Unit Tests - Why?

A unit test is basically testing one unit of code. One unit literally means one snippet of code. One snippet could mean 5 lines, a small function or in some cases even an entire small program. Usually though, in large corporate environments your code base is pretty massive so a good starting place is to think of a minimum of 1 unit test per function.

So you go to the function's code and see that it has 100 lines of code. The first 10 are just variable initialization, then there's a few calls to 3rd party libraries to get data, normalize the data and then log it. For e.g. Requests to REST APIs, convert the returned data into Json, add a new key with a timestamp to it and then log success or failure.

Finally once these calls succeed OR fail, your code returns True or False. So when you write a unit test, you're thinking (non intuitively) but you're thinking - "Let's assume that all these calls succeed and come to MY code, where I make the True/False decision. I want to make sure that my code reacts properly in either case.".

Which basically means, if there was a way to make all those calls (requests, json, log, add timestamp key) NOT run at all, in the first place and just provide a FAKE normalized, Json blob with a timestamp in it TO my True/False code - I'd do it. Because think of it, you're NOT testing the functionality of any of those calls - you're just interested in your code. So let's see a fake example here:

def code:

   a = 1
   b = 2
   c = 'https://fakesite.com'

   response = requests.get(a,b,c)
   json = json.convert(response)
   json['time'] = time.time()
   logger.info("Request completed")

   if json.size > 2 and json has.key('time'):
      return yay()
   else:
      return oops()

def oops():
    return 0

def yay():
    return 1

All you want to test is if oops() and yay() get called correctly. Nothing else. The end. So your yay() test (never mind how right now) looks like:

def test:
   response = patch('requests.get', 'fakerequests.get')
               #fake returns {}
   json = patch('json.convert', 'fakejson.convert')
               #fake returns {'a':1,'b':2}
   time = patch('time.time', 'faketime.time')
               # fake returns 1500
   patch('logger.info', 'fakelogger.info')
               # Doesn't matter
   json['time'] = time
               # Add key


   check code() == 1
               #That's what yay() will return and pass your test.

Remember, you could run your test without patching a single thing. As in, let all the 3rd party calls happen and test with real data, but that'll slow things down badly, specially if you have 1000s of tests to run.

And again, if you're still struggling (coz I did for a long time) it's a "unit" test - you isolate the bit you care about, assume everything around it works well, give it the input it needs to work well and then write your tests.

I hope that demystifies it a bit :).