Add check for bad index

bdsf/collapse.py

@@ -32,7 +32,8 @@ def __call__(self, img):
         chan0 = img.opts.collapse_ch0
         c_list = img.opts.collapse_av
         c_wts = img.opts.collapse_wt
-        if c_list == []: c_list = N.arange(img.shape[1])
+        if not c_list:
+            c_list = N.arange(img.shape[1])
         if len(c_list) == 1 and c_mode=='average':
             c_mode = 'single'
             chan0 = c_list[0]
@@ -43,6 +44,12 @@ def __call__(self, img):
         else:
           ch0images = ['ch0_arr']
 
+        # Check whether the collapse channel index is sensible
+        if chan0 < 0 or chan0 >= len(c_list):
+            raise RuntimeError('The channel index (set with the "collapse_ch0" option) '
+                               'must be greater than zero and less than the number of '
+                               'channels ({}).'.format(len(c_list)))
+
         # assume all Stokes images have the same blank pixels as I:
         blank = N.isnan(img.image_arr[0])
         hasblanks = blank.any()
@@ -64,9 +71,6 @@ def __call__(self, img):
               # Construct weights so that desired channel has weight of 1 and all
               # others have weight of 0.  The init_freq_collapse function will then
               # select the intended frequency
-              c_list = img.opts.collapse_av
-              if c_list == []:
-                c_list = N.arange(img.image_arr.shape[1])
               wtarr = N.zeros(len(c_list))
               wtarr[chan0] = 1.0
               init_freq_collapse(img, wtarr)