Why does collections.OrderedDict use try and except to initialize variables?

Question

Below is the source code of the collections module in Python 2.7. I feel confused by the way the OrderedDict initializes its __root variable. Why does it use try and except, is it necessary? Why can't it just use

self.__root = root = []                     # sentinel node
root[:] = [root, root, None]
self.__map = {}
self.__update(*args, **kwds) 

to initialize self.__root?

thanks a lot ...

class OrderedDict(dict):
'Dictionary that remembers insertion order'
# An inherited dict maps keys to values.
# The inherited dict provides __getitem__, __len__, __contains__, and get.
# The remaining methods are order-aware.
# Big-O running times for all methods are the same as regular dictionaries.

# The internal self.__map dict maps keys to links in a doubly linked list.
# The circular doubly linked list starts and ends with a sentinel element.
# The sentinel element never gets deleted (this simplifies the algorithm).
# Each link is stored as a list of length three:  [PREV, NEXT, KEY].
def __init__(self, *args, **kwds):
    '''Initialize an ordered dictionary.  The signature is the same as
    regular dictionaries, but keyword arguments are not recommended because
    their insertion order is arbitrary.
    '''
    if len(args) > 1:
        raise TypeError('expected at most 1 arguments, got %d' % len(args))
    try:
        self.__root
    except AttributeError:
        self.__root = root = []                     # sentinel node
        root[:] = [root, root, None]
        self.__map = {}
    self.__update(*args, **kwds)

Answer 1

I found a discussion here (worth noting that Raymond Hettinger is a python core developer).

Essentially it seems to be a precaution for cases where users call __init__ a second time (instead of update) like this:

In [1]: from collections import OrderedDict

In [2]: od = OrderedDict([(1,2), (3,4)])

In [3]: od
Out[3]: OrderedDict([(1, 2), (3, 4)])

In [4]: od.__init__([(5,6), (7,8)])

In [5]: od
Out[5]: OrderedDict([(1, 2), (3, 4), (5, 6), (7, 8)])

While very uncommon this is mainly important for consistency with dict.__init__ which can also be called a second time instead of dict.update:

In [6]: d = {1:2, 3:4}

In [7]: d.__init__([(5,6), (7,8)])

In [8]: d
Out[8]: {1: 2, 3: 4, 5: 6, 7: 8}  # warning: don't rely on this order!