fix collocation score computation on 2.7, remove words with empty cou… (#184)

* fix collocation score computation on 2.7, remove words with empty counts.

* copy dict keys on python3

test/test_wordcloud.py

@@ -36,13 +36,15 @@ Namespaces are one honking great idea -- let's do more of those!
 
 
 def test_collocations():
-    wc = WordCloud(collocations=False)
+    wc = WordCloud(collocations=False, stopwords=[])
     wc.generate(THIS)
 
-    wc2 = WordCloud(collocations=True)
+    wc2 = WordCloud(collocations=True, stopwords=[])
     wc2.generate(THIS)
 
-    assert_greater(len(wc2.words_), len(wc.words_))
+    assert_in("is better", wc2.words_)
+    assert_not_in("is better", wc.words_)
+    assert_not_in("way may", wc2.words_)
 
 
 def test_plurals_numbers():

wordcloud/tokenization.py

+from __future__ import division
 from itertools import tee
 from operator import itemgetter
 from collections import defaultdict
@@ -54,10 +55,18 @@ def unigrams_and_bigrams(words, normalize_plurals=True):
         word2 = standard_form[bigram[1].lower()]
 
         if score(count, counts[word1], counts[word2], n_words) > 30:
+            # bigram is a collocation
+            # discount words in unigrams dict. hack because one word might
+            # appear in multiple collocations at the same time
+            # (leading to negative counts)
             counts_unigrams[word1] -= counts_bigrams[bigram_string]
             counts_unigrams[word2] -= counts_bigrams[bigram_string]
-        # add joined bigram into unigrams
-        counts_unigrams[bigram_string] = counts_bigrams[bigram_string]
+            counts_unigrams[bigram_string] = counts_bigrams[bigram_string]
+    words = list(counts_unigrams.keys())
+    for word in words:
+        # remove empty / negative counts
+        if counts_unigrams[word] <= 0:
+            del counts_unigrams[word]
     return counts_unigrams