skills/ai-software-architect/references/gof-template-method.md

<!-- SPDX-FileCopyrightText: 2026 Leonardo Muffato (AUTOSOFT Engineering - www.autosoft-engineering.de) | SPDX-License-Identifier: MIT -->
# Template Method
## Intent
Fix an algorithm skeleton while allowing selected steps to vary.
## Problem and forces
Workflows share sequence and invariants but differ at controlled extension points.
## Applicability
Use when inheritance is already appropriate and the stable skeleton must govern variants.
## When not to use
Avoid when runtime composition, independent step reuse, or multiple inheritance axes are needed.
## Benefits
Centralizes sequence and prevents variants from violating fixed steps.
## Liabilities
Creates inheritance coupling and fragile hooks.
## Implementation considerations
Minimize protected hooks, document call order, and prevent bypass of invariants.
## Python example
**Example context:** A data-export application fixes the load-normalize-serialize workflow while allowing format-specific exporters to supply selected steps.

```python
from abc import ABC, abstractmethod


class DataExporter(ABC):
    def export(self) -> str:
        rows = self.load_rows()
        normalized = [row.strip() for row in rows]
        return self.serialize(normalized)

    @abstractmethod
    def load_rows(self) -> list[str]:
        raise NotImplementedError

    @abstractmethod
    def serialize(self, rows: list[str]) -> str:
        raise NotImplementedError


class CsvExporter(DataExporter):
    def load_rows(self) -> list[str]:
        return ["name, amount", "Ada, 10"]

    def serialize(self, rows: list[str]) -> str:
        return "\n".join(rows)
```
`DataExporter.export()` fixes the algorithm skeleton while subclasses implement only the permitted variation points.
## Credible alternatives
Strategy, pipeline composition, higher-order functions, or explicit orchestration.
## Related patterns
Factory Method, Strategy.
## Architecture interview questions
Which sequence is invariant, and is inheritance the correct lifetime relationship?