autospec_defense.py

with mock.patch("%s.restart_server" % module_under_test, autospec=True) as mock_restart:
    restart_servers_in_datacenter(servers, "sfo")
    mock_restart.assert_called_once()

autospec_defense_results

$ py.test beast.py
============================================================================
test session starts
============================================================================
platform linux2 -- Python 2.6.7 -- py-1.4.20 -- pytest-2.5.2
collected 1 items

beast.py F

============================================================================
FAILURES
============================================================================
_________________________________________________________________
test_restart_servers_in_datacenter_with_match
_________________________________________________________________

E           AttributeError: 'function' object has no attribute 'assert_called_once'

beast.py:22: AttributeError
============================================================================
1 failed in 0.01 seconds
============================================================================

empty_restart.py

def restart_servers_in_datacenter(servers, datacenter):
    pass

restart_server.py

def restart_server(server):
    """Safely restart a server by removing it from the load balancer,
    temporarily silencing its monitoring, telling its mom it will be home
    before dark, etc.

    (The implementation doesn't matter so we'll leave it out, but assume it
    makes remote API calls and invokes system commands and generally
    performs expensive operations.)
    """
    pass

def restart_servers_in_datacenter(servers, datacenter):
    for server in servers:
        if server.endswith("-%s" % datacenter):
            restart_server(server)

restart_server_test.py

import mock

module_under_test = __name__

def test_restart_servers_in_datacenter_with_match():
    servers = set(["web1-sfo", "web2-phx", "web3-jfk"])
    with mock.patch("%s.restart_server" % module_under_test) as mock_restart:
        restart_servers_in_datacenter(servers, "sfo")
        mock_restart.assert_called_once()

restart_server_test_output

$ py.test beast.py
============================================================================
test session starts
============================================================================
platform linux2 -- Python 2.6.7 -- py-1.4.20 -- pytest-2.5.2
collected 1 items

beast.py .

============================================================================
1 passed in 0.01 seconds
============================================================================

Instead of assert_called_once if the developer had mocked restart_server function and asserted if that function was called thrice with the arguments "web1-sfo", "web2-phx", "web3-jfk". Would that have worked?
With reference to this blog post: http://engineeringblog.yelp.com/2015/02/assert_called_once-threat-or-menace.html
/cc @tomelm