Logging in python, because mutable default arguments is a feature

logging.py

def log(x=None, acc=[]):
    if x is None:
        print('\n'.join(acc))
    else:
        acc.append(x)

log("Hello")
log("Goodbye")
log()

memarg.py

def fib(n, _mem=[1, 1]):
    for i in range(len(_mem), n + 1):
        _mem.append(_mem[-2] + _mem[-1])
    return _mem[n]

for i in range(10):
    print(fib(i))