backport to python 2.4

theano/scan.py

@@ -320,14 +320,20 @@ def scan(fn, sequences=[], outputs_info=[], non_sequences=[],
     for i,init_out in enumerate(outs_info):
         if init_out.get('taps', None) == [-1]:
             args += [init_out['initial'].type()]
-            val = slice_to_seqs[-1] if slice_to_seqs else -1
+            if slice_to_seqs:
+                val = slice_to_seqs[-1]
+            else:
+                val = -1
             slice_to_seqs += [ val+1 ]
             dummy_notshared_init_outs += 1
         elif init_out.get('taps',None):
             if numpy.any(numpy.array(init_out.get('taps',[])) > 0):
                 raise ValueError('Can not use future taps of outputs', init_out)
             slices = [ init_out['initial'][0].type() for k in init_out['taps'] ]
-            val = slice_to_seqs[-1] if slice_to_seqs else -1
+            if slice_to_seqs:
+                val = slice_to_seqs[-1]
+            else:
+                val = -1
             slice_to_seqs += [ val+1 for k in init_out['taps'] ]
             args  += slices
             dummy_notshared_init_outs += len(init_out['taps'])
@@ -423,7 +429,9 @@ def scan(fn, sequences=[], outputs_info=[], non_sequences=[],
         if isinstance(input.variable, theano.compile.SharedVariable) and input.update:
             new_var = input.variable.type()
             inner_fn_inputs.append(new_var)
-            val = slice_to_seqs[-1] if slice_to_seqs else -1
+            if slice_to_seqs:
+                val = slice_to_seqs[-1]
+            else: val = -1
             slice_to_seqs += [ val+1 ]
             inner_fn_out_states += [input.update]
             update_map[ input.variable ] = n_extended_outs
@@ -438,7 +446,9 @@ def scan(fn, sequences=[], outputs_info=[], non_sequences=[],
         if isinstance(input.variable, theano.compile.SharedVariable) and not input.update:
            shared_non_seqs += [input.variable]
            inner_fn_inputs += [input.variable.type() ]
-           val = slice_to_seqs[-1] if slice_to_seqs else -1
+           if slice_to_seqs:
+               val = slice_to_seqs[-1]
+           else: val = -1
            slice_to_seqs += [val +1]
            givens[input.variable] = inner_fn_inputs[-1]
         elif not isinstance(input.variable, theano.compile.SharedVariable):