python函数调用的问题

Question

最近在看《Python语言及其应用》这本书，里面提到了函数调用的一个坑，没有搞懂是怎么回事，求教一下。
先看代码和输出吧

def buggy(arg, result=[]):
    result.append(arg)
    print(result)
    
def work(arg):
    result = []
    result.append(arg)
    return result

def nonbuggy(arg, result=[]):
    if result is None:
        result = []
    result.append(arg)
    return result

def test(arg, result=[]):
    result.append(arg)
    return result

print('---buggy---')
buggy('a')
buggy('b')
print('---work---')
print(work('a'))
print(work('b'))
print('---nonbuggy---')
print(nonbuggy('a'))
print(nonbuggy('b'))
print('---test---')
print(test('a'))
print(test('b'))

输出

---buggy---
['a']
['a', 'b']
---work---
['a']
['b']
---nonbuggy---
['a']
['b']
---test---
['a']
['a', 'b']

不明白为什么buggy()和test()函数得到的结果会受到之前调用的影响。这是不是和python的内存模型有关系？

Answer 1

Default parameter value result = [] has been created during the definition period, and result points to the same object every time it is executed