In the sixth installment of the Software Design Patterns blog series I’ll talk about the Strategy Pattern. This pattern is interesting because it allows the consumer of this code to choose what “strategy” it wants to use to do the work that needs to be done. You could have a list of different algorithms that are interchangeable and depending on the need we can choose one or the other. You can use this pattern in conjunction with the Adapter Pattern.

Let’s see an example.

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)

def execute_replacement1(self):
    print(self.name + ' from execute 1')

def execute_replacement2(self):
    print(self.name + ' from execute 2')

If we see the StrategyExample class it has an execute function that just prints out the name property, but in the initializer it can receive a func parameter that would be a function that replaces the execute function if func is present.

Down below we also see two functions execute_replacement1 and execute_replacement2 that just prints out a property name plus something else, the purpose of these functions is to act as replacements for the execute functions for the StrategyExample class

Now let’s see how we can implement it

strat0 = StrategyExample()

strat1 = StrategyExample(execute_replacement1)
strat1.name = 'Strategy Example 1'

strat2 = StrategyExample(execute_replacement2)
strat2.name = 'Strategy Example 2'

strat0.execute()
strat1.execute()
strat2.execute()

We create 3 instances of the StrategyExample class, one with no replacement and the other two with replacements for the execute function with execute_replacement1 and execute_replacement2, after the three initializations we call execute on the three instances of the same class but see three different results, in this case the three implementations use the name property but in three different ways, this is the flexibility of the Strategy Pattern.

If you have any comments or questions feel free to reach out on twitter at @fcastellanos.