Recap: Command Pattern

A request as an object: queue it, log it, and undo it if it remembers what it changed.

Command turns a request into an object that can be held before, and after, it runs.

Intent. Encapsulate a request as an object, to parameterise, queue, log or undo it.
Participants. The command interface, the concrete command, the receiver that does the work, and the invoker that runs commands and may keep a history.
Undo. A stack of commands; each saves in execute what its change destroys, and restores it in undo.
In the wild. Java's Runnable and Callable, queued and run by an ExecutorService.
In Python. A function or functools.partial is a command that only runs; undo needs a class.
The trap. An undo that tries to read the old state from the receiver, which no longer has it.
When not. A single immediate call that is never stored, queued or reversed.

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