课堂上的怪异行为

Question

I've written a class that has a list as a variable. I have a function that adds to that list and a function that outputs that list.

class MyClass:
    myList = []

    def addToList(self, myNumber):
        self.myList.append(myNumber)

    def outputList(self):
        for someNumber in self.myList:
            print someNumber

Now for some weird reason, if I declare two separate objects of the class:

ver1 = MyClass()
ver2 = MyClass()

and then call addToList on ver1:

ver1.addToList(3)

and then output ver2's list:

ver2.outputList()

I get 3 as output for version 2's list! What is happening?

Answer 1

Your syntax is wrong. You are using a class-static variable. Consider this:

class MyClass:
    myList = []    # Static variable

    def __init__(self):
        self.myRealList = []   # Member variable

myList is actually a part of the MyClass definition, and thus is visible not only by the class name, but all instances of that class as well:

c = MyClass()
c.myList = [1]
print MyClass.myList  # will print [1]

You need to declare regular "member variables" in the __init__ constructor.

Don't feel bad, I came to python from a C/C++/C# world, made this same mistake, and it confused me too at first.

Answer 2

The problem is that mylist is initialized when the file is parsed. So each subsequent initializations of MyClass will have the same reference to the same mylist.

class MyClass:
    myList = []

Instead you should initialize it with the class.

class MyClass:
    def __init__(self):
        self.myList = []

Answer 3

myList is actually a class variable (I don't know if that's the Pythonic term for it) because it is declared outside of any function.

To make a variable instance-local (once again, I don't know the proper term), you should initialize it in your init method like "self.myList = []".

Please edit any bad formatting on my part, I am on my phone.

Answer 4

There are several things going on here. You can have instance or class variables in python. In your example myList is a class variable which is static across all instances. There are also instance variables that are local to a class and you can override class variables with instance variables. Here is an example:

class Foo:
    bar = 'Hello' # Class variable

    def __init__(self):
        self.foobar = 'Hello World' # instance variable

    def createInstanceBar(self, x):
        self.bar = x # Override class variable and create instance variable

a = Foo()
b = Foo()
print a.bar # Prints Hello
print b.bar # Prints Hello

print a.foobar # Prints Hello World
print b.foobar # Prints Hello World

a.createInstanceBar('Hello New World')
print a.bar # Prints Hello New World
print b.bar # Prints Hello