Python: block network connections for testing purposes?

Building on the very helpful answers from Thomas Orozco and driftcatcher here is a variant that works with Python's unittest and (after a small change) Django.

All you need to do is inherit your test case class from the enhanced NoSocketTestCase class and any access to the network will be detected and raises the SocketAccessError exception.

And this approach also works with Django. You only need to change the NoSocketTestCase class to inherit from django.test.TestCase instead of unittest.TestCase.

While not strictly answering OP's question I think this might be helpful for anyone who wants to block network access in unit tests.

no_sockets.py

import socket
from unittest import TestCase


class SocketAccessError(Exception):
    pass


class NoSocketsTestCase(TestCase):
    """Enhancement of TestCase class that prevents any use of sockets

    Will throw the exception SocketAccessError when any code tries to
    access network sockets
    """

    @classmethod
    def setUpClass(cls):
        cls.socket_original = socket.socket
        socket.socket = cls.guard
        return super().setUpClass()

    @classmethod
    def tearDownClass(cls):
        socket.socket = cls.socket_original
        return super().tearDownClass()

    @staticmethod
    def guard(*args, **kwargs):
        raise SocketAccessError('Attempted to access network')

test_no_sockets.py

import urllib.request
from .no_sockets import NoSocketsTestCase, SocketAccessError


class TestNoSocketsTestCase(NoSocketsTestCase):

    def test_raises_exception_on_attempted_network_access(self):

        with self.assertRaises(SocketAccessError):            
            urllib.request.urlopen('https://www.google.com')

A simple way to put a gag on the requests library:

from unittest import mock

requests_gag = mock.patch(
    'requests.Session.request',
    mock.Mock(side_effect=RuntimeError(
        'Please use the `responses` library to mock HTTP in your tests.'
    ))
)

with requests_gag:
    ...  # no Internet here

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.

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)