Service Layer Extraction from CLI

Extracted: 2026-02-11
Context: Refactoring a Typer/Click CLI module to separate business logic into a testable service layer

Problem

CLI modules (e.g., main.py with Typer) accumulate business logic alongside CLI concerns. This makes the logic untestable without CLI runners and blocks reuse from other interfaces (Web UI, SDK).

Solution

Extract business logic into service.py with these translation rules:

Conversion at the boundary (main.py):

# Catch service ValueError, convert to CLI output
try:
    files = collect_files(path)
except ValueError as e:
    console.print(f"[red]Error:[/red] {e}")
    raise typer.Exit(code=1) from e

# Pass enum .value to service layer
result = process_file(quality=quality.value, fmt=fmt.value)

When to Use

• CLI module exceeds ~300 LOC with mixed concerns
• Business logic needs testing without CliRunner
• Planning a second interface (Web UI, API, SDK)