memento

id	behavioral/memento
name	Memento
aliases	snapshot undo-token
guide_url
problem	Capture an object's state so it can be restored later, without exposing its internals.
symptoms	undo checkpoint and rollback save game restore previous state
verdict	use-with-care
caveats	Immutable state makes the pattern nearly free: a snapshot is just keeping the old object. Design the state to be frozen and mementos fall out. pickle.loads executes code while deserializing — only unpickle snapshots your own process produced; use JSON for anything crossing a trust boundary. Deep-copying big mutable graphs per keystroke is the obvious-first-attempt cost; snapshot the smallest state that matters.
stdlib_sightings	copy.deepcopy pickle.dumps dataclasses.replace

Memento

Keep "how it was" so you can go back — undo, checkpoints, rollback — without letting the keeper read what it keeps. Verdict: use with care — freeze the state and the pattern is nearly free.

Where	What
pattern/	The importable code: History, NoSnapshotError
docs/	Fundamentals · Implementation guide · External examples
examples/config_checkpoints/	Mini-project: validate-or-rollback config editing built on pattern/
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.behavioral.memento.examples.config_checkpoints.main