Python中浅拷贝的四种实现方法小结

方式一：使用切片 [:]

列表

# 浅拷贝 [:]
old_list = [1, 2, [3, 4]]
new_list = old_list[:]

old_list.append(5)
old_list[2][0] += 97

print("old list:", old_list, "old list id:", id(old_list), " old list[0] id:", id(old_list[2]))
print("new list:", new_list, "new list id:", id(new_list), " new list[0] id:", id(new_list[2]))


# 输出结果
old list: [1, 2, [100, 4], 5] old list id: 4537660608  old list[0] id: 4537659840
new list: [1, 2, [100, 4]] new list id: 4537711424  new list[0] id: 4537659840

方式二：使用工厂函数

工厂函数简介

• 工厂函数看上去像函数，但实际是一个类
• 调用时，生成该数据类型类型的一个实例

可变对象的工厂函数

• list()
• set()
• dict()

列表

old_list = [1, 2, [3, 4]]
new_list = list(old_list)

old_list.append(5)
old_list[2][0] += 97

print("old list:", old_list, "old list id:", id(old_list), " old list[0] id:", id(old_list[2]))
print("new list:", new_list, "new list id:", id(new_list), " new list[0] id:", id(new_list[2]))

集合

old_set = {1, 2, 3}
new_set = set(old_set)

old_set.add(4)

print("old set:", old_set, "old set id:", id(old_set))
print("new set:", new_set, "new set id:", id(new_set))


# 输出结果
old set: {1, 2, 3, 4} old set id: 4484723648
new set: {1, 2, 3} new set id: 4484723872

字典

old_dict = {"name": "小明"}
new_dict = dict(old_dict)

old_dict["second"] = "python"

print("old dict:", old_dict, "old dict id:", id(old_dict))
print("new dict:", new_dict, "new dict id:", id(new_dict))


# 输出结果
old dict: {'name': '小明', 'second': 'python'} old dict id: 4514161536
new dict: {'name': '小明'} new dict id: 4515690304

方式三：使用数据类型自带的 copy 方法

列表

old_list = [1, 2, [3, 4]]
new_list = old_list.copy()

old_list.append(5)
old_list[2][0] += 97

print("old list:", old_list, "old list id:", id(old_list), " old list[0] id:", id(old_list[2]))
print("new list:", new_list, "new list id:", id(new_list), " new list[0] id:", id(new_list[2]))


# 输出结果
old list: [1, 2, [100, 4], 5] old list id: 4309832000  old list[0] id: 4310372992
new list: [1, 2, [100, 4]] new list id: 4309735296  new list[0] id: 4310372992

集合

old_set = {1, 2, 3}
new_set = old_set.copy()

old_set.add(4)

print("old set:", old_set, "old set id:", id(old_set))
print("new set:", new_set, "new set id:", id(new_set))


# 输出结果
old set: {1, 2, 3, 4} old set id: 4309931392
new set: {1, 2, 3} new set id: 4309930944

字典

old_dict = {"name": "小明"}
new_dict = old_dict.copy()

old_dict["second"] = "python"

print("old dict:", old_dict, "old dict id:", id(old_dict))
print("new dict:", new_dict, "new dict id:", id(new_dict))

 

# 输出结果
old dict: {'name': '小明', 'second': 'python'} old dict id: 4308452288
new dict: {'name': '小明'} new dict id: 4308452224

源码

    def copy(self, *args, **kwargs): # real signature unknown
        """ return a shallow copy of the list. """
        pass

已经写的很清楚，这是浅拷贝

方式四：使用 copy 模块的 copy 方法

列表

from copy import copy

old_list = [1, 2, [3, 4]]
new_list = copy(old_list)

old_list.append(5)
old_list[2][0] += 97

print("old list:", old_list, "old list id:", id(old_list), " old list[0] id:", id(old_list[2]))
print("new list:", new_list, "new list id:", id(new_list), " new list[0] id:", id(new_list[2]))


# 输出结果
old list: [1, 2, [100, 4], 5] old list id: 4381013184  old list[0] id: 4381159936
new list: [1, 2, [100, 4]] new list id: 4381012800  new list[0] id: 4381159936

集合

from copy import copy

old_set = {1, 2, 3}
new_set = copy(old_set)

old_set.add(4)

print("old set:", old_set, "old set id:", id(old_set))
print("new set:", new_set, "new set id:", id(new_set))


# 输出结果
old set: {1, 2, 3, 4} old set id: 4381115552
new set: {1, 2, 3} new set id: 4381115776

字典

from copy import copy

old_dict = {"name": "小明"}
new_dict = copy(old_dict)

old_dict["second"] = "python"

print("old dict:", old_dict, "old dict id:", id(old_dict))
print("new dict:", new_dict, "new dict id:", id(new_dict))


# 输出结果
old dict: {'name': '小明', 'second': 'python'} old dict id: 4381159680
new dict: {'name': '小明'} new dict id: 4379632576

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