Python iteration of list of objects “not iterable”

Question

New to Python, but I have been researching this for a couple hours. Forgive me if I missed something obvious.

I have a class called LineItem, which has an attribute _lineItems, a list of LineItems that belong to the given LineItem. A sub-list, basically.

I want to print out a LineItem and all of its sub-items (and the sub-items own sub-items), but I'm having trouble with the iteration.

from decimal import * 
class LineItem(object):
    """
    Instance attributes:
      amount: Decimal
      _lineItems: list of child internal lineitems (possibly an empty list)
      isInternal: bool
    """

    def __init__(self, **kw):
        self.amount = Decimal(0)
        self._lineItems = []
        self.isInternal = False
        for k, v in kw.items():
            setattr(self, k, v)

An example LineItem that's giving me the trouble is defined below as ext2, with three children.

# External line item with one level of children
int1 = LineItem(amount=Decimal('1886.75'), description='State Dues',
         isInternal=True)
int2 = LineItem(amount=Decimal('232.50'), description='National Dues',
         isInternal=True)
int3 = LineItem(amount=Decimal('50'), description='Processing Fee',
         isInternal=True)
ext2 = LineItem(amount=Decimal('2169.25'), description='Dues',
         _lineItems=[int1, int2, int3])

I have this recursive function for iterating all the sub-items (and printing them numbered, like 1, 2, 2.1 as the first subitem of the second item, etc)

def print_line_item(LineItems):
    count = 1
    for a in LineItems:
        print count, ' ', a.description, ' (', a.amount, ')'
        if a._lineItems != []:
            for b in a._lineItems:
                print count, '.', print_line_item(b),
        count+=1

but when I try to use it

def main():
    print_line_item([ext1, ext2, ext3]) #ext1 has no children, prints fine
if __name__=="__main__":
    main()

I get

line 56, in print_line_item
    print count, '.', print_line_item(b),
line 51, in print_line_item
    for a in LineItems:
TypeError: 'LineItem' object is not iterable

Okay, so somehow I'm screwing up lists.

if I add a couple print statements:

def print_line_item(LineItems):
    count = 1
    for a in LineItems:
        print count, ' ', a.description, ' (', a.amount, ')'
        if a._lineItems != []:
            print a._lineItems
            for b in a._lineItems:
                print b
                print count, '.', print_line_item(b),
        count+=1

I get proof that a._lineItems is indeed a list, printed as follows:

[<__main__.LineItem object at 0x0227C430>, <__main__.LineItem object at 0x0227C5F0>, <__main__.LineItem object at 0x0227C670>]

and that the b I'm trying to pass to the recursing call is a memory address of a single LineItem

<__main__.LineItem object at 0x0227C430>

Sooo how am I actually supposed to do what I'm trying to do? I tried a couple things with .iter or ___iter___ but no luck.

Also, the if a._lineItems != [] doesn't seem to be working either (nor variations on that). I get printed lines of "None"

Answer 1

It makes sense you're getting a not-iterable message--you're essentially recursing into print_line_item for each item in a list--and sooner or later, you'll hit something in a list that isn't iterable itself--and you just go on and call print_line_item() on it, which will try to iterate over it.

If you want to ask "is this item a list?" you could use isinstance(some-object, list). Or, if you want to allow for other iterable-but-not-list-things, you can use if isinstance(some-object, collections.Iterable) (you'll have to import collections).

Answer 2

def print_line_item(LineItems):
    count = 1
    for a in LineItems:
        print count, ' ', a.description, ' (', a.amount, ')'
        if a._lineItems != []:
            for b in a._lineItems:
                print count, '.', print_line_item(b),
        count+=1

It might be correct version, not tested.

def print_line_item(LineItems, precedingNumber='1'):
    count = 1
    for a in LineItems:
        print precedingNumber, '.', count, ' ', a.description, ' (', a.amount, ')'
        print_line_item(a._lineItems, precedingNumber + '.' + count),
        count+=1