Python Strange Behavior with List & Append

Question

The following code is an issue I encountered and am looking for an explanation. The behavior of the code different than what I expected. Below the code will be my expected output, and the actual output. One last thing to note, is that I understand this code may be 'strange', and that using range(1) is a bit odd to say the least. The reason for this is that this exact occurrence in a program (the ranges were variables but at these values) caused problems.. so I made this simple code to replicate it.

userList = []

class User():
    listA = []
    listB = []

    def setup(self):
        for i in range(1):
            self.listA.append('a')
            self.listB.append('b')

for i in range(5):
    user = User()
    userList.append(user)

for i in range(len(userList)):
    userList[i].setup()

for i in range(len(userList)):
    print str(userList[i].listA)
    print str(userList[i].listB)

Expected Output

['a']
['b']
['a']
['b']
['a']
['b']
['a']
['b']
['a']
['b']

Actual Output

['a','a','a','a','a']
['b','b','b','b','b']
['a','a','a','a','a']
['b','b','b','b','b']
['a','a','a','a','a']
['b','b','b','b','b']
['a','a','a','a','a']
['b','b','b','b','b']
['a','a','a','a','a']
['b','b','b','b','b']

Discussion

I appreciate any explanation as to why this is happening. I'm not sure if the built-in append() function is somehow affecting all Users, or if each User is somehow sharing their fields. Running on Python 2.7.3.

Answer 1

Compare this to your code

class User():
    def setup(self):
        self.listA = []                          # instance variable
        self.listB = []                          # instance variable
        for i in range(1):
            self.listA.append('a')
            self.listB.append('b')

Note that it's not necessary to "declare" any variables at the class level