def foo1(x, item=[]):
    item.append(x)
    return item

def foo2(x, item=None):
    if item is None:
        item = []
    item.append(x)
    return item

print(foo1(1))
print(foo2(2))

item = []
print(foo1(3, item))
print(foo1(4, item))

 

输出：

[1]
[2]
[3]
[3, 4]