Cryptic python error 'classobj' object has no attribute '__getitem__'. Why am I getting this? - class

Question

I really wish I could be more specific here but I have read through related questions and none of them seem to relate to the issue that I am experiencing here and I have no understanding of the issue i am experiencing. This is for a homework assignment so I am hesitant to put up all my code for the program, here is a stripped down version. Compile this and you will see the issue.
import copy
class Ordering:
def __init__(self, tuples):
self.pairs = copy.deepcopy(tuples)
self.sorted = []
self.unsorted = []
for x in self.pairs:
self.addUnsorted(left(x))
self.addUnsorted(right(x))
def addUnsorted(self, item):
isPresent = False
for x in self.unsorted:
if x == item:
isPresent = True
if isPresent == False:
self.unsorted.append(left(item))
Here I have created a class, Ordering, that takes a list of the form [('A', 'B'), ('C', 'B'), ('D', 'A')] (where a must come before b, c must come before b, etc.) and is supposed to return it in partial ordered form. I am working on debugging my code to see if it works correctly but I have not been able to yet because of the error message I get back.
When I input the follwing in my terminal:
print Ordering[('A', 'B'), ('C', 'B'), ('D', 'A')]
I get back the following error message:
Traceback (most recent call last): File "<stdin>", line 1, in (module) Type Error: 'classobj' object has no attribute '__getitem__'
Why is this?!

Answer 1

To access an element of a list, use square brackets. To instantiate a class, use parens.
In other words, do not use:
print Ordering[('A', 'B'), ('C', 'B'), ('D', 'A')]
Use:
print Ordering((('A', 'B'), ('C', 'B'), ('D', 'A')))
This will generate another error from deeper in the code but, since this is a homework assignment, I will let you think about that one a bit.
How to use __getitem__:
As a minimal example, here is a class that returns squares via __getitem__:
class HasItems(object):
def __getitem__(self, key):
return key**2
In operation, it looks like this:
>>> a = HasItems()
>>> a[4]
16
Note the square brackets.

Answer 2

Answer to "Why is this?"
Your demo-code is not complete ( ref. comment above ), however the issue with .__getitem__ method is clearly related with a statement to print an object ( which due to other reasons did fail to respond to a request to answer to a called .__getitem__ method ) rather than the Class itself.
>>> aList = [ ('A','B'), ('C','D'), ('E','F')] # the stated format of input
>>> aList # validated to be a list
[('A', 'B'), ('C', 'D'), ('E', 'F')]
>>> type( aList ) # cross-validated
<type 'list'>
>>> for x in aList: # iterator over members
... print x, type( x ) # show value and type
... left( x ) # request as in demo-code
...
('A', 'B') <type 'tuple'>
Traceback (most recent call last): <<< demo-code does not have it
File "<stdin>", line 3, in <module>
NameError: name 'left' is not defined
>>> dir( Ordering ) # .__getitem__ method missing
[ '__doc__', '__init__', '__module__', 'addUnsorted']
>>> dir( aList[0] ) # .__getitem__ method present
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__',
'__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__',
'__getslice__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__',
'__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
'count', 'index']