content: from multiprocessing import Value
  
  from copy import copy
  from selenium.webdriver.common.keys import Keys
  
  from dash import Dash, no_update, Input, Output, State, html, dcc
  from dash.exceptions import PreventUpdate
  
  from dash.testing.wait import until
  
  
  def test_cbpu001_aborted_callback(dash_duo):
      """Raising PreventUpdate OR returning no_update prevents update and
      triggering dependencies."""
  
      initial_input = "initial input"
      initial_output = "initial output"
  
      app = Dash(__name__)
      app.layout = html.Div(
          [
              dcc.Input(id="input", value=initial_input),
              html.Div(initial_output, id="output1"),
              html.Div(initial_output, id="output2"),
          ]
      )
  
      callback1_count = Value("i", 0)
      callback2_count = Value("i", 0)
  
      @app.callback(Output("output1", "children"), [Input("input", "value")])
      def callback1(value):
          callback1_count.value += 1
          if callback1_count.value > 2:
              return no_update
          raise PreventUpdate("testing callback does not update")
          return value
  
      @app.callback(
          Output("output2", "children"),
          [Input("output1", "children")],
          prevent_initial_call=True,
      )
      def callback2(value):
          callback2_count.value += 1
          return value
  
      dash_duo.start_server(app)
  
      input_ = dash_duo.find_element("#input")
      for i, key in enumerate("xyz"):
          input_.send_keys(key)
          until(
              lambda: callback1_count.value == i + 2,
              timeout=3,
              msg="callback1 runs 4x (initial page load and 3x through send_keys)",
          )
  
      dash_duo.wait_for_text_to_equal("#input", "initial inputxyz")
  
      assert (
          callback2_count.value == 0
      ), "callback2 is never triggered, even on initial load"
  
      # double check that output1 and output2 children were not updated
      assert dash_duo.find_element("#output1").text == initial_output
      assert dash_duo.find_element("#output2").text == initial_output
  
      assert not dash_duo.get_logs()
  
      dash_duo.percy_snapshot(name="aborted")
  
  
  def test_cbpu002_multi_output_no_update(dash_duo):
      app = Dash(__name__)
  
      app.layout = html.Div(
          [
              html.Button("B", "btn"),
              html.P("initial1", "n1"),
              html.P("initial2", "n2"),
              html.P("initial3", "n3"),
          ]
      )
  
      @app.callback(
          [Output("n1", "children"), Output("n2", "children"), Output("n3", "children")],
          [Input("btn", "n_clicks")],
      )
      def show_clicks(n):
          # partial or complete cancellation of updates via no_update
          return [
              no_update if n and n > 4 else n,
              no_update if n and n > 2 else n,
              # make a new instance, to mock up caching and restoring no_update
              copy(no_update),
          ]
  
      dash_duo.start_server(app)
  
      dash_duo.multiple_click("#btn", 10, 0.2)
  
      dash_duo.wait_for_text_to_equal("#n1", "4")
      dash_duo.wait_for_text_to_equal("#n2", "2")
      dash_duo.wait_for_text_to_equal("#n3", "initial3")
  
  
  def test_cbpu003_no_update_chains(dash_duo):
      app = Dash(__name__)
  
      app.layout = html.Div(
          [
              dcc.Input(id="a_in", value="a"),
              dcc.Input(id="b_in", value="b"),
              html.P("", id="a_out"),
              html.P("", id="a_out_short"),
              html.P("", id="b_out"),
              html.P("", id="ab_out"),
          ]
      )
  
      @app.callback(
          [Output("a_out", "children"), Output("a_out_short", "children")],
          [Input("a_in", "value")],
      )
      def a_out(a):
          return a, a if len(a) < 3 else no_update
  
      @app.callback(Output("b_out", "children"), [Input("b_in", "value")])
      def b_out(b):
          return b
  
      @app.callback(
          Output("ab_out", "children"),
          [Input("a_out_short", "children")],
          [State("b_out", "children")],
      )
      def ab_out(a, b):
          return a + " " + b
  
      dash_duo.start_server(app)
  
      a_in = dash_duo.find_element("#a_in")
      b_in = dash_duo.find_element("#b_in")
  
      b_in.send_keys("b")
      a_in.send_keys("a")
      dash_duo.wait_for_text_to_equal("#a_out", "aa")
      dash_duo.wait_for_text_to_equal("#b_out", "bb")
      dash_duo.wait_for_text_to_equal("#a_out_short", "aa")
      dash_duo.wait_for_text_to_equal("#ab_out", "aa bb")
  
      b_in.send_keys("b")
      a_in.send_keys("a")
      dash_duo.wait_for_text_to_equal("#a_out", "aaa")
      dash_duo.wait_for_text_to_equal("#b_out", "bbb")
      dash_duo.wait_for_text_to_equal("#a_out_short", "aa")
      # ab_out has not been triggered because a_out_short received no_update
      dash_duo.wait_for_text_to_equal("#ab_out", "aa bb")
  
      b_in.send_keys("b")
      a_in.send_keys(Keys.END)
      a_in.send_keys(Keys.BACKSPACE)
      dash_duo.wait_for_text_to_equal("#a_out", "aa")
      dash_duo.wait_for_text_to_equal("#b_out", "bbbb")
      dash_duo.wait_for_text_to_equal("#a_out_short", "aa")
      # now ab_out *is* triggered - a_out_short got a new value
      # even though that value is the same as the last value it got
      dash_duo.wait_for_text_to_equal("#ab_out", "aa bbbb")
  