Apriori use sets (#393)

docs/sources/CHANGELOG.md

@@ -26,7 +26,7 @@ The CHANGELOG for the current development version is available at
 
 ##### Changes
 
-- -
+- Itemsets generated with `apriori` are now sets ([#344](https://github.com/rasbt/mlxtend/issues/344) by [William Laney](https://github.com/WLaney))
 
 
 ##### Bug Fixes

mlxtend/frequent_patterns/apriori.py

@@ -130,7 +130,7 @@ def apriori(df, min_support=0.5, use_colnames=False, max_len=None):
     all_res = []
     for k in sorted(itemset_dict):
         support = pd.Series(support_dict[k])
-        itemsets = pd.Series([i for i in itemset_dict[k]])
+        itemsets = pd.Series([set(i) for i in itemset_dict[k]])
 
         res = pd.concat((support, itemsets), axis=1)
         all_res.append(res)
@@ -139,8 +139,8 @@ def apriori(df, min_support=0.5, use_colnames=False, max_len=None):
     res_df.columns = ['support', 'itemsets']
     if use_colnames:
         mapping = {idx: item for idx, item in enumerate(df.columns)}
-        res_df['itemsets'] = res_df['itemsets'].apply(lambda x: [mapping[i]
-                                                      for i in x])
+        res_df['itemsets'] = res_df['itemsets'].apply(lambda x: set([mapping[i]
+                                                      for i in x]))
     res_df = res_df.reset_index(drop=True)
 
     return res_df

mlxtend/frequent_patterns/tests/test_apriori.py

@@ -52,3 +52,13 @@ def test_max_len():
 
     res_df2 = apriori(df, max_len=2)
     assert len(res_df2.iloc[-1, -1]) == 2
+
+
+def test_itemsets_type():
+    res_colindice = apriori(df, use_colnames=False)  # This is default behavior
+    for i in res_colindice['itemsets']:
+        assert isinstance(i, set) is True
+
+    res_colnames = apriori(df, use_colnames=True)
+    for i in res_colnames['itemsets']:
+        assert isinstance(i, set) is True