Template Method

The Template Method pattern defines the skeleton of an algorithm in a method, deferring some steps to subclasses. It allows one of the algorithm’s steps to be overridden by a subclass without changing the algorithm’s structure. This promotes code reuse and reduces redundancy by centralizing common logic while providing flexibility for specific variations.

This pattern is particularly useful when you have a process with several steps that are largely the same across different scenarios, but certain steps need to be customized. It’s also valuable when you want to enforce a specific order of operations, ensuring consistency while enabling extension. The abstract class implements the overall algorithm, while concrete classes provide implementations for the abstract steps.

Usage

The Template Method pattern is frequently used in scenarios like:

• Framework Development: Creating the basic structure of a framework where the core logic is defined, and clients fill in the specific details.
• Report Generation: Generating reports with a common format, but different sections depending on the type of report.
• Data Processing Pipelines: Implementing a pipeline with stages that are consistent but have varying data transformation logic.
• Game Development: Defining the basic flow of a game level, but allowing different levels to have their specific events or behaviors.

Examples

1. Java I/O Streams: The InputStream class in Java employs the Template Method pattern. The read() method is the template method, defining the overall process of reading data. Subclasses like FileInputStream and ByteArrayInputStream override the read() method to provide the specific implementation for reading from a file or a byte array, respectively. The core logic of handling buffering and error checking remains in the InputStream class.

2. Django’s Class-Based Views (CBVs): Django’s CBVs use this pattern extensively. A base class like View defines the as_get_view(), as_post_view() methods (the template method) outlining the request handling process. Then, different types of views (e.g., DetailView, ListView) inherit from View and override specific methods like get() or post() to perform tailored actions. The overall request-response cycle is managed in the base View class, ensuring consistency across all views.

3. Scikit-learn Estimators: In the Python machine learning library scikit-learn, estimators (like LogisticRegression or DecisionTreeClassifier) often follow the Template Method pattern. The fit() method is a template method that defines the learning process. Subclasses implement more specific fitting algorithms while relying on the common infrastructure and validation mechanisms defined in the base estimator class.