Help with recursive functions

[perfringo]

I don't understand what the objective is, so no help here. However, I would like to remind what will happen if there is parameter level = []:

>>> def example(level=[]):
...     level.append(1)
...     return level
... 
>>> example()
[1]
>>> example()
[1, 1]
>>> example()
[1, 1, 1]
>>> def example_2(num, level=[]):
...     level.append(num)
...     return level
... 
>>> example_2(1)
[1]
>>> example_2(2)
[1, 2]
>>> example_2('3')
[1, 2, '3']
>>> def example_3(num, level=[]):
...     level.append(num)
...     return [num * num for num in level]
... 
>>> example_3(1)
[1]
>>> example_3(2)
[1, 4]