import altair as alt

click = alt.selection_point("click", empty=False)
ctrl_click = alt.selection_point("ctrl_click", on="click[event.ctrlKey]", empty=True)

>>> (click & ctrl_click).to_dict()
{'and': [{'param': 'click', 'empty': False},
  {'param': 'ctrl_click', 'empty': True}]}

ctrl_click_false = alt.selection_point("ctrl_click_false", on="click[event.ctrlKey]", empty=False)

>>> (ctrl_click_false & ctrl_click).to_dict()
{'and': [{'param': 'ctrl_click_false', 'empty': False},
  {'param': 'ctrl_click', 'empty': True}]}