PyPro #95 Strategy Design Pattern

Strategy Design Pattern in Python

Strategy Design Pattern in Python

Write a Python program to implement Strategy Design Pattern.

import sys
import types
 
# if construct to select the right amount of arguments to method MethodType
# MethodType binds a function to an instance of a class, or to all instances of the class.
# The function being bound either gets added in the class definition, or replaces the contents of a method defined in the class.
# The MethodType takes two arguments in Python 3, first, the method to be bound, second, the instance to which the method is to be bound.
# In Python 2, there is third positional argument, which seeks the class to which the instance passed as second argument, belongs.
if sys.version_info[0] > 2:                                                                                 # Python 3
    createBoundMethod = types.MethodType
else:
    def createBoundMethod(func, obj):                                                               # Python 2
        return types.MethodType(func, obj, obj.__class__)
         
class Strategy:
    '''Strategy Class: houses default strategy; provides mechanism to replace the execute() method with a supplied method'''
 
    def __init__(self, strategyName = 'Default Strategy', replacementFunction=None):
        '''If strategyName is provided, sets strategy name to string provided;
          If a reference to new strategy function is provided, replace the execute() method with the supplied function.'''
        self.name = strategyName
        if replacementFunction:
            self.execute = createBoundMethod(replacementFunction, self)       
 
    def execute(self):
        '''Contains default strategy behavior.
          If a reference to a new strategy function is provided, then the body of this function gets replaced by body of supplied method'''
        print("Executing {}...".format(self.name))
 
# Behavior of Strategy One
def strategyOne(self):
    '''Contains behavior of Strategy One.'''
    print("Executing {}...".format(self.name))
 
# Behavior of Strategy Two  
def strategyTwo(self):
    '''Contains behavior of Strategy Two.'''
    print("Executing {}...".format(self.name))
 
     
     
## USING THE ABOVE SETUP ##
# Instantiating Default Strategy
defaultStrategy = Strategy()
 
# Instantiating Strategy One
strategyONE = Strategy('Strategy One', strategyOne)
 
# Instantiating Strategy Two
strategyTWO = Strategy('Strategy Two', strategyTwo)
 
# Executing different strategies
defaultStrategy.execute()
strategyONE.execute()
strategyTWO.execute()
 
### OUTPUT ###
Executing Default Strategy...
Executing Strategy One...
Executing Strategy Two...

See also:

• Python 101
• Object Oriented Python
• Design Patterns in Python
• 50+ Handy Standard Library Modules
• 60+ Handy Third Library Modules
• 50+ Tips & Tricks for Python Developers
• 50+ Know-How(s) Every Pythonista Must Know