python: mock a module

Question

Is it possible to mock a module in python using unittest.mock? I have a module named config, while running tests I want to mock it by another modul

Answer 1

foo.py:

import config

VAR1 = config.CONF_VAR1

def bar():
    return VAR1

test.py:

import unittest
import unittest.mock as mock

import test_config


class Test(unittest.TestCase):

    def test_one(self):
        with mock.patch.dict('sys.modules', config=test_config):
            import foo
            self.assertEqual(foo.bar(), 'test_VAR1')

As you can see, the patch works even for code executed during import foo.

Answer 2

Consider this following setup

configuration.py:

import os

class Config(object):
    CONF_VAR1 = "VAR1"
    CONF_VAR2 = "VAR2"

class TestConfig(object):
    CONF_VAR1 = "test_VAR1"
    CONF_VAR2 = "test_VAR2"


if os.getenv("TEST"):
    config = TestConfig
else:
    config = Config

now everywhere else in your code you can use:

from configuration import config
print config.CONF_VAR1, config.CONF_VAR2

And when you want to mock your coniguration file just set the environment variable "TEST".

Extra credit: If you have lots of configuration variables that are shared between your testing and non-testing code, then you can derive TestConfig from Config and simply overwrite the variables that need changing:

class Config(object):
    CONF_VAR1 = "VAR1"
    CONF_VAR2 = "VAR2"
    CONF_VAR3 = "VAR3"

class TestConfig(Config):
    CONF_VAR2 = "test_VAR2"
    # CONF_VAR1, CONF_VAR3 remain unchanged

Answer 3

If your application ("app.py" say) looks like

import config
print config.var1, config.var2

And gives the output:

$ python app.py
VAR1 VAR2

You can use mock.patch to patch the individual config variables:

from mock import patch

with patch('config.var1', 'test_VAR1'):
    import app

This results in:

$ python mockimport.py
test_VAR1 VAR2

Though I'm not sure if this is possible at the module level.

Answer 4

If you're always accessing the variables in config.py like this:

import config
...
config.VAR1

You can replace the config module imported by whatever module you're actually trying to test. So, if you're testing a module called foo, and it imports and uses config, you can say:

from mock import patch
import foo
import config_test
....
with patch('foo.config', new=config_test):
   foo.whatever()

But this isn't actually replacing the module globally, it's only replacing it within the foo module's namespace. So you would need to patch it everywhere it's imported. It also wouldn't work if foo does this instead of import config:

from config import VAR1

You can also mess with sys.modules to do this:

import config_test
import sys
sys.modules["config"] = config_test
# import modules that uses "import config" here, and they'll actually get config_test

But generally it's not a good idea to mess with sys.modules, and I don't think this case is any different. I would favor all of the other suggestions made over it.