Python tests. Patch method from library in venv

I know how to patch my methods:

@patch('common.connections.upload_image')
def test_upload(self, mocked_upload_image):
    mocked_upload_image.return_value = 'Mocked url'

This will patch my method upload_image in module connections in folder common (I am using Django, folder common lies in root).

But I want to patch serializer from Django Rest framework, which lies in venv.lib.python2.7.site-packages.rest_framework.serializers.BaseSerializer.is_valid

So I tried:

@patch('venv.lib.python2.7.site-packages.rest_framework.serializers.BaseSerializer.is_valid')
def test_upload(self, mocked_is_valid):
    mocked_is_valid.return_value = True # this doesn't work

But it doesn't work. Says ImportError: No module named venv. My venv folder lies in the same folder, where the above mentioned common lies.

So how to patch this library method then?