Python: block network connections for testing purposes?

Question

I'm trying to test a package that provides interfaces to a few web services. It has a test suite that is supposed to test most functions without connecting to the internet. However, there are some lingering tests that may attempt to connect to the internet / download data, and I'd like to prevent them from doing so for two reasons: first, to make sure my test suite works if no network connection is available; second, so that I'm not spamming the web services with excess queries.

An obvious solution is to unplug my machine / turn off wireless, but when I'm running tests on a remote machine that obviously doesn't work.

So, my question: Can I block network / port access for a single python process? ("sandbox" it, but just blocking network connections)

(afaict, pysandbox doesn't do this)

EDIT: I'm using py.test so I need a solution that will work with py.test, in case that affects any proposed answers.

Answer 1

Monkey patching socket ought to do it:

import socket
def guard(*args, **kwargs):
    raise Exception("I told you not to use the Internet!")
socket.socket = guard

Make sure this runs before any other import.

Answer 2

Update: There is now a pytest plugin that does the same thing as this answer! You can read the answer just to see how things work, but I strongly recommend using the plugin instead of copying-pasting my answer :-) See here: https://github.com/miketheman/pytest-socket

---

I found Thomas Orozco's answer to be very helpful. Following on keflavich, this is how I integrated into my unit test suite. This works for me with thousands of very different unit test-cases (<100 that need socket though) ... and in and out of doctests.

I posted it here. Including below for convenience. Tested with Python 2.7.5, pytest==2.7.0. (To test for yourself, run py.test --doctest-modules in directory with all 3 files cloned.)

_socket_toggle.py

from __future__ import print_function
import socket
import sys

_module = sys.modules[__name__]

def disable_socket():
    """ disable socket.socket to disable the Internet. useful in testing.

    .. doctest::
        >>> enable_socket()
        [!] socket.socket is enabled.
        >>> disable_socket()
        [!] socket.socket is disabled. Welcome to the desert of the real.
        >>> socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        Traceback (most recent call last):
        ...
        RuntimeError: I told you not to use the Internet!
        >>> enable_socket()
        [!] socket.socket is enabled.
        >>> enable_socket()
        [!] socket.socket is enabled.
        >>> disable_socket()
        [!] socket.socket is disabled. Welcome to the desert of the real.
        >>> socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        Traceback (most recent call last):
        ...
        RuntimeError: I told you not to use the Internet!
        >>> enable_socket()
        [!] socket.socket is enabled.
    """
    setattr(_module, '_socket_disabled', True)

    def guarded(*args, **kwargs):
        if getattr(_module, '_socket_disabled', False):
            raise RuntimeError("I told you not to use the Internet!")
        else:
            # SocketType is a valid public alias of socket.socket,
            # we use it here to avoid namespace collisions
            return socket.SocketType(*args, **kwargs)

    socket.socket = guarded

    print(u'[!] socket.socket is disabled. Welcome to the desert of the real.')


def enable_socket():
    """ re-enable socket.socket to enable the Internet. useful in testing.
    """
    setattr(_module, '_socket_disabled', False)
    print(u'[!] socket.socket is enabled.')

conftest.py

# Put this in the conftest.py at the top of your unit tests folder,
# so it's available to all unit tests
import pytest
import _socket_toggle


def pytest_runtest_setup():
    """ disable the interet. test-cases can explicitly re-enable """
    _socket_toggle.disable_socket()


@pytest.fixture(scope='function')
def enable_socket(request):
    """ re-enable socket.socket for duration of this test function """
    _socket_toggle.enable_socket()
    request.addfinalizer(_socket_toggle.disable_socket)

test_example.py

# Example usage of the py.test fixture in tests
import socket
import pytest

try:
    from urllib2 import urlopen
except ImportError:
    import urllib3
    urlopen = urllib.request.urlopen


def test_socket_disabled_by_default():
    # default behavior: socket.socket is unusable
    with pytest.raises(RuntimeError):
        urlopen(u'https://www.python.org/')


def test_explicitly_enable_socket(enable_socket):
    # socket is enabled by pytest fixture from conftest. disabled in finalizer
    assert socket.socket(socket.AF_INET, socket.SOCK_STREAM)