Add example for data structure type checking

Author: gjbex

source-code/typing/README.md

@@ -38,3 +38,5 @@ Type checking can be done using [mypy](http://mypy-lang.org/index.html).
    matplotlib with type hints.
 1. `classes.py`: illustration of using type hints with a user-defined
    class.
+1. `classes_incorrect.py`: illustration of using type hints with a
+   user-defined class with errors.

source-code/typing/classes_incorrect.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+
+from typing import Any
+
+
+class MyClass:
+
+    def __init__(self, data: Any) -> None:
+        self.data = data
+
+    @property
+    def data(self) -> int:
+        return self.data
+
+    @data.setter
+    def data(self, value: Any) -> None:
+        self.data = int(value)
+
+    def __repr__(self) -> str:
+        return f'data: {self.data}'
+
+
+def print_data(data: MyClass) -> None:
+    print(data)
+
+
+def print_all(datas: list[MyClass]) -> None:
+    for data in datas:
+        print_data(data)
+
+
+if __name__ == '__main__':
+    data = MyClass('25')
+    print_data(data)
+    datas = [data]
+    print_all(datas)
+    datas.append('354')
+    print_all(datas)