Question

有两个列表变量：listA和listB。两个列表都存储了三个MyClass实例。 listA和listB之间的区别在于listB的实例将self.attrA设置为＆＃34; myValue＆＃34;。在脚本结束时，我循环遍历listA和listB以检查它们的实例self.id属性是否匹配。如果他们这样做，我想用相应的listB实例更新（覆盖）listA实例（因此listA实例都将self.myAttr设置为＆＃34; myValue＆＃34;。奇怪的是listA实例即使在他们设定为平等：`

inst_A = inst_B

错误在哪里？

class MyClass(object):
    def __init__(self, arg):
        super(MyClass, self).__init__()
        self.id=None
        self.attrA=None
        if 'id' in arg.keys(): 
            self.id=arg['id']
        if 'attrA' in arg.keys(): 
            self.attrA=arg['attrA']

listA=[]
for i in range(3): 
    listA.append( MyClass({'id':i}))

listB=[]
for i in range(3): 
    listB.append( MyClass({'id':i, 'attrA':'myValue'}))

for inst_A in listA:
    for inst_B in listB:
        if inst_A.id==inst_B.id:
            inst_A=inst_B

for inst_A in listA:
    print inst_A.attrA

Answer 1

你的for循环不会改变你的列表，它会改变你的迭代变量。

for inst_A in listA: # this creates a new name called inst_A which points
                     # to a value in listA
    for inst_B in listB:
        if inst_A.id == inst_B.id:
            # this assignment changes the inst_A name to now point to inst_B
            inst_A = inst_B

    # At the bottom of the loop, inst_A is recycled, so the value it was 
    # assigned to (inst_B) is forgotten

尝试：

for i in range(len(listA)): 
    for inst_B in listB:
        if listA[i].id == inst_B.id:
            # almost the same as above, except here we're changing the value
            # of the i-th entry in listA
            listA[i] = inst_B

Python：基于另一个List-Variable中的相同类实例，在一个List-Variable中更新类实例

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