[Python] How to patch an attribute in an object

The question is how to patch an attribute of an instance within a with statement. I tried with following example which doesn't work. It prints as in the comment.

from mock import patch, PropertyMock

class Foo(object):
f = {'a': 1}

new_foo = Foo()

with patch.object(new_foo, 'f', new_callable=PropertyMock) as mock:
mock.return_value = {'b': 2}
print new_foo.f
# <PropertyMock name='f' id='4474801232'>

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