Question

I can’t understand how works the code below:

class Host:
    name = None
    fileList = []
    def __init__(self, hostName):
        self.name = hostName    
    def AddInfo(self,file):
        self.fileList.append(file)          
    def Cleanup(self):
        self.fileList   = []

I create 2 instances:

h1 = Host("hostName1")
h1.AddInfo("h1")
h1.Cleanup()
print h1.fileList, Host.fileList

h2 = Host("hostName2")
h2.AddInfo("h2")
print h2.fileList, Host.fileList

the result is:

h1.fileList = [], Host.fileList = ['h1']
h2.fileList = ['h1', 'h2'], Host.fileList = ['h1', 'h2']

why Host.fileList value is changed – I assigned new values to the instance only? why h2.fileList has such value – I was expecting ['h2'] here?

Answer 1

This is probably easist to see by stepping through your code:

h1 = Host("hostName1")  #After __init__:h1.filelist -> Host.filelist
h1.AddInfo("h1")        #After AddInfo: h1.filelist -> Host.filelist
h1.Cleanup()            #After Cleanup: h1.filelist is a new list

h2 = Host("hostName2")  #After __init__:h2.filelist -> Host.filelist (which already has 'h1' inserted)
h2.AddInfo("h2")        #After AddInfo: h2.filelist -> Host.filelist

Inside your methods, when using self.filelist, python looks to see if the instance has a filelist attribute first. If the instance doesn’t have that attribute, python looks for the attribute on the class the instance belongs to. when you do self.filelist=[] in Cleanup, you are giving the instance the attribute filelist.