Document the optional arguments facilites for COp and rename to a

doc/extending/extending_theano_c.txt

@@ -893,6 +893,12 @@ If you pass a function name to the ``__init__()`` method of the
         types for the argument is dependant on the Types (that is
         theano Types) of your inputs and outputs.
 
+*       You can sepcify the number of inputs and outputs for your op
+        by setting the `_cop_num_inputs` and `_cop_num_outputs`
+        attributes on your op.  The main function will always be
+        called with that number of arguments, using NULL to fill in
+        for missing values at the end.
+
 For example, the main C function of an op that takes two TensorTypes
 (which has ``PyArrayObject *`` as its C type) as inputs and returns
 both their sum and the difference between them would have four

theano/gof/op.py

@@ -1435,11 +1435,11 @@ class COp(Op):
         # function. The argstring will be of format :
         # "input0, input1, input2, &output0, &output1"
         inp = list(inp)
-        numi = getattr(self, '_cop_numi', len(inp))
+        numi = getattr(self, '_cop_num_inputs', len(inp))
         while len(inp) < numi:
             inp.append('NULL')
         out = ["&%s" % o for o in out]
-        numo = getattr(self, '_cop_numo', len(out))
+        numo = getattr(self, '_cop_num_outputs', len(out))
         while len(out) < numo:
             out.append('NULL')
         return ", ".join(inp + out)