diff --git a/py/desispec/large_trace_shifts.py b/py/desispec/large_trace_shifts.py
index 01174d1ba..28768a167 100644
--- a/py/desispec/large_trace_shifts.py
+++ b/py/desispec/large_trace_shifts.py
@@ -85,22 +85,21 @@ def detect_spots_in_image(image) :
 
 
 def match_same_system(x1,y1,x2,y2,remove_duplicates=True) :
-    """
+    '''
     match two catalogs, assuming the coordinates are in the same coordinate system (no transfo)
     Args:
         x1 : float numpy array of coordinates along first axis of cartesian coordinate system
         y1 : float numpy array of coordinates along second axis in same system
         x2 : float numpy array of coordinates along first axis in same system
         y2 : float numpy array of coordinates along second axis in same system
-
     returns:
         indices_2 : integer numpy array. if ii is a index array for entries in the first catalog,
                             indices_2[ii] is the index array of best matching entries in the second catalog.
                             (one should compare x1[ii] with x2[indices_2[ii]])
                             negative indices_2 indicate unmatched entries
         distances : distances between pairs. It can be used to discard bad matches
+    '''
 
-    """
     xy1=np.array([x1,y1]).T
     xy2=np.array([x2,y2]).T
     tree2 = KDTree(xy2)