每週一個 Python 模塊 | copy

時間 2019-11-21

標籤每週一個 python 模塊 copy 欄目 Python 简体版

原文原文鏈接

copy 模塊包括兩個功能，copy() 和 deepcopy()，用於複製現有對象。python

淺拷貝

copy() 建立的淺表副本是一個新容器，是對原始對象內容的引用。git

import copy
import functools


@functools.total_ordering
class MyClass:

    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

    def __gt__(self, other):
        return self.name > other.name


a = MyClass('a')
my_list = [a]
dup = copy.copy(my_list)

print(' my_list:', my_list)
print(' dup:', dup)
print(' dup is my_list:', (dup is my_list))
print(' dup == my_list:', (dup == my_list))
print('dup[0] is my_list[0]:', (dup[0] is my_list[0]))
print('dup[0] == my_list[0]:', (dup[0] == my_list[0]))

# output
# my_list: [<__main__.MyClass object at 0x101f9c160>]
# dup: [<__main__.MyClass object at 0x101f9c160>]
# dup is my_list: False
# dup == my_list: True
# dup[0] is my_list[0]: True
# dup[0] == my_list[0]: True
複製代碼

對於淺拷貝，MyClass 實例並不複製，所以dup 和 my_list 引用的是同一個對象。github

深拷貝

將調用替換爲 deepcopy() 會使輸出明顯不一樣。數據結構

import copy
import functools


@functools.total_ordering
class MyClass:

    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

    def __gt__(self, other):
        return self.name > other.name


a = MyClass('a')
my_list = [a]
dup = copy.deepcopy(my_list)

print(' my_list:', my_list)
print(' dup:', dup)
print(' dup is my_list:', (dup is my_list))
print(' dup == my_list:', (dup == my_list))
print('dup[0] is my_list[0]:', (dup[0] is my_list[0]))
print('dup[0] == my_list[0]:', (dup[0] == my_list[0]))

# output
# my_list: [<__main__.MyClass object at 0x101e9c160>]
# dup: [<__main__.MyClass object at 0x1044e1f98>]
# dup is my_list: False
# dup == my_list: True
# dup[0] is my_list[0]: False
# dup[0] == my_list[0]: True
複製代碼

列表的第一個元素再也不是相同的對象引用，可是當比較兩個對象時，它們仍然是相等的。app

自定義複製行爲

可使用 __copy__()和__deepcopy__() 方法來自定義複製行爲。spa

__copy__() 不須要參數，返回該對象的淺拷貝副本。
__deepcopy__()使用 memo 字典調用，並返回該對象的深拷貝對象。任何須要深度複製的成員屬性，都應與 memo 字典一塊兒傳遞給 copy.deepcopy()。

如下示例說明了如何調用方法。code

import copy
import functools


@functools.total_ordering
class MyClass:

    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

    def __gt__(self, other):
        return self.name > other.name

    def __copy__(self):
        print('__copy__()')
        return MyClass(self.name)

    def __deepcopy__(self, memo):
        print('__deepcopy__({})'.format(memo))
        return MyClass(copy.deepcopy(self.name, memo))


a = MyClass('a')

sc = copy.copy(a)
dc = copy.deepcopy(a)

# output
# __copy__()
# __deepcopy__({})
複製代碼

memo 字典用於跟蹤已經複製的值，以免無限遞歸。orm

深度複製中的遞歸

爲避免重複遞歸數據結構的問題，deepcopy() 使用字典來跟蹤已複製的對象。這個字典被傳遞給__deepcopy__() 方法，所以能夠在這裏檢查重複遞歸問題。cdn

下一個示例顯示了互連數據結構（若有向圖）如何經過實現__deepcopy__()方法來防止遞歸。

import copy


class Graph:

    def __init__(self, name, connections):
        self.name = name
        self.connections = connections

    def add_connection(self, other):
        self.connections.append(other)

    def __repr__(self):
        return 'Graph(name={}, id={})'.format(
            self.name, id(self))

    def __deepcopy__(self, memo):
        print('\nCalling __deepcopy__ for {!r}'.format(self))
        if self in memo:
            existing = memo.get(self)
            print(' Already copied to {!r}'.format(existing))
            return existing
        print(' Memo dictionary:')
        if memo:
            for k, v in memo.items():
                print(' {}: {}'.format(k, v))
        else:
            print(' (empty)')
        dup = Graph(copy.deepcopy(self.name, memo), [])
        print(' Copying to new object {}'.format(dup))
        memo[self] = dup
        for c in self.connections:
            dup.add_connection(copy.deepcopy(c, memo))
        return dup


root = Graph('root', [])
a = Graph('a', [root])
b = Graph('b', [a, root])
root.add_connection(a)
root.add_connection(b)

dup = copy.deepcopy(root)

# output
# Calling __deepcopy__ for Graph(name=root, id=4326183824)
# Memo dictionary:
# (empty)
# Copying to new object Graph(name=root, id=4367233208)
# 
# Calling __deepcopy__ for Graph(name=a, id=4326186344)
# Memo dictionary:
# Graph(name=root, id=4326183824): Graph(name=root, id=4367233208)
# Copying to new object Graph(name=a, id=4367234720)
# 
# Calling __deepcopy__ for Graph(name=root, id=4326183824)
# Already copied to Graph(name=root, id=4367233208)
# 
# Calling __deepcopy__ for Graph(name=b, id=4326183880)
# Memo dictionary:
# Graph(name=root, id=4326183824): Graph(name=root, id=4367233208)
# Graph(name=a, id=4326186344): Graph(name=a, id=4367234720)
# 4326183824: Graph(name=root, id=4367233208)
# 4367217936: [Graph(name=root, id=4326183824), Graph(name=a, id=4326186344)]
# 4326186344: Graph(name=a, id=4367234720)
# Copying to new object Graph(name=b, id=4367235000)
複製代碼