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Testing an Airflow DAG end-to-end wtih fixed input from within a python unit test while mocking Configuration

So, I have a DAG and some specified input, and i expect a certain output. I want to run all operators end to end.
Currently I'm trying to write some unit test in python that will mock over the some configuration,
and then I also want to overwrite some input parameters in dag_run.conf.
This seems to be cumbersome, since i did not find a way yet to both mock Variables.get, BaseHook.get_connection and trigger a dag in the python unit test.
If I use from airflow.api.client.local_client import Client then I cannot mock over Variable.get and Hooks.
How would i best do that?
Is there some further documentation on how Airflow works internally (I'm suspecting it starts different processes that overwrite the mocks?)
Or maybe I'm just not doing this the way it was intended to be done?

Python/Django unittest, how to handle outside calls?

I read multiple times that one should use mock to mimic outside calls and there should be no calls made to any outside service because your tests need to run regardless of outside services.
This totally makes sense....BUT
What about outside services changing? What good is a test, testing that my code works like it should if I will never know when it breaks because of the outside service being modified/updated/removed/deprecated/etc...
How can I reconcile this? The pseudocode is below
function post_tweet:
data = {"tweet":"tweetcontent"}
send request to twitter
receive response
return response
If I mock this there is no way I will be notified that twitter changed their API and now I have to update my test...

There are different levels of testing.
Unit tests are testing, as you might guess from the name, a unit. Which is for example a function or a method, maybe a class. If you interpret it wider it might include a view to be tested with Djangos test client. Unittests never test external stuff like libraries, dependencies or interfaces to other Systems. Theses thing will be mocked.
Integration tests are testing if your interfaces and usage of outside libraries, systems and APIs is implemented properly. If the dependency changes, you will notice have to change your code and unit tests.
There are other levels of tests as well, like behavior tests, UI tests, usability tests. You should make sure to separate theses tests classes in your project.

How to approach this unit-testing issue based on an external class?

I have a class Node (add_node.py) in which I create nodes that connect to a websocket. Now I want to write unit-tests for checking whether or not the connection was successful, and for checking the response of the server.
So I created a node_tests.py file which the following content:
import unittest
import json
import re
from add_node import Node
class TestNodes(unittest.TestCase):
def test_node_creation(self):
self.node = Node(a='1', b='2', c=True)
self.response = json.loads(self.node.connect())
self.assertIn('ok', self.response['r'])
def test_node_c(self):
self.assertTrue(self.response['c'])
if __name__ == '__main__':
unittest.main()
The first method is working but the second is failing because there is no attribute 'response'. So how could I approach this problem?
Also, is it ok to do it they way I'm doing it? Importing the class and writing multiple test within the same Test class?

The point of a unit test is to verify the functionality of a single isolated unit. Exactly what a unit is can be debated. Some would argue it's a single public method on a single class. I think most would agree that is not a very useful definition though. I like to think of them as use-cases. Your tests should do one use-case, and then make one assertion about the results from that use-case. This means that sometimes it's OK to let classes collaborate, and sometimes it's better to isolate a class and use test doubles for it's dependencies.
Knowing when to isolate is something you learn over time. I think the most important points to consider are that every test you write should
Fully define the context in which it's run (without depending on global state or previously run tests)
Be fast, a few milliseconds tops (this means you don't touch external resources like the file system, a web server or some database)
Not test a bunch of other things that are covered by other tests.
This third point is probably the hardest to balance. Obviously several tests will run the same code if they're making different assertions on the same use-case. You should keep the tests small though. Let's say we want to test the cancellation process of orders in an e-commerce application. In this case we probably don't want to import and set up all the classes used to create, verify, etc. an order before we cancel it. In that case it's probably a better idea to just create an order object manually or maybe use a test double.
In your case, if what you actually want to do is to test the real connection and the responses the real server gives, you don't want a unit test, you want an integration test.
If what you actually want is to test the business logic of your client class, however, you should probably create a fake socket/server where you can yourself define the responses and whether or not the connection is successful. This allows you to test that your client behaves correctly depending on it's communications with the remote server, without actually having to depend on the remote server in your test suite.

Writing unit tests for various os/system level checks

I have a few functions for performing actions in python, and I am debating whether or not it makes sense to write unit tests for these.
There is one function that resolves a host using the socket module. I don't think it makes sense to write a test for the socket module itself, but seems like it may make sense to write a functional test to see if DNS is working.
I have another function that opens a file and puts the contents into a list.
Does anyone have any guidance or links on performing unit tests/functional tests for items like this? Most of what I have read pertains to application level testing only.
Thank you in advance.

First of all, if you don't have tests at all, better start with high-level end-to-end functional tests and end with unit tests gathering coverage statistics on every new test you write.
While writing a test to a function which uses system or network libraries, you usually want to isolate your test, to make it independent and straight as much as possible by mocking out system and network calls (see Mock library).
By using mock library you can and should test how does your application/function respond to situations where there is a socket or system error.
Also see:
python unittest howto
How to Mock an HTTP request in a unit testing scenario in Python
An Introduction to Mocking in Python
Hope that helps.

unit testing for an application server

I wrote an application server (using python & twisted) and I want to start writing some tests. But I do not want to use Twisted's Trial due to time constraints and not having time to play with it now. So here is what I have in mind: write a small test client that connects to the app server and makes the necessary requests (the communication protocol is some in-house XML), store in a static way the received XML and then write some tests on those static data using unitest.
My question is: Is this a correct approach and if yes, what kind of tests are covered with this approach?
Also, using this method has several disadvantages, like: not being able to access the database layer in order to build/rebuild the schema, when will the test client going to connect to the server: per each unit test or before running the test suite?

You should use Trial. It really isn't very hard. Trial's documentation could stand to be improved, but if you know how to use the standard library unit test, the only difference is that instead of writing
import unittest
you should write
from twisted.trial import unittest
... and then you can return Deferreds from your test_ methods. Pretty much everything else is the same.
The one other difference is that instead of building a giant test object at the bottom of your module and then running
python your/test_module.py
you can simply define your test cases and then run
trial your.test_module
If you don't care about reactor integration at all, in fact, you can just run trial on a set of existing Python unit tests. Trial supports the standard library 'unittest' module.

"My question is: Is this a correct approach?"
It's what you chose. You made a lot of excuses, so I'm assuming that your pretty well fixed on this course. It's not the best, but you've already listed all your reasons for doing it (and then asked follow-up questions on this specific course of action). "correct" doesn't enter into it anymore, so there's no answer to this question.
"what kind of tests are covered with this approach?"
They call it "black-box" testing. The application server is a black box that has a few inputs and outputs, and you can't test any of it's internals. It's considered one acceptable form of testing because it tests the bottom-line external interfaces for acceptable behavior.
If you have problems, it turns out to be useless for doing diagnostic work. You'll find that you need to also to white-box testing on the internal structures.
"not being able to access the database layer in order to build/rebuild the schema,"
Why not? This is Python. Write a separate tool that imports that layer and does database builds.
"when will the test client going to connect to the server: per each unit test or before running the test suite?"
Depends on the intent of the test. Depends on your use cases. What happens in the "real world" with your actual intended clients?
You'll want to test client-like behavior, making connections the way clients make connections.
Also, you'll want to test abnormal behavior, like clients dropping connections or doing things out of order, or unconnected.

I think you chose the wrong direction. It's true that the Trial docs is very light. But Trial is base on unittest and only add some stuff to deal with the reactor loop and the asynchronous calls (it's not easy to write tests that deal with deffers). All your tests that are not including deffer/asynchronous call will be exactly like normal unittest.
The Trial command is a test runner (a bit like nose), so you don't have to write test suites for your tests. You will save time with it. On top of that, the Trial command can output profiling and coverage information. Just do Trial -h for more info.
But in any way the first thing you should ask yourself is which kind of tests do you need the most, unit tests, integration tests or system tests (black-box). It's possible to do all with Trial but it's not necessary allways the best fit.

haven't used twisted before, and the twisted/trial documentation isn't stellar from what I just saw, but it'll likely take you 2-3 days to implement correctly the test system you describe above. Now, like I said I have no idea about Trial, but I GUESS you could probably get it working in 1-2 days, since you already have a Twisted application. Now if Trial gives you more coverage in less time, I'd go with Trial.
But remember this is just an answer from a very cursory look at the docs