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The Command Interface

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Step 1: The Command Interface

Every undoable action in an editor is a Command: an object that knows how to do something and how to undo it. Bundling the action and its inverse together is what makes undo possible later. The editor never needs to know the details.

The document is a tiny object holding a string. Implement the first concrete command, Insert.

  • Insert(pos, text): execute(doc) inserts text into doc.text at index pos; undo(doc) removes exactly those characters again
Hints
Starter code
class Document:
    def __init__(self, text=""):
        self.text = text


class Command:
    def execute(self, doc):
        raise NotImplementedError

    def undo(self, doc):
        raise NotImplementedError


class Insert(Command):
    def __init__(self, pos, text):
        self.pos = pos
        self.text = text

    def execute(self, doc):
        """Insert self.text into doc.text at self.pos."""
        # TODO: doc.text = doc.text[:self.pos] + self.text + doc.text[self.pos:]
        pass

    def undo(self, doc):
        """Remove the characters this command inserted."""
        # TODO: doc.text = doc.text[:self.pos] + doc.text[self.pos + len(self.text):]
        pass
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