apply-strategy-pattern

Apply Strategy Pattern

Define a family of algorithms, encapsulate each one, and make them interchangeable — letting the algorithm vary independently from the clients that use it.

Why This Is Best Practice

Adopted by: Java's Comparator interface (Strategy for sorting — used in every Collections.sort() call), Python's sorted(key=...) (a Strategy as a first-class function), scikit-learn's estimator API (every ML algorithm is a Strategy implementing fit()/predict() — the library's entire design), and AWS SDK's configurable retry strategies (RetryPolicy interface). Impact: GoF documents that Strategy eliminates conditional logic that selects algorithms at runtime. scikit-learn's adoption of Strategy is directly responsible for its plug-and-play model comparison: swapping RandomForestClassifier for SVC requires one line — because both implement the same Strategy interface. Why best: if/elif dispatch — the alternative — couples the client to every algorithm variant and requires modification for every new algorithm. Strategy makes each algorithm independently changeable, testable, and composable without touching the client.

Sources: Gamma et al. (1994) pp. 315–323; Java Comparator specification; scikit-learn API documentation

Steps

Step 1: Define the strategy interface

from abc import ABC, abstractmethod

class SortStrategy(ABC):
    @abstractmethod
    def sort(self, data: list) -> list: ...

Step 2: Implement concrete strategies — one class per algorithm

class BubbleSortStrategy(SortStrategy):
    def sort(self, data: list) -> list:
        data = list(data)
        n = len(data)
        for i in range(n):
            for j in range(0, n - i - 1):
                if data[j] > data[j + 1]:
                    data[j], data[j + 1] = data[j + 1], data[j]
        return data

class QuickSortStrategy(SortStrategy):
    def sort(self, data: list) -> list:
        if len(data) <= 1:
            return data
        pivot = data[len(data) // 2]
        left = [x for x in data if x < pivot]
        mid = [x for x in data if x == pivot]
        right = [x for x in data if x > pivot]
        return self.sort(left) + mid + self.sort(right)

Step 3: Context holds a strategy reference — delegates to it

class Sorter:
    def __init__(self, strategy: SortStrategy):
        self._strategy = strategy

    def set_strategy(self, strategy: SortStrategy) -> None:
        self._strategy = strategy

    def sort(self, data: list) -> list:
        return self._strategy.sort(data)

Step 4: Swap strategies at runtime without changing context

sorter = Sorter(BubbleSortStrategy())
print(sorter.sort([3, 1, 4, 1, 5]))   # [1, 1, 3, 4, 5]

sorter.set_strategy(QuickSortStrategy())
print(sorter.sort([3, 1, 4, 1, 5]))   # [1, 1, 3, 4, 5] — same result, different algorithm

Step 5: In Python, use first-class functions as lightweight strategies

from typing import Callable

class Sorter:
    def __init__(self, strategy: Callable[[list], list] = sorted):
        self._strategy = strategy

    def sort(self, data: list) -> list:
        return self._strategy(data)

# Use built-ins or lambdas as strategies — no class needed
sorter = Sorter(strategy=lambda d: sorted(d, reverse=True))

Use class-based strategies when state is needed; function strategies for stateless algorithms.

When NOT to Use

• Only one algorithm ever — if the algorithm never varies, a direct method call is simpler. Strategy pays off only when multiple alternatives exist.
• Trivial variation — if the difference between strategies is a single parameter (ascending vs descending sort), a reverse=True flag is cleaner than two strategy classes.

Common Mistakes

Passing strategy selection logic into the strategy. A Strategy that asks "which variant should I use?" is no longer a Strategy — it's a dispatcher. Each Strategy implements exactly one algorithm.

State leakage between calls. Strategies that store results from previous sort() calls produce bugs when reused. Strategies should be stateless or clearly document what state they maintain.

Using Strategy when Template Method is the right pattern. If all algorithms share the same skeleton but differ only in specific steps, apply-template-method-pattern is more appropriate than several full Strategy implementations.