input: Write a python function to get the current date and time formatted as 'YYYY-MM-DD HH:MM:SS'.
  assert get_current_datetime() != ""
  assert isinstance(get_current_datetime(), str)
  assert len(get_current_datetime()) == 19
output: def get_current_datetime():
      from datetime import datetime
      return datetime.now().strftime('%Y-%m-%d %H:%M:%S')