Add illustration of NewType

Author: gjbex

source-code/typing/README.md

@@ -24,8 +24,8 @@ Type checking can be done using [mypy](http://mypy-lang.org/index.html).
 1. `dict_incorrect.py`: code that counts the words in a text read from standard
    input.  The counts are subsequently normalized to `float`, which is a type
    error.
-1. `dict_correct_type_statement.py`: same code as `dict_correct.py`, but
-   with a type variable as type statement.
+1. `dict_correct_type_statement.py`: same code as `dict_correct.py`, but with a
+   type variable as type statement.
 1. `people_incorrect.py`: code that defines a `People` class, stores some in a
    list with mistakes.
 1. `duck_typing.py`: example code illustrating duck typing.
@@ -42,4 +42,7 @@ Type checking can be done using [mypy](http://mypy-lang.org/index.html).
 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.
-1. `tree.py`: illustration of using type hints on more sophisticated classes, as well as generic types.
+1. `tree.py`: illustration of using type hints on more sophisticated classes,
+   as well as generic types.
+1. `new_types.py`: illustration of using `NewType` to create specific types
+   with specific semantics. 

source-code/typing/new_types.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python
+
+from typing import NewType
+
+PersonId = NewType('PersonId', int)
+Age = NewType('Age', int)
+
+class Person:
+    _person_id: PersonId
+    _age: Age
+
+    def __init__(self, person_id: PersonId, age: Age) -> None:
+        self._person_id = person_id
+        self._age = age
+
+    @property
+    def person_id(self) -> PersonId:
+        return self._person_id
+
+    @person_id.setter
+    def person_id(self, person_id: PersonId) -> None:
+        self._person_id = person_id
+
+    @property
+    def age(self) -> Age:
+        return self._age
+
+    @age.setter
+    def age(self, age: Age) -> None:
+        self._age = age
+
+    def __repr__(self) -> str:
+        return f'Person(person_id={self.person_id}, age={self.age})'
+
+
+if __name__ == '__main__':
+    person = Person(PersonId(1), Age(20))
+    print(person)
+    person.person_id = PersonId(2)
+    person.age = Age(30)
+    print(person)
+    print(person.person_id)
+    print(person.age)
+    print(person.person_id + 1)
+    print(person.age + 1)
+    print(person.person_id + person.age)
+    person.age = PersonId(40)
+    person.age = 25