Treat columns as ordered throughout code

The set() does not make anything any easier,
lists are closer to underlying representation.

dataset/persistence/table.py

@@ -25,7 +25,7 @@ def columns(self):
         """
         Get a listing of all columns that exist in the table.
         """
-        return set(self.table.columns.keys())
+        return list(self.table.columns.keys())
 
     def drop(self):
         """
@@ -184,7 +184,10 @@ def delete(self, **_filter):
         self.database.executable.execute(stmt)
 
     def _ensure_columns(self, row, types={}):
-        for column in set(row.keys()) - set(self.table.columns.keys()):
+        # Keep order of inserted columns
+        for column in row.keys():
+            if column in self.table.columns.keys():
+                continue
             if column in types:
                 _type = types[column]
             else:

docs/quickstart.rst

@@ -76,8 +76,8 @@ database:
 
 Now, let's list all columns available in the table ``user``:
 
-   >>> print db['user'].columns
-   set([u'id', u'name', u'email', u'pwd', u'country'])
+   >>> print(db['user'].columns)
+   [u'id', u'name', u'email', u'pwd', u'country']
 
 Using ``len()`` we can get the total number of rows in a table:
 

test/test_persistence.py

@@ -229,8 +229,7 @@ def test_drop_warning(self):
 
     def test_columns(self):
         cols = self.tbl.columns
-        assert isinstance(cols, set), 'columns should be a set'
-        assert len(cols) == 4, 'column count mismatch'
+        assert len(list(cols)) == 4, 'column count mismatch'
         assert 'date' in cols and 'temperature' in cols and 'place' in cols
 
     def test_iter(self):