diff --git a/tablib/formats/_csv.py b/tablib/formats/_csv.py
index a66f2e0..6dba44b 100644
--- a/tablib/formats/_csv.py
+++ b/tablib/formats/_csv.py
@@ -25,17 +25,15 @@ def export_set(dataset):
 	return stream.getvalue()
 
 
-def import_set(in_stream, headers=True):
+def import_set(dset, in_stream, headers=True):
 	"""Returns dataset from CSV stream."""
 
-	data = tablib.core.Dataset()
+	dset.wipe()
 
 	rows = csv.reader(in_stream.split())
 	for i, row in enumerate(rows):
 
 		if (i == 0) and (headers):
-			data.headers = row
+			dset.headers = row
 		else:
-			data.append(row)
-
-	return data
\ No newline at end of file
+			dset.append(row)
diff --git a/tablib/formats/_json.py b/tablib/formats/_json.py
index 18885c7..d60cd21 100644
--- a/tablib/formats/_json.py
+++ b/tablib/formats/_json.py
@@ -17,13 +17,11 @@ def export_book(databook):
 	return json.dumps(databook._package())
 	
 
-def import_set(in_stream):
+def import_set(dset, in_stream):
 	"""Returns dataset from JSON stream."""
 	
-	data = tablib.core.Dataset()
-	data.dict = json.loads(in_stream)
-
-	return data
+	dset.wipe()
+	dset.dict = json.loads(in_stream)
 
 
 def import_book(in_stream):
diff --git a/tablib/formats/_xls.py b/tablib/formats/_xls.py
index 67bd31c..979fa62 100644
--- a/tablib/formats/_xls.py
+++ b/tablib/formats/_xls.py
@@ -5,7 +5,7 @@ import cStringIO
 
 
 title = 'xls'
-extentions = ('xls')
+extentions = ('xls',)
 
 
 def export_set(dataset):
diff --git a/tablib/formats/_yaml.py b/tablib/formats/_yaml.py
index d54e29a..6cec90c 100644
--- a/tablib/formats/_yaml.py
+++ b/tablib/formats/_yaml.py
@@ -18,13 +18,11 @@ def export_book(databook):
 	return yaml.dump(databook._package())
 
 
-def import_set(in_stream):
+def import_set(dset):
 	"""Returns dataset from YAML stream."""
 
-	data = tablib.core.Dataset()
-	data.dict = yaml.load(in_stream)
-
-	return data
+	dset.wipe()
+	dset.dict = yaml.load(in_stream)
 
 
 def import_book(in_stream):