text editor

class Editor:
    def __init__(self):
        self.s = ''
        self.hist = [self.s]
        self.hi = 0 # history index -> current state

    def append(self, t):
        self.s += t
        self._save()

    def delete(self, n):
        self.s = self.s[:-n]
        self._save()

    def undo(self):
        if self.hi:
            self.hi -= 1
            self.s = self.hist[self.hi]

    def redo(self):
        if self.hi < len(self.hist) - 1:
            self.hi += 1
            self.s = self.hist[self.hi]

    def _save(self):
        self.hi += 1
        if self.hi <= len(self.hist):
            self.hist[self.hi:] = [self.s]
        else:
            self.hist.append(self.s)

if __name__ == '__main__':
    e = Editor()
    e.append('Hello, World')
    print(e.s)
    e.delete(5)
    e.append('sekai')
    e.append('!')
    print(e.s)
    e.undo()
    e.undo()
    e.undo()
    print(e.s)
    e.redo()
    e.redo()
    e.append('~')
    print(e.s)