Python assignment and depth copy

Assignment value:

 >> A = [1, 2, 3 >>> b = a  >>> a = [4, 5, 6]// Assigns a new value to a  >>> a[
     4, 5, 6]  >>> b[ 1, 2, 3  #   b does not follow a when the value of a is changed. Change  >>> a = [1, 2, 3 >>> b = a< /span>>>> a[0], a[1], a[2] = 4, 5, 6// Change the elements in the original list  >>> a[ 4, 5, 6]  >>> b [ 4, 5, 6]  #   

In both of the above code, the value of a has changed. The difference is that the first piece of code is directly assigned a new value (from [1, 2, 3] to [4, 5, 6]), and the second paragraph changes each element in the list individually. The effect on B is different, one does not change the value of B, and the other changes. How to use the above reason to explain the strange difference?

For the first time, [1, 2, 3] is regarded as an object. A = [1, 2, 3] is equivalent to labeling the item A. B = A is a label for the item and a B.

First case:

A = [4, 5, 6] is equivalent to tearing a label from [1, 2, 3] and pasting it onto [4, 5, 6]. In this process, [1, 2, 3] This item does not disappear. b from start to finish well on [1, 2, 3], since this reference has not changed. The value of B does not change naturally.

Second case:

A[0], a[1], a[2] = 4, 5, 6 is directly changed [1, 2, 3] the item itself. Re-modified every part of it inside. After the internal modification, [1, 2, 3] itself became [4, 5, 6]. And in the process, A and B are not moving, they are also affixed to the item. So the natural value of a B is changed to [4, 5, 6].

deep, shallow copy:

>>>ImportCopy>>> origin = [1, 2, [3, 4]]#There are three elements inside Origin: 1, 2,[3, 4]>>> COP1 =Copy.copy (Origin)>>> COP2 =Copy.deepcopy (Origin)>>> COP1 = =cop2true>>> COP1 isCop2false#Cop1 and Cop2 look the same, but they are no longer the same object>>> Origin[2][0] ="hey!">>>origin[1, 2, ['hey!', 4]]>>>cop1[1, 2, ['hey!', 4]]>>>cop2[1, 2, [3, 4]]#Change the Sub list in Origin [3, 4] to an element, observing Cop1 and COP2

Copy is not completely copied for a complex object's child objects, what is a child object of a complex object? Like nested sequences in a sequence, nested sequences in a dictionary are sub-objects of complex objects. For sub-objects, Python stores it as a public image, and all copies of it are treated as a reference, so that when one of the references changes the mirror and the other reference uses the mirror, the image is changed.

So look at the origin[2 here], that is [3, 4] this list. According to the definition of shallow copy, the cop1[2] points to the same list [3, 4]. Well, if we change this list here, it will cause both origin and cop1 to change at the same time. This is why the top origin[2][0] = "hey!", Cop1 also became [1, 2, [' hey! ', 4]].

deepcopy will copy each layer of the complex object to a single individual. this time origin[2] and cop2[2] Although the values are equal to [3, 4], but it is not the same list. That we replicate in the ordinary sense.

Reference Http://iaman.actor/blog/2016/04/17/copy-in-python

Python assignment and depth copy