Issue creating class objects from within the method of another class, and adding them to lists Python

Question

So this has been confusing me for a couple of days now, I have three classes one called Page:

class Page:

   def __init__(self, pageName, sectionIBelongTo="Uncategorised"):
        self.mySection = sectionIBelongTo
        #each page belongs to only one section
        self.name = pageName

Which must have an assigned section object:

class Section:

    childPages = []

    def __init__(self, padName):
        self.name = padName

    def addSection(self, pageObject):
        self.childPages.append(pageObject)

Section also has a list of all child notes. This is all managed through a single Book class object:

class Book:
    Sections = []

    def __init__(self):
        print "notebook created"

    def addSection(self, secName):
        sectionToAdd = Section(secName)

        self.Sections.append(sectionToAdd)

    def addPage(self, bufferPath, pageName, pageSection="Uncategorised"):
        #Create a page and add it to the appropriate section


        for section in self.Sections:
            if section.name == pageSection:
                sectionToSet = section
        #Search list of sections for matching name.

        newPage = Page(pageName, sectionToSet)
        #Create new page and assign it the appropriate section object

        self.Sections[self.Sections.index(sectionToSet)].addSection(newPage)
        #Add page to respective section's list of pages.

Which as you can see holds a list of all of its sections. So I import these classes from another file and try to fill up my book like so:

myBook = Book()

myBook.addSection("Uncategorised")
myBook.addSection("Test")
myBook.addSection("Empty")
#Create three sections

myBook.addPage("belongs to uncategorised")
#Add page with no section parameter (uncategorised).
myBook.addPage("Belongs to test", "Test")
#Add page to section "Test"
myBook.addPage("Belongs to uncategorised again")
#Another uncategorised page

for x in range(0, 3):
    print "Populated section '", myBook.Sections[x].name, "', with: ", len(myBook.Sections[x].childPages), " child pages."

The output shows that all three sections are created fine, but every section has 3 child pages, if I print the names of the pages it appears that every page has been added to every section.

I would hugely appreciate anyone being able to spot my silly mistake.

Thanks in advance! :)

Mike Müller · Accepted Answer · 2017-01-25 12:57:56Z

1

Making childPages an instance attribute, not a class attribute, should solve your problem:

class Section:

    def __init__(self, padName):
        self.name = padName
        self.childPages = []

    def addSection(self, pageObject):
        self.childPages.append(pageObject)

answered Jan 25, 2017 at 12:57

Mike Müller

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