Merge pull request faif#257 from gyermolenko/refactor_strategy

Refactor Strategy

Author: faif

behavioral/strategy.py

@@ -3,71 +3,48 @@
 
 """
 *What is this pattern about?
-This pattern aims to encapsulate each algorithm and allow them to be
-interchangeable. Separating algorithms allows the client to scale
-with larger and more complex algorithms, since the client and the
-strategies are kept independent of each other.
-
-Having the algorithms as an integral part of the client can cause the
-client to be larger and harder to maintain. This is more evident when
-supporting multiple algorithms. The separation of client and algorithm
-allows us to easily replace and vary the algorithm.
-
-*What does this example do?
-Below the 'StrategyExample' is an example of the client while the two
-functions; 'execute_replacement1' and 'execute_replacement2' are
-examples of the implementation or strategy. In the example we can see
-that the client can vary it's 'execute' method by changing the
-strategy which is responsible for implementation.
-
-http://stackoverflow.com/questions/963965/how-is-this-strategy-pattern
- -written-in-python-the-sample-in-wikipedia
-In most of other languages Strategy pattern is implemented via creating some
-base strategy interface/abstract class and subclassing it with a number of
-concrete strategies (as we can see at
-http://en.wikipedia.org/wiki/Strategy_pattern), however Python supports
-higher-order functions and allows us to have only one class and inject
-functions into it's instances, as shown in this example.
+Define a family of algorithms, encapsulate each one, and make them interchangeable.
+Strategy lets the algorithm vary independently from clients that use it.
 
 *TL;DR80
 Enables selecting an algorithm at runtime.
 """
 
-import types
-
-
-class StrategyExample:
-    def __init__(self, func=None):
-        self.name = 'Strategy Example 0'
-        if func is not None:
-            self.execute = types.MethodType(func, self)
-
-    def execute(self):
-        print(self.name)
 
+class Order:
+    def __init__(self, price, discount_strategy=None):
+        self.price = price
+        self.discount_strategy = discount_strategy
 
-def execute_replacement1(self):
-    print(self.name + ' from execute 1')
+    def price_after_discount(self):
+        if self.discount_strategy:
+            discount = self.discount_strategy(self)
+        else:
+            discount = 0
+        return self.price - discount
 
+    def __repr__(self):
+        fmt = "<Price: {}, price after discount: {}>"
+        return fmt.format(self.price, self.price_after_discount())
 
-def execute_replacement2(self):
-    print(self.name + ' from execute 2')
 
+def ten_percent_discount(order):
+    return order.price * 0.10
 
-if __name__ == '__main__':
-    strat0 = StrategyExample()
 
-    strat1 = StrategyExample(execute_replacement1)
-    strat1.name = 'Strategy Example 1'
+def on_sale_discount(order):
+    return order.price * 0.25 + 20
 
-    strat2 = StrategyExample(execute_replacement2)
-    strat2.name = 'Strategy Example 2'
 
-    strat0.execute()
-    strat1.execute()
-    strat2.execute()
+if __name__ == "__main__":
+    order0 = Order(100)
+    order1 = Order(100, discount_strategy=ten_percent_discount)
+    order2 = Order(1000, discount_strategy=on_sale_discount)
+    print(order0)
+    print(order1)
+    print(order2)
 
 ### OUTPUT ###
-# Strategy Example 0
-# Strategy Example 1 from execute 1
-# Strategy Example 2 from execute 2
+# <Price: 100, price after discount: 100>
+# <Price: 100, price after discount: 90.0>
+# <Price: 1000, price after discount: 730.0>

tests/test_strategy.py

@@ -14,7 +14,7 @@ def test_print_output(self):
         """
         output = subprocess.check_output(["python", "behavioral/strategy.py"])
         expected_output = os.linesep.join(
-            ['Strategy Example 0', 'Strategy Example 1 from execute 1', 'Strategy Example 2 from execute 2', '']
+            ['<Price: 100, price after discount: 100>', '<Price: 100, price after discount: 90.0>', '<Price: 1000, price after discount: 730.0>', '']
         )
         # byte representation required due to EOF returned subprocess
         expected_output_as_bytes = expected_output.encode(encoding='UTF-8')