python列表作为默认参数的问题

# coding=utf-8
def foo(a, x=[]):
    x.append(a)
    return x


print(foo(1))
print(foo(2))
# 预想输出
# [1]
# [2]
# 实际输出
# [1]
# [1, 2]

正确示例：

# 正确示例
def foo1(a, x=None):
    if x is None:
        x = []
    x.append(a)
    return x


print(foo1(1))
print(foo1(2))
# [1]
# [2]