Override a "private" method in a python module

I want to test a function in python, but it relies on a module-level "private" function, that I don't want called, but I'm having trouble overriding/mocking it. Scenario:

module.py

_cmd(command, args):
  # do something nasty

function_to_be_tested():
  # do cool things
  _cmd('rm', '-rf /')
  return 1

test_module.py

import module
test_function():
  assert module.function_to_be_tested() == 1

Ideally, in this test I dont want to call _cmd. I've looked at some other threads, and I've tried the following with no luck:

 test_function():
   def _cmd(command, args):
     # do nothing
     pass
   module._cmd = _cmd

although checking module._cmd against _cmd doesn't give the correct reference. Using mock:

from mock import patch

def _cmd_mock(command, args):
  # do nothing
  pass

@patch('module._cmd', _cmd_mock) 
test_function():
   ...

gives the correct reference when checking module._cmd, although `function_to_be_tested' still uses the original _cmd (as evidenced by it doing nasty things).

This is tricky because _cmd is a module-level function, and I dont want to move it into a module