I have reduced to this simple code an unexpected behavior in a larger python code:

def f(n, s=[]):
    s.append(n)
    return s

print(f(3))
print(f(5))

The output given is:
[3]
[3, 5]

I expected:
[3]
[5]

If the empty list is given explictly as argument, f(3, s=[]), it works as expected. I imagine that this is a bad practice, has this happened to anyone?

(The function looks useless but the original had some random variables sampled inside)