gistfile1.py

# The general idea is to attach a sequence to an object upon instantiation.
# When creating kwargs to pass to dict(), Python evaluates each expression
# in the order written, and thus each Field instance in this example has
# a _number attribute that corresponds to the order in which it was written
# in source code.

class Field(object):
    _last_number = 0
    def __init__(self, **kw):
        self.__dict__.update(kw)
        self._number = Field._last_number + 1
        Field._last_number += 1

bag_o_fields = dict(
    foo=Field(a=1),
    bar=Field(x=9),
    abc=Field(q=5),
    )

def item_number(item):
    return item[1]._number

for field_name, field in sorted(bag_o_fields.items(), key=item_number):
    print field_name, field.__dict__

# Output:
#  foo {'a': 1, '_number': 1}
#  bar {'x': 9, '_number': 2}
#  abc {'q': 5, '_number': 3}