Be wary of the *duplicate character (operator) in python

There is a special symbol "*" in python, which can be used as a multiplication operator for numerical operations and as a repetition operator for objects. However, you must pay attention when using it as a repetition operator

Note that: *Each repeated object has the same id, which means it points to the same address in the memory. You must pay attention when operating each object.

For example:

>>> alist = [range(3)]*4
>>> alist
[[0, 1, 2], [0, 1, 2], [0, 1, 2], [0, 1, 2]]

The above initializes a two-level list to simulate the matrix. The matrix is ​​4X3. For the convenience of description, the matrix is ​​denoted as A here.

Now I want to assign a value of 1 to A11, using the following code:

alist[0][0]=1

Then the result we want should be:

[[1, 1, 2], [0, 1, 2], [0, 1, 2], [0, 1, 2]]

But unfortunately, what we get is:

[[1, 1, 2], [1, 1, 2], [1, 1, 2], [1, 1, 2]]

What is going on? Why is A21 assigned a value? Why are other Ai1 What has changed?

The reason is this:

We have already said it at the beginning of the article. * Each repeated object has the same id, which means it points to the same address in the memory. You must pay attention when operating each object. .

We used the repetition operator "*" when initializing. When this operator performs repeated operations on objects, all repeated objects will point to the same memory address. So when you change one of the values,

Other values ​​will naturally be updated. The explanation in python is the following command and output:

>>> id(alist[0])
18858192
>>> id(alist[1])
18858192
>>> id(alist[2])
18858192
>>> id(alist[3])
18858192
>>>

As you can see, the ids are all the same, which means that these 4 lists are the same "list" .

In this case, what should we do if we want to simulate a matrix? In addition to the special numpy package, you can of course append new lists to the upper list one by one, for example:

>>> blist=[]
>>> for i in range(4):
    blist.append([j for j in range(3)])
>>> blist
[[0, 1, 2], [0, 1, 2], [0, 1, 2], [0, 1, 2]]

In this way, let’s try again The above assignment operation:

>>> blist[0][0]=1
>>> blist
[[1, 1, 2], [0, 1, 2], [0, 1, 2], [0, 1, 2]]
>>>