apply-memento-pattern

Apply Memento Pattern

Capture an object's internal state in an external object (memento) so the state can be restored later, without exposing internal details.

Why This Is Best Practice

Adopted by: Every text editor's undo system (VS Code, IntelliJ, Vim) — each undo operation restores a memento of prior editor state. Database savepoints (SAVEPOINT in SQL) are mementos stored by the transaction engine. Git commits are mementos of filesystem state — each commit captures a snapshot restorable at any time. Redux time-travel debugging (DevTools' "jump to state") replays memento snapshots. Impact: GoF documents that without Memento, undo requires either breaking encapsulation (the undo manager reads private fields) or duplicating all object state externally. Memento lets the object produce its own opaque snapshot, maintaining encapsulation while enabling undo. Why best: The alternative — Command pattern with undo methods — requires every command to implement a perfect inverse operation. For complex state, that is error-prone and sometimes impossible (file content after a regex replace can't always be inverted). Memento captures state directly, making restoration exact.

Sources: Gamma et al. (1994) pp. 283–291; Git internals documentation; Redux DevTools

Steps

Step 1: Define the memento — an opaque snapshot of originator state

from dataclasses import dataclass

@dataclass(frozen=True)
class EditorMemento:
    text: str
    cursor_position: int
    # frozen=True: memento is immutable — protects the snapshot

Make the memento's internal structure opaque to the caretaker (undo history). frozen=True (or a private constructor) enforces this.

Step 2: Add save() and restore() to the originator

class TextEditor:
    def __init__(self):
        self._text = ""
        self._cursor = 0

    def type(self, text: str) -> None:
        self._text = self._text[:self._cursor] + text + self._text[self._cursor:]
        self._cursor += len(text)

    def save(self) -> EditorMemento:
        return EditorMemento(self._text, self._cursor)

    def restore(self, memento: EditorMemento) -> None:
        self._text = memento.text
        self._cursor = memento.cursor_position

    @property
    def content(self) -> str:
        return self._text

The originator creates and restores its own mementos — it's the only one that knows what state needs capturing.

Step 3: Build the caretaker (undo history) — stores mementos, never reads them

class UndoHistory:
    def __init__(self, editor: TextEditor):
        self._editor = editor
        self._history: list[EditorMemento] = []

    def save(self) -> None:
        self._history.append(self._editor.save())

    def undo(self) -> None:
        if self._history:
            self._editor.restore(self._history.pop())

The caretaker holds mementos but treats them as opaque objects — it never inspects or modifies their contents.

Step 4: Use at every state-changing operation

editor = TextEditor()
history = UndoHistory(editor)

history.save()
editor.type("Hello")

history.save()
editor.type(" World")

print(editor.content)   # "Hello World"

history.undo()
print(editor.content)   # "Hello"

history.undo()
print(editor.content)   # ""

Step 5: For large state, use incremental or compressed mementos

If storing full snapshots is expensive (large documents, game world state), store deltas:

@dataclass(frozen=True)
class IncrementalMemento:
    operation: str   # "insert" or "delete"
    position: int
    text: str
    # reconstruct full state by replaying forward or backward

When NOT to Use

• When state is small and Commands support perfect undo — if every operation has a cheap inverse, apply-command-pattern with undo() is simpler and uses less memory.
• When snapshots are prohibitively expensive — if a full-state snapshot requires deep-copying gigabytes, use incremental/delta snapshots or event sourcing.

Common Mistakes

Caretaker that reads memento internals. The caretaker should treat mementos as black boxes. If it reads memento.text to display a preview, it couples to the internal structure — which must then stay stable.

Mutable mementos. If a memento can be modified after capture, the "snapshot" is no longer reliable. Always use immutable (frozen) mementos.

Not saving before every mutation. If the caretaker misses a save() call, an intermediate state is lost from the undo chain. Consider making save() automatic in the originator's mutating methods.