A simple text editor, along with tests

simple_text_editor.py

import functools


class TextEditor:
  def __init__(self):
    self.text = ''
    self.left = 0
    self.right = 0
    self.clipboard = ''
    self.operation_stack = OperationStack()

  def return_text(method):
    @functools.wraps(method)
    def wrapper(self, *args, **kwargs):
      method(self, *args, **kwargs)
      return self.text
    return wrapper    

  @return_text
  def append(self, string):
    if not string:
      return
    self._store_and_perform(AppendOperation(self, string))

  @return_text
  def move(self, position):
    self.left = self.right = int(position)

  @return_text
  def backspace(self):
    if self.left == self.right == 0:
      return
    self._store_and_perform(BackspaceOperation(self))

  @return_text
  def select(self, left, right):
    self.left, self.right = max(0, int(left)), min(len(self.text), int(right))

  @return_text
  def copy(self):
    self.clipboard = self.text[self.left:self.right]

  @return_text
  def paste(self):
    self.append(self.clipboard)

  @return_text
  def undo(self):
    try:
      self.operation_stack.last_done().undo()
    except IndexError:
      pass

  @return_text
  def redo(self):
    try:
      self.operation_stack.last_undone().do()
    except IndexError:
      pass

  def _store_and_perform(self, operation):
    self.operation_stack.store(operation)
    operation.do()

  del return_text

class BackspaceOperation:
  def __init__(self, editor):
    self.editor = editor
    self.left = self.right = None
  
  def do(self):
    editor = self.editor
    self.left = self.left or editor.left
    self.right = self.right or editor.right
    left = self.left - (self.left == self.right)
    self.removed = editor.text[left:self.right]
    editor.text = f'{editor.text[:left]}{editor.text[self.right:]}'
    editor.left = editor.right = left

  def undo(self):
    editor = self.editor
    left = self.left - (self.left == self.right)
    editor.text = f'{editor.text[:left]}{self.removed}{editor.text[left:]}'
    editor.left, editor.right = self.left, self.right


class AppendOperation:
  def __init__(self, editor, string):
    self.editor = editor
    self.string = string
    self.left = self.right = None

  def do(self):
    editor = self.editor
    self.left = self.left or editor.left
    self.right = self.right or editor.right
    self.removed = editor.text[self.left:self.right]
    editor.text = f'{editor.text[:self.left]}{self.string}{editor.text[self.right:]}'
    editor.left = editor.right = editor.left + len(self.string)

  def undo(self):
    editor = self.editor
    editor.text = f'{editor.text[:self.left]}{self.removed}{editor.text[self.left + len(self.string):]}'
    editor.left, editor.right = self.left, self.right

class OperationStack:
  def __init__(self):
    self.stack = []
    self.cursor = 0

  def store(self, operation):
    del self.stack[self.cursor:]
    self.stack.append(operation)
    self.cursor += 1

  def last_done(self):
    self.cursor = max(0, self.cursor - 1)
    return self.stack[self.cursor]

  def last_undone(self):
    last = self.stack[self.cursor]
    self.cursor += 1
    return last

operations = {'APPEND': TextEditor.append,
             'MOVE': TextEditor.move,
             'BACKSPACE': TextEditor.backspace,
             'SELECT': TextEditor.select,
             'COPY': TextEditor.copy,
             'PASTE': TextEditor.paste,
             'UNDO': TextEditor.undo,
             'REDO': TextEditor.redo}


def process_queries(*queries, text_editor=None):
  text_editor = text_editor or TextEditor()
  return [operations[operation](text_editor, *parameters) for operation, *parameters in queries]


def test(name, *, queries, expected):
  output = process_queries(*queries)
  for i, (query, out, exp) in enumerate(zip(queries, output, expected), start=1):
    assert out == exp, '\n'.join(("\n"
      f"Test {name} failed at query {i} of {len(queries)}",
      f"{'Query:':9} {query}",
      f"Expected: '{exp}'",
      f"{'Got:':9} '{out}'"))


test(name='1.1',
     queries=[["APPEND", "Hey"], ["APPEND", " there"],["APPEND", "!"]],
     expected=['Hey', 'Hey there', 'Hey there!'])

test(name='1.2',
    queries=[["APPEND", "Hey"], ["APPEND", " you"], ["APPEND", ", don't"], ["APPEND", " "],["APPEND", "let me down"]],
    expected=["Hey", "Hey you", "Hey you, don't", "Hey you, don't ", "Hey you, don't let me down"])

test(name='2',     
     queries=[["APPEND", "Hey you"],["MOVE", "3"], ["APPEND", ","]],
     expected=['Hey you', 'Hey you', 'Hey, you'])

test(name='3.1',
     queries=[["APPEND", "Hey you"], ["BACKSPACE"], ["BACKSPACE"], ["MOVE", "0"], ["BACKSPACE"]],
     expected = ["Hey you", "Hey yo", "Hey y", "Hey y", "Hey y"])

test(name='3.2',
     queries=[["APPEND", "!"], ["BACKSPACE"], ["BACKSPACE"]],
     expected = ['!', '', ''])

test(name='4',
     queries=[["APPEND", "Hello cruel world!"], ["SELECT", "5", "11"], ["APPEND", ","], ["SELECT", "5", "12"], ["BACKSPACE"], ["SELECT", "4", "6"], ["MOVE", "1"]],
     expected=["Hello cruel world!", "Hello cruel world!", "Hello, world!", "Hello, world!", "Hello!", "Hello!", "Hello!"])

test(name='5-6',
     queries=[["APPEND", "Hello, world!"], ["SELECT", "5", "12"], ["COPY"], ["MOVE", "12"], ["PASTE"], ["PASTE"]],
     expected = ["Hello, world!", "Hello, world!", "Hello, world!", "Hello, world!", "Hello, world, world!", "Hello, world, world, world!"])

test(name='7',
    queries=[["APPEND", "Hello, world!"], ["SELECT", "7", "12"], ["BACKSPACE"], ["UNDO"], ["APPEND", "you"], ["MOVE", 6], ["BACKSPACE"], ["UNDO"]],
    expected=["Hello, world!", "Hello, world!", "Hello, !", "Hello, world!", "Hello, you!", "Hello, you!", "Hello you!", "Hello, you!"])

test(name='8',
    queries=[["APPEND", "Hello, world!"], ["SELECT", "7", "12"], ["APPEND", "you"], ["SELECT", "7", "10"], ["BACKSPACE"], ["MOVE", "6"], ["UNDO"], ["UNDO"], ["UNDO"], ["REDO"], ["REDO"], ["REDO"]],
    expected=["Hello, world!", "Hello, world!", "Hello, you!", "Hello, you!", "Hello, !", "Hello, !", "Hello, you!", "Hello, world!", "", "Hello, world!", "Hello, you!", "Hello, !"])

test(name='9',
    queries=[["UNDO"], ["APPEND", "Hello, world!"], ["UNDO"], ["UNDO"], ["REDO"], ["REDO"]],
    expected=["", "Hello, world!", "", "", "Hello, world!", "Hello, world!"])

test(name='10',
     queries=[["APPEND", "Hello, world!"], ["SELECT", "5", "12"], ["COPY"], ["PASTE"], ["PASTE"], ["UNDO"], ["UNDO"], ["UNDO"], ["REDO"], ["REDO"], ["REDO"]],
     expected=["Hello, world!", "Hello, world!", "Hello, world!", "Hello, world!", "Hello, world, world!", "Hello, world!", "Hello, world!", "", "Hello, world!", "Hello, world!", "Hello, world, world!"])

test(name='11',
     queries=[["APPEND", "Hello"], ["UNDO"], ["UNDO"], ["APPEND", "Hi"], ["UNDO"], ["REDO"], ["REDO"]],
     expected=["Hello", "", "", "Hi", "", "Hi", "Hi"])


print('All tests passed')