examples.md

Memento — where it lives outside this repo

Cited, real implementations to study (or point an agent at) when designing or reviewing snapshot/rollback code.

Python standard library

• dataclasses.replace + frozen dataclasses. Immutability makes snapshots free: replace builds the next state, the previous object is the memento. The foundation this module's History assumes. docs.python.org/3/library/dataclasses.html#dataclasses.replace
• pickle / copy.deepcopy. pickle.dumps produces an opaque snapshot restorable with loads, even in another process; deepcopy is the in-memory equivalent for state you can't freeze. Security (CWE-502): pickle.loads executes code while deserializing — only unpickle snapshots your own process produced and stored where untrusted input cannot reach; use JSON for anything crossing a trust boundary. docs.python.org/3/library/pickle.html#module-pickle

Databases

• SQLAlchemy Session.begin_nested() — a SAVEPOINT as a memento: checkpoint mid-transaction, roll back to it on failure while the outer transaction survives. The validate-or-rollback flow of the mini-project, at database scale. docs.sqlalchemy.org/en/latest/orm/session_transaction.html#using-savepoint
• SQLite SAVEPOINT — the same idea in the database the stdlib ships. sqlite.org/lang_savepoint.html

Everyday tools

• Editor undo persistence — Vim's undo files (:help undo-persistence) are mementos written to disk: state snapshots that outlive the process. (unverified)

What to notice across all of them

Each one keeps the caretaker ignorant: the SAVEPOINT name, the pickle bytes, the undo file are all opaque handles. The moment restoring requires interpreting the snapshot, you are maintaining two copies of the object's logic — the pattern's encapsulation promise is the part worth defending.