No, undo needs real inverse behavior. Sending an email or shipping a parcel cannot simply be erased by an undo method.

A command captures the receiver and operation parameters. AppendCommand saves a before and after value and permits one execute followed by an undo only when the document still equals its expected after value. A practical editor would coordinate a command history and clear or rebase it on conflicting edits. Commands can support queues, audit, and undo, but those guarantees require explicit supporting state.

What the function promises: A command permits one active execution; successful undo allows execution again. Undo refuses when the document differs from the expected result. Equal text does not prove no intervening edits occurred; a real editor should track revisions. The example is local and single-threaded.

class Document:
    def __init__(self, text=""):
        self.text = text

class AppendCommand:
    def __init__(self, document, text):
        self.document, self.text = document, text
        self.executed = False
    def execute(self):
        if self.executed:
            raise ValueError("Command already executed")
        self.before = self.document.text
        self.after = self.before + self.text
        self.document.text = self.after
        self.executed = True
    def undo(self):
        if not self.executed or self.document.text != self.after:
            raise ValueError("Cannot safely undo this command")
        self.document.text = self.before
        self.executed = False

doc = Document("Hi")
command = AppendCommand(doc, "!")
command.execute()
assert doc.text == "Hi!"
command.undo()
assert doc.text == "Hi"

Work and storage: O(text length) copied string work and snapshot storage.

When to recognize it: Remember the previous text. Undo is safe only if no later unrelated edit has changed the document since this command ran.

Check before applying it: Not every command has a true inverse. Define retry and undo behavior before advertising them.

No. A compensating action such as refunding is a new business operation, may fail, and may not erase history. That distinction leads to sagas.