Mutable list problems

mutability.py

# first version
def boo1(l=[]):
    l = list(l)
    l.append(1)
    return l

# second version
def boo2(l=[]):
    result = []
    result.extend(l)
    result.append(1)
    return result


m = [1, 2, 3, 4]
print boo1(m)
print boo1(m)
print boo1(m)
print boo1(m)

print boo2(m)
print boo2(m)
print boo2()