Add clean example using a factory function

Author: gjbex

source-code/typing/README.md

@@ -1,42 +1,40 @@
-# Typing
-Python 3.5 introducd optional type annotation for functions, and that
-functionality was extended in Python 3.6.
+# Typing Python 3.5 introducd optional type annotation for functions, and
+that functionality was extended in Python 3.6.
 
-The `mypy` static type checker can use this annotation to detect type
-errors.
+The `mypy` static type checker can use this annotation to detect type errors.
 
 Type checking can be done using [mypy](http://mypy-lang.org/index.html).
 
 ## What is it?
 1. `mypy.ini`: mypy configuration file.
 1. `correct.py`: code that has type annotations, and no type errors.
-1. `incorrect_01.py`: code that has type annotations, and passes a string
-    to a function that expects an `int`.
+1. `incorrect_01.py`: code that has type annotations, and passes a string to
+   a function that expects an `int`.
 1. `incorrect_02.py`: code that has type annotations, and the result of a
-    function that returns an `int` is assigned to a `str` variable.
+   function that returns an `int` is assigned to a `str` variable.
 1. `incorrect_03.py`: code that has type annotations, and the result of a
-    function that returns an `int`, assigns it to a variable that is
-    later used as a `str`.
-1. `dict_correct.py`: code that counts the words in a text read from
-    standard input.
+   function that returns an `int`, assigns it to a variable that is later
+   used as a `str`.
+1. `dict_correct.py`: code that counts the words in a text read from standard
+   input.
 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. `people_incorrect.py`: code that defines a `People` class, stores some
-    in a list with mistakes.
+   standard input.  The counts are subsequently normalized to `float`, which
+   is a type error.
+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.
 1. `duck_typing_incorrect.py`: example code illustrating duck typing, but
    with an error.
-1. `typed_duck_typing.py`: example code illustrating duck typing
-   using type hints.
+1. `typed_duck_typing.py`: example code illustrating duck typing using type
+   hints.
+1. `typed_duck_typing_clean.py`: example code illustrating duck typing using
+   type hints with a factory function.
 1. `typed_duck_typing_incorrect.py`: example code illustrating duck typing
    using type hints with an error.
-1. `typed_duck_typing_false_positive.py`: example code illustrating
-   duck typing using type hints for which mypy 0.910 generates a
-   false positive.
+1. `typed_duck_typing_false_positive.py`: example code illustrating duck
+   typing using type hints for which mypy 0.910 generates a false positive.
 1. `numpy_typing.py`: illustration of a script using both numpy and
    matplotlib with type hints.
-1. `classes.py`: illustration of using type hints with a user-defined
-   class.
+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/typed_duck_typing_clean.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python
+import argparse
+from typing import Protocol
+
+
+class SoundMaker(Protocol):
+
+    def make_sound(self) -> None:
+        pass
+
+
+class Duck:
+
+    def make_sound(self) -> None:
+        print('quack')
+
+
+class AlarmClock:
+
+    def make_sound(self) -> None:
+        print('ring-ring')
+
+
+def sound_maker_factory(sound_type: str) -> SoundMaker:
+    if sound_type == 'duck':
+        return Duck()
+    elif sound_type == 'alarm':
+        return AlarmClock()
+    else:
+        raise ValueError(f'unknown sound type: {sound_type}')
+
+
+def sound_repeater(sound_maker: SoundMaker, nr_repeats: int) -> None:
+    for _ in range(nr_repeats):
+        sound_maker.make_sound()
+
+
+if __name__ == '__main__':
+    arg_parser = argparse.ArgumentParser(description='sound maker')
+    arg_parser.add_argument('--type', choices=['duck', 'alarm'],
+                            help='sound source')
+    arg_parser.add_argument('--n', type=int, default=1,
+                            help='number of sounds to make')
+    options = arg_parser.parse_args()
+    sound_maker: SoundMaker = sound_maker_factory(options.type)
+    sound_repeater(sound_maker, options.n)