README.md

@@ -1,2 +1,55 @@
 # 017-followers
 
+Day 17 of the Udemy *100 Days of Code* course: a small example of how to
+define and use a Python class.
+
+This is a demonstration only. It has no plan, no tests and no dependencies.
+
+## What it shows
+
+The `User` class in [src/main.py](src/main.py) demonstrates:
+
+- a constructor (`__init__`) that sets attributes with default values
+- instance attributes (`id`, `username`, `followers`, `following`)
+- a method (`follow`) that changes the state of two objects at once
+
+```python
+class User:
+
+    def __init__(self, user_id, username):
+        self.id = user_id
+        self.username = username
+        self.followers = 0
+        self.following = 0
+
+    def follow(self, user):
+        user.followers += 1
+        self.following += 1
+```
+
+When `user_1.follow(user_2)` runs, `user_1.following` goes up by one and
+`user_2.followers` goes up by one.
+
+## Run it
+
+```bash
+python src/main.py
+```
+
+Expected output:
+
+```text
+0
+1
+1
+0
+```
+
+The four lines are `user_1.followers`, `user_1.following`,
+`user_2.followers` and `user_2.following`.
+
+## Layout
+
+| Path | Purpose |
+| --- | --- |
+| `src/main.py` | The `User` class and the demo that uses it |

src/main.py

@@ -0,0 +1,32 @@
+"""Demo of a simple `User` class with follower counts."""
+
+
+class User:
+    """A user that can follow other users."""
+
+    def __init__(self, user_id: str, username: str) -> None:
+        self.id = user_id
+        self.username = username
+        self.followers = 0
+        self.following = 0
+
+    def follow(self, user: "User") -> None:
+        """Follow another user, updating both users' counters."""
+        user.followers += 1
+        self.following += 1
+
+
+def main() -> None:
+    user_1 = User(user_id="001", username="Tirsvad")
+    user_2 = User(user_id="002", username="Tirsvad")
+
+    user_1.follow(user_2)
+
+    print(user_1.followers)
+    print(user_1.following)
+    print(user_2.followers)
+    print(user_2.following)
+
+
+if __name__ == "__main__":
+    main()