Question

I have created a word object, which consists of just two methods, and takes just two parameters. In spite of this apparent simplicity it is behaving in a way that’s beyond my comprehension: if I create two instances of the same object, with the same first argument (“dissembling” in this case) the second instance somehow interferes with the first. Printing the instances reveals that they are indeed separate, so why are the interacting in this way?

# Example tested with Python 2.7.3

from collections import namedtuple
DefinitionTuple = namedtuple("Definition", "word word_id text pos")


class Word(object):

    def __init__(self, word, defs=None):
        """"""
        self.definitions = []       
        self.word = word

        if defs != None:
            for each in defs:
                try:
                    each.pos
                    if each.word.lower() == self.word.lower():
                        self.definitions.append(each)
                except AttributeError:
                    raise AttributeError("Definitions must be named tuples")

            self.orderDefinitions()


    def orderDefinitions(self):
        """"""  
        ordered = sorted(self.definitions, key=lambda definition: definition.pos)
        for i,each in enumerate(ordered):
            each.pos = (i+1)

        self.definitions = ordered


class Definition(object):
    """"""
    def __init__(self, definition):
        """Incoming arg is a single namedtuple"""       
        self.word = definition.word
        self.word_id = definition.word_id
        self.text = definition.text
        self.pos = definition.pos       


if __name__ == "__main__":
    nt1 = DefinitionTuple("dissemble", 5, "text_string_a", 1)
    nt2 = DefinitionTuple("dissemble", 5, "text_string_b)", 2)
    nt3 = DefinitionTuple("dissemble", 5, "text_string_c", 3)   

    # Definiton objects
    def_1 = Definition(nt1)
    def_2 = Definition(nt2)
    def_3 = Definition(nt3)



    dissemble = Word("dissemble", [def_1, def_2, def_3])
    print "first printing: "
    for each in dissemble.definitions:
        print each.pos, each.text

    # create a new instance of Word ...
    a_separate_instance = Word("dissemble", [def_3])    

    # ... and now the 'pos' ordering of my first instance is messed up!
    print "\nnow note how numbers differ compared with first printing:"     
    for each in dissemble.definitions:
        print each.pos, each.text

Answer 1

You create a new instance of Word, but you reuse the same instance of def_3:

a_separate_instance = Word("dissemble", [def_3])    

which is stateful. If we look inside using vars:

print vars(def_3)
# create a new instance of Word ...
a_separate_instance = Word("dissemble", [def_3])    
print vars(def_3)

We see

{'text': 'text_string_c', 'word': 'dissemble', 'pos': 3, 'word_id': 5}
{'text': 'text_string_c', 'word': 'dissemble', 'pos': 1, 'word_id': 5}

due to orderDefinitions.