How do you create and test a custom command with the Flask CLI?
Instruction: Use Flask’s built-in CLI to implement a complete non-destructive database initialization example. Show invocation, argument validation, explicit failure behavior, and how to test the command.
Context: Assesses Flask CLI registration, application context, Click options, database error propagation, and command testing.
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For current Flask, use its built-in Click-based CLI. Flask-Script is a legacy extension; a Manager(app) declaration alone is not an executable command entry point. With Flask's CLI, registering a command on app.cli also supplies an application context.
Save this complete Flask 3.x example as app.py...