar', my_instance.bar()
assert my_instance.prop == 'prop', my_instance.prop

patcher.stop()

What I'm getting instead:

$ python mock_test.py
Traceback (most recent call last):
  File "mock_test.py", line 22, in <module>
    assert my_instance.bar() == 'bar', my_instance.bar()
AssertionError: <MagicMock name='MyClass().bar()' id='140162491496976'>

Why is the bar method now returning a MagicMock object? How can I patch this class so it stubs only the methods I want and leave everything else alone?