从元组创建的字典列表中的字典值未正确更新答案

【问题标题】：Dictionary value in Dictionary list created from tuple doesn't update correctly从元组创建的字典列表中的字典值未正确更新
【发布时间】：2015-07-15 02:54:18
【问题描述】：

当我在列表中的字典（从元组创建）上创建新字典值时，字典值会更新所有相似的字典条目，而不是我想要的单个条目：

import itertools
import random

seq = [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]

# Create combinations of ABCD -> AB, AC, AD, BA, BB, BC [...] ABD [...]  ABCD
allCombinations = []
for i in range(2, len(seq) + 1):
    combinationTuple = list(itertools.combinations(seq, i))  # Produces tuples (immutable) of dictionaries
    allCombinations += combinationTuple
# allCombinations = [({'Item': 'A'}, {'Item': 'B'}), ({'Item': 'A'}, {'Item': 'C'}), ({'Item': 'A'}, {'Item': 'D'}), ({'Item': 'B'}, {'Item': 'C'}), ({'Item': 'B'}, {'Item': 'D'}), ({'Item': 'C'}, {'Item': 'D'}), ({'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}), ({'Item': 'A'}, {'Item': 'B'}, {'Item': 'D'}), ({'Item': 'A'}, {'Item': 'C'}, {'Item': 'D'}), ({'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}), ({'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'})]

combinationsList = [list(i) for i in allCombinations]  # Make into a list so new dict-key can be saved
# combinationsList = [[{'Item': 'A'}, {'Item': 'B'}], [{'Item': 'A'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}], [{'Item': 'B'}, {'Item': 'D'}], [{'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]]

for item in combinationsList:
    for i in range(0, len(item) - 1):
        item[i]["newkey"] = random.random()
        # i want:
        # combinationsList = [[{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'B'}], [{'Item': 'A'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}], [{'Item': 'B'}, {'Item': 'D'}], [{'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]]
        # but what i get is
        # combinationsList = [[{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'B'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'C'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}], [{'Item': 'B'}, {'Item': 'D'}], [{'Item': 'C'}, {'Item': 'D'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'B'}, {'Item': 'D'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}], [{'newkey': 0.06184604397709914, 'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]]
        print(combinationsList)
        exit()  # exit only for demo purposes

"""
# What's really odd is if i explicitly make combinationsList, run the same code, it works:

combinationsList = [[{'Item': 'A'}, {'Item': 'B'}], [{'Item': 'A'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}], [{'Item': 'B'}, {'Item': 'D'}], [{'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}], [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]]

for item in combinationsList:
    for i in range(0, len(item) - 1):
        item[i]["newkey"] = random.random()
        # works perfectly
        print(combinationsList)
        exit()  # exit only for demo purposes
"""

【问题讨论】：

标签： python python-3.x dictionary iteration tuples

【解决方案1】：

您的元组包含对来自seq 的字典的引用；它们不是那些字典的副本。您必须明确地从每个字典创建新字典：

for i in range(2, len(seq) + 1):
    combinationTuple = [tuple(d.copy() for d in c) for c in itertools.combinations(seq, i)]
    allCombinations += combinationTuple

仅仅因为元组是不可变的并不意味着它们的内容仍然不能共享。

演示：

>>> import itertools
>>> seq = [{'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]
>>> not_copied = list(itertools.combinations(seq, 2))
>>> not_copied[0]
({'Item': 'A'}, {'Item': 'B'})
>>> not_copied[0][0]['foo'] = 'bar'
>>> seq
[{'foo': 'bar', 'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]
>>> copied = [tuple(d.copy() for d in c) for c in itertools.combinations(seq, 2)]
>>> copied[0]
({'foo': 'bar', 'Item': 'A'}, {'Item': 'B'})
>>> del copied[0][0]['foo']
>>> seq
[{'foo': 'bar', 'Item': 'A'}, {'Item': 'B'}, {'Item': 'C'}, {'Item': 'D'}]
>>> not_copied[0][0] is seq[0]
True
>>> copied[0][0] is seq[0]
False

【讨论】：