Strategy
- Python course
- Advanced
- Lesson 63/83
- 5 min
Learn the Strategy pattern.
Strategy
The Strategy pattern defines different methods of an operation as callable objects; the client picks one at runtime. In Python, functions are naturally callable, so using functions as strategies is easy.
def add(a, b):
return a + b
def mul(a, b):
return a * b
def calculate(strategy, a, b):
return strategy(a, b)
print(calculate(add, 3, 4)) # 7
print(calculate(mul, 3, 4)) # 12Strategy turns conditional branches (if/elif) into callable objects; this makes the code easier to extend.
Function strategies
In Python, functions are first-class objects; they can be passed as parameters and stored in a list. This makes the Strategy pattern easy to apply.
strategies = [add, mul, lambda a, b: a - b]
for s in strategies:
print(s(10, 3)) # 13, 30, 7A class-based strategy is also possible; however, a function is usually shorter and more readable.
Practice Calculate with Strategy
Define def add(a, b): and def mul(a, b): (add does return a + b, mul does return a * b). Write def calculate(strategy, a, b): that does return strategy(a, b). Print the results with print(calculate(add, 3, 4)) and print(calculate(mul, 3, 4)).
Starter code
# define add and mul functions
# write calculate: it calls the strategy parameter
# call calculate with add and mul strategies, print the resultsSample solution
def add(a, b):
return a + b
def mul(a, b):
return a * b
def calculate(strategy, a, b):
return strategy(a, b)
print(calculate(add, 3, 4))
print(calculate(mul, 3, 4))Expected output
7 12
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the Strategy concepts
| Term | Meaning |
|---|---|
strategy parameter |
Is callable |
add, mul |
Are strategy functions |
calculate(strat, a, b) |
Calls the strategy |
if/elif |
Stays in code without strategy |
2 How does calculate(strategy, a, b) work in the Strategy pattern?
- strategy(a, b) is called Correct answer
- strategy is ignored
- strategy is printed
- strategy is deleted
Explanation: calculate calls the strategy parameter; it is a function.
3 Complete the calculate body
def calculate(strategy, a, b):
return ____(a, b)def calculate(strategy, a, b):
return strategy(a, b)Explanation: return strategy(a, b) calls the strategy.