Python iterate object with a list of objects

Question

Is it possible to iterate though an object that contains a list of objects.

For example, I have the following class

Class Page(object)

    def __init__(self, name):

        self.name = name
        self.pages = []

I then create a new Page object and add other page objects to it.

page = Page('FirstPage')

apagepage = Page('FirstChild')

anotherpagepage = Page('SecondChild')

apagepage.pages.append(Page('FirstChildChild'))

apagepage.pages.append(Page('SecondChildChild'))

page.pages.append(apagepage)

page.pages.append(anotherpagepage)

What I would like to do is

for thispage in page:
    print thispage.name

And get the following output

FirstPage
FirstChild
SecondChild
FirstChildChild
SecondChildChild

So I get all the 1st level, then the 2nd, then the 3rd.

However, the following output would be find as well

FirstPage
FirstChild
FirstChildChild
SecondChildChild
SecondChild

You should look for questions about flattening a list

John La Rooy
– John La Rooy

2012-10-23 04:23:46 +00:00
Commented Oct 23, 2012 at 4:23 — John La Rooy
– John La Rooy, Commented Oct 23, 2012 at 4:23

Michael Anderson · Accepted Answer · 2012-10-23 05:52:52Z

3

You can define a __str__

from itertools import chain

class Page(object):
    def __init__(self, name):
        self.name = name
        self.pages = []
    def __str__(self):
        return "\n".join(chain([self.name], map(str, self.pages)))

then you can just print page

Alternatively you can define an iterator if you wish to use your the for loop

class Page(object):
    def __init__(self, name):
        self.name = name
        self.pages = []
    def __iter__(self):
        yield self
        for page in self.pages:
            for i in page:
                yield i

edited Oct 23, 2012 at 5:52

Michael Anderson

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answered Oct 23, 2012 at 4:35

John La Rooy

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4 Comments

Aleksi Torhamo Over a year ago

__repr__ should strive to return a representation that can be evaluated to get the original object. You should instead use __str__ for human-readable representations like this.

jfs Over a year ago

you could traverse a tree-like object in different ways. Some might prefer that iter(page) would return only immediate children and use explicit page.walk() for recursive traversals.

nerd Over a year ago

@J.F.Sebastian If I'm understanding your correctly, could I use iter to iterate all the direct children. And then some how do a walk to iterate though all the children. That would be just like the os.path.walk function. I'm not sure if this will help me or not but I would be very interested in learning how to do this. Thanks

jfs Over a year ago

@nerd: I've posted answer to show what I mean

K Z · Accepted Answer · 2012-10-23 05:51:21Z

1

Do it recursively:

class Page(object):
    def __init__(self, name):
        self.name = name
        self.pages = []

    def __iter__(self):
        for p in self.get_children_pages(self):
            yield p

    def get_children_pages(self, start_page):
        result = [start_page.name]
        for this_page in start_page.pages:
            result.extend(self.get_children_pages(this_page))
        return result


>>>>for p in page:
....    print p

FirstPage
FirstChild
FirstChildChild
SecondChildChild
SecondChild

edited Oct 23, 2012 at 5:51

answered Oct 23, 2012 at 4:28

K Z

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Comments

JamesSwift · Accepted Answer · 2012-10-23 06:03:16Z

0

Do it recursively

def print_all_pages(page):
    print page.name
    if len(page.pages) == 0 then:
        return
    else:
        for child_page in page.pages:
            print_all_pages(child_page)

def print_pages(pages_collection):
    for page in pages_collection:
        print_all_pages(page)

edited Oct 23, 2012 at 6:03

answered Oct 23, 2012 at 4:48

JamesSwift

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2 Comments

K Z Over a year ago

1. I don't think this is what OP wants 2. your code does not actually work. You may want check the exit condition

JamesSwift Over a year ago

@KayZhu - fixed syntax errors, my b. Seems that it is what he was looking for though... can't say for sure, of course.

jfs · Accepted Answer · 2012-10-24 10:52:25Z

0

class Page(object):
    def __init__(self, name, pages=None):
        self.name = name
        self.pages = pages if pages is not None else []
    def __iter__(self):
        return iter(self.pages) # only immediate children
    def walk(self, topdown=True): # all pages recursively
        if topdown:
            yield self
        for page in self:
            yield from page.walk(topdown)
        if not topdown:
            yield self

You can replace yield from on Python <3.3 with:

for subpage in page.walk(topdown):
    yield subpage

in this case.

Example

page = Page('FirstPage', [
        Page('FirstChild', [
                Page('FirstChildChild'),
                Page('SecondChildChild'),
                ]),
        Page('SecondChild'),
        ])

for p in page.walk():
    print(p.name)

Output

FirstPage
FirstChild
FirstChildChild
SecondChildChild
SecondChild

edited Oct 24, 2012 at 10:52

answered Oct 23, 2012 at 10:36

jfs

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1 Comment

nerd Over a year ago

Thank you. This solution is both ideal and elegant.

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