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It is not really a SageMath problem, but a Python one. I do not know why xL is not the empty list each time that test() is called, according to the default value of xL in your definition of test. However, this happens even if such a default value is not an empty list, as you can check with any Python interpreter:

>>> def test(xL=[0]):
...     xL.append(len(xL))
...     return xL
... 
>>> for i in range(3):
...     print(test())
... 
[0, 1]
[0, 1, 2]
[0, 1, 2, 3]

However, it one uses concatenation instead of the append method, one gets the expected behaviour:

>>> def test(xL=[0]):
...     return xL + [len(xL)]
... 
>>> for i in range(3):
...     print(test())
... 
[0, 1]
[0, 1]
[0, 1]

>>> def test(xL=[0]):
...     xL.append(len(xL))
...     return xL
... 
>>> for i in range(3):
...     print(test())
... 
[0, 1]
[0, 1, 2]
[0, 1, 2, 3]

However, it if one uses concatenation instead of the append method, one gets the expected behaviour:

>>> def test(xL=[0]):
...     return xL + [len(xL)]
... 
>>> for i in range(3):
...     print(test())
... 
[0, 1]
[0, 1]
[0, 1]