dict python的奇怪行为

Question

我有一个不符合预期的 dict 代码片段a = {"d1":{"a":1,"b":2,"c":4},"d2":{"a":1,"b":2,"c":4},"d3":{"a":1,"b":2,"c":4}}b = {"d1":{"a":1,"b":0},"d2":{"a":0,"c":4},"d3":{"a":1,"b":2,"c":4}}c = dict.fromkeys(a.keys(),{})print(c)for doc in b.keys():  for word in b[doc].keys():    c[doc][word] = a[doc][word]*b[doc][word]print(c)输出是：{'d1': {}, 'd2': {}, 'd3': {}}{'d1': {'a': 1, 'b': 4, 'c': 16}, 'd2': {'a': 1, 'b': 4, 'c': 16}, 'd3': {'a': 1, 'b': 4, 'c': 16}}代替：{'d1': {}, 'd2': {}, 'd3': {}}{'d1': {'a': 1, 'b': 0}, 'd2': {'a': 0, 'c': 16}, 'd3': {'a': 1, 'b': 4, 'c': 16}}我现在很困惑，任何见解都会有所帮助。

Answer 1

问题是因为您使用可变对象作为fromkeys.

这在这里更清楚：

d = dict.fromkeys(['a', 'b'], [])

d['a'].append(1)

print(d)

输出

{'a': [1], 'b': [1]}

Answer 2

使用字典理解来创建c：

c = {k: {} for k in a.keys()}

for doc in b.keys():

  for word in b[doc].keys():

    c[doc][word] = a[doc][word]*b[doc][word]

print(c)

# {'d1': {'a': 1, 'b': 0}, 'd2': {'a': 0, 'c': 16}, 'd3': {'a': 1, 'b': 4, 'c': 16}}

请注意使用fromkeysvs 字典理解时的区别：

c = dict.fromkeys(a.keys(),{})

print([id(o) for o in c.values()])

# [53649152, 53649152, 53649152]

# same object reference id!

c = {k: {} for k in a.keys()}

print([id(o) for o in c.values()])

# [53710208, 53649104, 14445232]

# each object has different reference id

Answer 3

for对您的循环进行了修改：

for doc in b.keys():

  for word in b[doc].keys():

    if doc not in c:

        c[doc]={}

    c[doc][word] = a[doc][word]*b[doc][word]

print(c)

#{'d1': {'a': 1, 'b': 0}, 'd2': {'a': 0, 'c': 16}, 'd3': {'a': 1, 'b': 4, 'c': 16}}