dictionary shared between objects for no reason?

Question

The following code is supposed to create a new (modified) version of a frequency distribution (nltk.FreqDist). Both variables should then be the same length.

It works fine when a single instance of WebText is created. But when multiple WebText instances are created, then the new variable seems to be shared by all the objects.

For example:

import nltk
from operator import itemgetter

class WebText:

    freq_dist_weighted = {}

    def __init__(self, text):
        tokens = nltk.wordpunct_tokenize(text) #tokenize
        word_count = len(tokens)
        freq_dist = nltk.FreqDist(tokens)


        for word,frequency in freq_dist.iteritems():
            self.freq_dist_weighted[word] = frequency/word_count*frequency
        print len(freq_dist), len(self.freq_dist_weighted)

text1 = WebText("this is a test")
text2 = WebText("this is another test")
text3 = WebText("a final sentence")

results in

4 4
4 5
3 7

Which is incorrect. Since I am just transposing and modifying values, there should be the same numbers in each column. If I reset the freq_dist_weighted just before the loop, it works fine:

import nltk
from operator import itemgetter

class WebText:

    freq_dist_weighted = {} 

    def __init__(self, text):
        tokens = nltk.wordpunct_tokenize(text) #tokenize
        word_count = len(tokens)
        freq_dist = nltk.FreqDist(tokens)
        self.freq_dist_weighted = {}

        for word,frequency in freq_dist.iteritems():
            self.freq_dist_weighted[word] = frequency/word_count*frequency
        print len(freq_dist), len(self.freq_dist_weighted)

text1 = WebText("this is a test")
text2 = WebText("this is another test")
text3 = WebText("a final sentence")

results in (correct):

4 4
4 4
3 3

This doesn't make sense to me.

I don't see why I would have to reset it, since it's isolated within the objects. Am I doing something wrong?

Answer 1

Your comment is blatantly wrong. Objects in a class scope are only initialized when the class is created; if you want a different object per instance then you need to move it into the initializer.

class WebText:
    def __init__(self, text):
        self.freq_dist_weighted = {} #### RESET the dictionary HERE ####
         ...