python中可变类型在修改中需谨慎

由于python中tuple与str类型是不可修改的，dict和list是可以修改的，因而在对其进行一系列修改操作的时候需千万谨慎，其中涉及了python的assignment、copy和deepcopy一些内容。
可参考：

1. https://www.geeksforgeeks.org/copy-python-deep-copy-shallow-copy/
2. https://www.runoob.com/w3cnote/python-understanding-dict-copy-shallow-or-deep.html

下面是一个例子，可以对比一下有无.copy()的区别

>>> dict_old = [{'a':'1'}, {'b':'2'}, {'c':'3'}]
>>> dict_new = {}
>>> list_new = []
>>> for dict_old_item in dict_old:
>>>     dict_new.update(dict_old_item)
>>>     list_new.append(dict_new)
>>> print(list_new)
[{'a': '1', 'b': '2', 'c': '3'},
 {'a': '1', 'b': '2', 'c': '3'},
 {'a': '1', 'b': '2', 'c': '3'}]

>>> dict_old = [{'a':'1'}, {'b':'2'}, {'c':'3'}]
>>> dict_new = {}
>>> list_new = []
>>> for dict_old_item in dict_old:
>>>     dict_new.update(dict_old_item)
>>>     list_new.append(dict_new.copy()) # 添加.copy()
>>> print(list_new)
[{'a': '1'},
 {'a': '1', 'b': '2'},
 {'a': '1', 'b': '2', 'c': '3'}]