[BUG] Scan now correctly raise error when it get negative steps or 0 steps.

fix gh-1455 by making all version raise an error. (But not always the same error). This also fix the problem where test_rop where randomly returning some bad answer, but making that case return an error (0 step in scan). We still need to find why the scan save mem sometimes generated scan with 0 step.

[BUG] Scan now correctly raise error when it get negative steps or 0 steps.
2bf6d218 · Frederic Bastien · Frederic · 5bda83ef · 2bf6d218 · 2bf6d218
--- a/theano/scan_module/scan_op.py
+++ b/theano/scan_module/scan_op.py
@@ -835,15 +835,14 @@ class Scan(PureOp):
        n_steps = args[0]
        seqs = []
        if n_steps < 0:
-            n_steps = abs(n_steps)
+            # History, in the past, this was used for backward
-            for idx, seq in enumerate(args[1:self.seqs_arg_offset]):
+            # scan. Now we reverse the inputs outside of scan.
-                if seq.shape[0] < n_steps:
+            raise IndexError(
-                    raise ValueError(('Sequence is shorter then the required '
+                "Scan was asked to run for negative number of step %d" %
-                                     'number of steps : (n_steps, seq, '
+                n_steps)
-                                      'seq.shape):'), n_steps,
+        elif n_steps == 0:
-                                      node.inputs[1 + idx],
+            raise NotImplementedError(
-                                      seq.shape)
+                "We didn't implemented yet the case where scan do 0 iteration")
-                seqs.append(seq[::-1])
        else:
            for idx, seq in enumerate(args[1:self.seqs_arg_offset]):
                if seq.shape[0] < n_steps:

--- a/theano/scan_module/scan_perform.c
+++ b/theano/scan_module/scan_perform.c
--- a/theano/scan_module/scan_perform.pyx
+++ b/theano/scan_module/scan_perform.pyx
--- a/theano/scan_module/scan_perform_ext.py
+++ b/theano/scan_module/scan_perform_ext.py