Unittest on AWS python Lambda

Question

I have this python lambda on handler.py

def isolate_endpoints(event=None, context=None):

    endpoint_id = event['event']['endpoint_id']

    client = get_edr_client()
    response = client.isolate_endpoints(endpoint_id=endpoint_id)

    return response # E.g {"reply":{"status":"success", "error_msg":null}}

I want to write a unit test for this lambda. However after reading on the unittests and having seen actual implementations of the tests I can comfortably say I have no idea what is being tested exactly. (I know the theory but having hard time understanding the implementation of mocks, magicmocks etc.)

The unittest that I have right now is on test_calls and looks like this:

@mock.patch('project_module.utils.get_edr_client')
def test_isolate_endpoints(get_edr_client: MagicMock):
    client = MagicMock()
    get_edr_client.return_value = client
    mock_event = {"event": {"endpoint_id":"foo"}}
    resp = isolate_endpoints(event=mock_event) # Is it right to call this lambda directly from here?
    assert resp is not None
    expected = [call()] ## what is this ?? # What is supposed to follow this?

    assert client.isolate_endpoints.call_args_list == expected

definition & body of get_edr_client in utils.py:

from EDRAPI import EDRClient
def get_edr_client():
    return EDRClient(api_key="API_KEY")

rdas · Accepted Answer · 2022-08-31 09:23:54Z

1

I'll try to explain each aspect of the test you have written

@mock.patch('project_module.utils.get_edr_client')

This injects dependencies into your test. You are essentially patching the name 'project_module.utils.get_edr_client' with an auto-created MagicMock object that's passed to you as the test argument get_edr_client. This is useful to bypass external dependencies in your test code. You can pass mock implementations of objects that are used in your code that's under test, but don't need to be tested in this test itself.

def test_isolate_endpoints(get_edr_client: MagicMock):
    client = MagicMock()
    get_edr_client.return_value = client

Setup: You are setting up the external mock you patched into the test. Making it return more mock objects so that the code under test works as expected.

    mock_event = {"event": {"endpoint_id":"foo"}}
    resp = isolate_endpoints(event=mock_event)

Invoke: Calling the code that you want to test, with some (possibly mock) argument. Since you want to test the function isolate_endpoints, that's what you should be calling. In other tests you could also try passing invalid arguments like isolate_endpoints(event=None) to see how your code behaves in those scenarios.

    assert resp is not None

Verify: Check if the value returned from your function is what you expect it to be. If your function was doing 2+2 this is the part where you check if the result equals 4 or not.

    expected = [call()]

    assert client.isolate_endpoints.call_args_list == expected

Verify side-effects: Your function has side-effects apart from returning a value. So you should check if those are happening properly or not. You expect the function to call isolate_endpoint on the client mock you injected earlier once. So you're checking if that method was called once with no arguments. The call() is for matching one invocation of the method with no arguments. The [call()] means only one invocation with no arguments. Then you just check if that is indeed the case with the assertion.

A much cleaner way to do the same this is to do this instead:

client.isolate_endpoints.assert_called_once()

Which does the same thing.

answered Aug 31, 2022 at 9:23

rdas

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5 Comments

rustam Shirinov Over a year ago

Since I copied the implementation from another project after trying to understand as much as I could, I still have questions regarding the code I pasted. How should I call call() in this case? with which function's arguments? why Do I need to set up external mock? Why do I need it to return more values if the only thing I am testing is one functionality which is isolate_endpoints?

rustam Shirinov Over a year ago

Also when I run the tests as they are I get an error saying 'HTTPSConnectionPool: Unable to connect to EDRAPI..' What I understand from this is that in the tests the test actually needs to run the functionality? meaning it needs to connect to the EDRAPI and verify if it's valid connection? I thought the whole point of mocks is to mock the behaviour and say that 'My get_edr_client' will return a EDRClient object. But in this case I don't see it?

rdas Over a year ago

Your error is coming because the mock object is not being patched properly. Check your module name & paths.

rdas Over a year ago

call() is an mocking instrument that represents a single call f a mock function with no arguments - Why would you need to call it?

rustam Shirinov Over a year ago

Okay, the error was me patching the function by using the name of the file which it was defined it. But for some reason specifying the name of the file where it was used instead of the definition fixed the error. Now I also know when & where to use the call().

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