Question

Simple question, I’ve scaled down a problem I’m having where a list which I’ve retrieve from an object is changing when I append more data to the object. Not to the list.

Can anyone help my understand the behavior of python?

class a():
    def __init__(self):
        self.log = []
    def clearLog(self):
        del self.log[:]
    def appendLog(self, info):
        self.log.append(str(info))
    def getLog(self):
        return self.log

if __name__ == '__main__':
    obj = a()
    obj.appendLog("Hello")

    # get an instance as of this moment....
    list = obj.getLog()
    print list

    obj.appendLog("World")

    # print list, BUT we want the instance that was obtained
    # before the new appendage.   
    print list

OutPut:

['Hello']
['Hello', 'World']

Answer 1

The only place you create a new list is in the constructor, with the statement:

self.log = []

Later, when you do:

list = obj.getLog()

just puts a reference to the same list in a new variable (note, don’t use list
as a variable name, since it shadows the type). It does not create or clone a list in any way. If you want to clone it, do:

def getLog(self):
    return list(self.log)

You can also use a tuple (read-only sequence), if that’s appropriate:

def getLog(self):
    return tuple(self.log)

This may help minimize confusion about which should be modified.