Add Usage chapter to README.md

Author: Maxcode123

README.md

@@ -17,7 +17,119 @@ This minimal library aims to simplify behavioural testing with Python's standard
 pip install unittest-extensions
 ```
 # Usage
+Suppose you have some code that looks like this:
 
+```py
+from dataclasses import dataclass
+
+@dataclass
+class User:
+    name: str
+    surname: str
+
+    def is_relative_to(self, user: "User") -> bool:
+        return self.surname.casefold() == user.surname.casefold()
+```
+This is a dummy example, meaning that how exactly the User and their methods are implemented does not really matter; what we actually care about here is how to test this code given the above implementation.   
+Say we'd like to test the `is_relative_to` method with pairs of User names and surnames using the standard `unittest` library.
+So, here's an example of how we could do that as simply as possible:
+## unittest
+
+```py
+from unittest import main, TestCase
+
+
+class TestIsRelativeToSameName(TestCase):
+    def test_same_name(self):
+        user1 = User("Niklas", "Strindberg")
+        user2 = User("Niklas", "Ibsen")
+        self.assertFalse(user1.is_relative_to(user2))
+
+    def test_same_empty_name(self):
+        user1 = User("", "Stringberg")
+        user2 = User("", "Ibsen")
+        self.assertFalse(user1.is_relative_to(user2))
+
+
+class TestIsRelativeToSameSurname(TestCase):
+    def test_same_surname(self):
+        user1 = User("August", "Nietzsche")
+        user2 = User("Henrik", "Nietzsche")
+        self.assertTrue(user1.is_relative_to(user2))
+
+    def test_same_empty_surname(self):
+        user1 = User("August", "")
+        user2 = User("Henrik", "")
+        self.assertTrue(user1.is_relative_to(user2))
+
+    def test_same_surname_case_sensitive(self):
+        user1 = User("August", "NiEtZsChE")
+        user2 = User("Henrik", "nietzsche")
+        self.assertTrue(user1.is_relative_to(user2))
+
+    def test_surname1_contains_surname2(self):
+        user1 = User("August", "Solzenietzsche")
+        user2 = User("Henrik", "Nietzsche")
+        self.assertFalse(user1.is_relative_to(user2))
+
+
+if __name__ == "__main__":
+    main()
+```
+
+The cases we check here are rather limited but still there is some duplication in our code; we use many lines to create our User subjects. Of course we can avoid that
+by creating custom assertion methods that receive only the parameters that change
+between tests, but that's why a testing library is here for.  
+Here's how we could write the above code with `unittest-extensions`:
+
+## unittest-extensions
+```py
+from unittest import main
+
+from unittest_extensions import TestCase, args
+
+
+class TestIsRelativeToSameName(TestCase):
+    def subject(self, name1, name2):
+        return User(name1, "Strindberg").is_relative_to(User(name2, "Ibsen"))
+
+    @args({"name1": "Niklas", "name2": "Niklas"})
+    def test_same_name(self):
+        self.assertResultFalse()
+
+    @args({"name1": "", "name2": ""})
+    def test_same_empty_name(self):
+        self.assertResultFalse()
+
+
+class TestIsRelativeToSameSurname(TestCase):
+    def subject(self, surname1, surname2):
+        return User("August", surname1).is_relative_to(User("Henrik", surname2))
+
+    @args({"surname1": "Nietzsche", "surname2": "Nietzsche"})
+    def test_same_surname(self):
+        self.assertResultTrue()
+
+    @args({"surname1": "", "surname2": ""})
+    def test_same_empty_surname(self):
+        self.assertResultTrue()
+
+    @args({"surname1": "NiEtZsChE", "surname2": "Nietzsche"})
+    def test_same_surname_case_sensitive(self):
+        self.assertResultTrue()
+
+    @args({"surname1": "Nietzsche", "surname2": "Solszenietzsche"})
+    def test_surname2_contains_surname1(self):
+        self.assertResultFalse()
+
+
+if __name__ == "__main__":
+    main()
+```
+
+The number of lines is still the same, but the testing code has become clearer:  
+1. The subject of our assertions in each test case is documented in the `subject` method
+2. Each test method contains only information we care about, i.e. the input data (names/surnames) we test and the result of the assertion (true/false).
 
 # License
 [MIT License](https://opensource.org/license/mit/)