Notes for using class variables and member variables in python: python Variables

Notes for using class variables and member variables in python: python Variables

Preface

Recently I was writing a project in python and found a disgusting bug, that is, the data between two instances generated by the same class could affect each other, which made me very puzzled. Later I thought that java classes have class variables and instance variables, so I read the relevant information and found that python also has similar class variables and instance variables. Let's take a look at the details.

See the following sample code:

class A: x = 0 def __init__(self): self.y = 0

X is a class variable, and y is an instance variable.

In principle, there is no error, but some disgusting problems are found in actual use (that is, I found a three-day bug )... For example, the following code:

class A: x = [] y = 0 def __init__(self): pass def add(self): self.x.append('1') self.y+=1a=A() print a.x,a.yprint A.x,A.ya.add()print a.x,a.yprint A.x,A.yb=A() print b.x,b.yprint A.x,A.y

It is obvious that both x and y are class variables, and add is used to modify x and y respectively. Create an instance a, modify the value of instance a, and construct instance B.

I thought this result was obvious, but his output was:

[] 0[] 0['1'] 1['1'] 0['1'] 0['1'] 0

Where is the problem? Obviously, both x and y are class variables. In the second group of print, why is a. x and B. x the same, but a. y and B. y are different?

After thinking for a long time, I realized a truth... For python, class variables are indeed common to all classes. But that is when we use the same reference. For example, the append method of the [] object shares a class variable. But for the value assignment statement, if a value assignment statement is used for class variables in the class, python will generate a copy of the object. Subsequent operations will be based on this copy without affecting the original Class Object. This explains the above phenomenon.

In order to prevent the variables from being shared when you forget the differences between the class variables and the instance variables, the best way is to reinitialize the variables when they are used in each class, this will not cause an accident.

Summary

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