Mocking global/module level variables in Python - python-3.x

Question

I have a module that looks like this:
import psycopg2
client = vault_client(vault_url, vault_certs_path, credentials)
vault_data = client.read(vault_path)['data']
def do_thing
connection = psycopg2.connect(
dbname=vault_data['database'],
host=vault_data['cluster_name'],
...
)
How do I test this do_thing method. I need to mock out vault_data and the import of psycopy2. I need to ensure:
That the psycopg2.connect methods receives the right arguments
How do I mock the vault_client method to return a mock that then returns a dictionary when the read method is called on the mock?
I have this but the real methods get called:
#mock.patch("sources.segment.handler")
#mock.patch("sources.segment.handler.psycopg2")
def test_attempts_to_connect_to_redshift(self, mock_psycopg2, mock_handler):
mock_handler.vault_client.return_value = {
"data": {
"database": "some_database",
"cluster_name": "some_cluster_name",
"port": "some_port",
"username": "some_username",
"password": "some_password",
}
}
do_thing()
mock_psycopg2.connect.assert_called_with("some database")
...

Answer 1

It seems that the problem here is that it's too late to mock something by the time the module with do_thing() has already been imported. You can try the following trick:
import sys
from unittest.mock import MagicMock
psycopg2_mock = MagicMock()
sys.modules['psycopg2'] = psycopg2_mock
import module_with_do_thing
del sys.modules['psycopg2']
class TestSomething(unittest.TestCase):
...
def test_attempts_to_connect_to_redshift(self):
...
assert psycopg2_mock.has_required_properties
Moving the import module_with_do_thing line to the actual test method with the right patch might work as well.