2017-07-09-python浅拷贝和深拷贝

1. 浅拷贝
2. 深拷贝
3. 总结

1 浅拷贝

    import copy

    will = ["Will", 28, ["Python", "C#", "JavaScript"]]
    wilber = copy.copy(will)

    print '<<<<<<<<<<<<<<<<'
    print id(will)
    print will
    print [id(ele) for ele in will]
    print '================'
    print id(wilber)
    print wilber
    print [id(ele) for ele in wilber]
    print '>>>>>>>>>>>>>>>>'

    will[0] = "Wilber"
    will[2].append("CSS")
    print '<<<<<<<<<<<<<<<<'
    print id(will)
    print will
    print [id(ele) for ele in will]
    print '================'
    print id(wilber)
    print wilber
    print [id(ele) for ele in wilber]
    print '>>>>>>>>>>>>>>>>'

总结一下，当我们使用下面的操作的时候，会产生浅拷贝的效果：

使用切片[:]操作

      a = [1, 2, 3, 4]
      b = a[1:]
      print '<<<<<<<<<<<<<<<<'
      print a
      print id(a)
      print [id(elm) for elm in a]
      print '================'
      print b
      print id(b)
      print [id(elm) for elm in b]
      print '>>>>>>>>>>>>>>>>'

      b[0] = 'xiepeng'

      print '<<<<<<<<<<<<<<<<'
      print a
      print id(a)
      print [id(elm) for elm in a]
      print '================'
      print b
      print id(b)
      print [id(elm) for elm in b]
      print '>>>>>>>>>>>>>>>>'

使用工厂函数（如list/dir/set）
使用copy模块中的copy()函数

2 深拷贝

    import copy

    will = ["Will", 28, ["Python", "C#", "JavaScript"]]
    wilber = copy.deepcopy(will)

    print '<<<<<<<<<<<<<<<<'
    print id(will)
    print will
    print [id(ele) for ele in will]
    print '================'
    print id(wilber)
    print wilber
    print [id(ele) for ele in wilber]
    print '>>>>>>>>>>>>>>>>'

    will[0] = "Wilber"
    will[2].append("CSS")
    print '<<<<<<<<<<<<<<<<'
    print id(will)
    print will
    print [id(ele) for ele in will]
    print '================'
    print id(wilber)
    print wilber
    print [id(ele) for ele in wilber]
    print '>>>>>>>>>>>>>>>>'

其实，对于拷贝有一些特殊情况：

对于非容器类型（如数字、字符串、和其他不可变的'原子'类型的对象）没有拷贝这一说，就是说，对于这些类型，"obj is copy.copy(obj)" 、"obj is copy.deepcopy(obj)"
如果元祖变量只包含原子类型对象，则不能深拷贝

    import copy
    print(2 is copy.copy(2))
    print(2 is copy.deepcopy(2))

    print("=" * 70)
    book = ('python', 'C#', 'JavaScript')
    print(book)
    copies = copy.deepcopy(book)
    print book is copies

    print("=" * 70)
    book = ('python', 'C#', 'JavaScript', [])
    print(book)
    copies = copy.deepcopy(book)
    print book is copies

3 总结

我理解浅拷贝和深拷贝，可以先理解一下可变对象和不可变对象：

对于不可变对象，两种拷贝效果是一样的，都是指向的同一块内存。
对于可变对象是有区别的：
1. 浅拷贝：直接指向同一份内存，一处修改，处处体现。
2. 深拷贝：将内存中的内容全copy一份出来，每份单独存在，相互不影响。

2017-07-09-python浅拷贝和深拷贝

Table of Contents

1 浅拷贝

2 深拷贝

3 总结