Python – How to write Strategy Pattern in Python differently than example in Wikipedia

design-patternspython

In the 2009 Wikipedia entry for the Strategy Pattern, there's a example written in PHP.

Most other code samples do something like:

a = Context.new(StrategyA.new)
a.execute #=> Doing the task the normal way

b = Context.new(StrategyB.new)
b.execute #=> Doing the task alternatively

c = Context.new(StrategyC.new)
c.execute #=> Doing the task even more alternative

In the Python code a different technique is used with a Submit button. I wonder what the Python code will look like if it also did it the way the other code samples do.

Update: Can it be shorter using first-class functions in Python?

Best Solution

The example in Python is not so different of the others. To mock the PHP script:

class StrategyExample:
    def __init__(self, func=None):
        if func:
             self.execute = func

    def execute(self):
        print("Original execution")

def executeReplacement1():
    print("Strategy 1")

def executeReplacement2():
    print("Strategy 2")

if __name__ == "__main__":
    strat0 = StrategyExample()
    strat1 = StrategyExample(executeReplacement1)
    strat2 = StrategyExample(executeReplacement2)

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

Output:

Original execution
Strategy 1
Strategy 2

The main differences are:

  • You don't need to write any other class or implement any interface.
  • Instead you can pass a function reference that will be bound to the method you want.
  • The functions can still be used separately, and the original object can have a default behavior if you want to (the if func == None pattern can be used for that).
  • Indeed, it's clean short and elegant as usual with Python. But you lose information; with no explicit interface, the programmer is assumed as an adult to know what they are doing.

Note that there are 3 ways to dynamically add a method in Python:

  • The way I've shown you. But the method will be static, it won't get the "self" argument passed.

  • Using the class name:

    StrategyExample.execute = func

Here, all the instance will get func as the execute method, and will get self passed as an argument.

  • Binding to an instance only (using the types module):

    strat0.execute = types.MethodType(executeReplacement1, strat0)

    or with Python 2, the class of the instance being changed is also required:

    strat0.execute = types.MethodType(executeReplacement1, strat0, StrategyExample)

This will bind the new method to strat0, and only strat0, like with the first example. But start0.execute() will get self passed as an argument.

If you need to use a reference to the current instance in the function, then you would combine the first and the last method. If you do not:

class StrategyExample:
    def __init__(self, func=None):
        self.name = "Strategy Example 0"
        if func:
             self.execute = func

    def execute(self):
        print(self.name)

def executeReplacement1():
    print(self.name + " from execute 1")

def executeReplacement2():
    print(self.name + " from execute 2")

if __name__ == "__main__":
    strat0 = StrategyExample()
    strat1 = StrategyExample(executeReplacement1)
    strat1.name = "Strategy Example 1"
    strat2 = StrategyExample(executeReplacement2)
    strat2.name = "Strategy Example 2"

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

You will get:

Traceback (most recent call last):
  File "test.py", line 28, in <module>
    strat1.execute()
  File "test.py", line 13, in executeReplacement1
    print self.name + " from execute 1"
NameError: global name 'self' is not defined

So the proper code would be:

import sys
import types

if sys.version_info[0] > 2:  # Python 3+
    create_bound_method = types.MethodType
else:
    def create_bound_method(func, obj):
        return types.MethodType(func, obj, obj.__class__)

class StrategyExample:
    def __init__(self, func=None):
        self.name = "Strategy Example 0"
        if func:
             self.execute = create_bound_method(func, self)

    def execute(self):
        print(self.name)

def executeReplacement1(self):
    print(self.name + " from execute 1")

def executeReplacement2(self):
    print(self.name + " from execute 2")

if __name__ == "__main__":
    strat0 = StrategyExample()
    strat1 = StrategyExample(executeReplacement1)
    strat1.name = "Strategy Example 1"
    strat2 = StrategyExample(executeReplacement2)
    strat2.name = "Strategy Example 2"

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

This will output the expected result:

Strategy Example 0
Strategy Example 1 from execute 1
Strategy Example 2 from execute 2

Of course, in the case the functions cannot be used stand alone anymore, but can still be bound to any other instance of any object, without any interface limitation.

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