python - 如何附加到深度嵌套在字典中的列表并保留嵌套结构？

Question

我正在尝试附加到埋藏在深层嵌套字典中的列表。但是，每当我尝试附加到列表时，它都会使字典变平:

import copy

my_d = {'this': 'that'}
deeply_nested = {'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}}}
print 'initial: ', deeply_nested

for i in range(10):
    for k, v in deeply_nested.iteritems():
        tmp = copy.deepcopy(v)
        while not isinstance(tmp, list):
            for v in tmp.values():
                 tmp = copy.deepcopy(v)
        tmp.append(my_d)
        deeply_nested[k] = copy.deepcopy(tmp)

print 'final: ', deeply_nested

结果:

initial:  {'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}}}
final:  {'and': [{'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}]}

最终版本缺少嵌套键，应该是:

{'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': [{'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}]}}}}}}}

Answer 1

我认为您可能误解了复制的工作原理。由于 python 具有可变性，因此实现复制的目的是为了避免更改原始值。

复制旨在将对象/值复制为新对象，因此您可以对新对象进行操作，而不更改原始对象。通过创建列表的副本，您将永远无法按预期追加到原始列表。

而是遍历你的字典到列表，并且因为列表是一个可变对象，所以直接更改它。该更改将反射(reflect)在词典中。

import copy

my_d = {'this': 'that'}
deeply_nested = {'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}}}
print 'initial: ', deeply_nested

for i in range(10):
    tmp = deeply_nested.values()[0]
    while not isinstance(tmp,list):
        tmp = tmp.values()[0] #this will reference the list in the dict, changes to tmp are changes to that list.
    tmp.append(my_d)

print 'final: ', deeply_nested

结果

initial:  {'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}}}

final:  {'and': {'another': {'string': {'separated': {'by': {'underscore': {'my_l': [{'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}, {'this': 'that'}]}}}}}}}

至于你的程序失败的原因，只要看看你采取的步骤:

1. k 变为 and，v 变为 {'another': {'string': {'separated': {'by': {'underscore': {'my_l': [ ]}}}}}}
2. tmp 将 v 复制到一个新对象中(对 tmp 所做的所有更改都不会反射(reflect)在 v 上)

3. tmp 向下遍历剩余的字典:

   {'another': {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}}
   {'string': {'separated': {'by': {'underscore': {'my_l': []}}}}}
   {'separated': {'by': {'underscore': {'my_l': []}}}}
   {'by': {'underscore': {'my_l': []}}}
   {'underscore': {'my_l': []}}
   {'my_l': []}
   []
   

4. 您附加到最后一个值，它是您要更改的实际列表的副本。因此实际列表不会改变，只有 tmp 会改变。 tmp 变为 [{'this': 'that'}]

5. 你说deeply_nested[k] = copy.deepcopy(tmp)。从步骤 1.k=any 开始。从步骤 4 开始，tmp=[{'this': 'that'}]，因此:

   {'and': [{'this': 'that'}]}
   

   冲洗并重复 10 次即可得到所需的效果。