made sequence more flexible

lib/ansible/runner/lookup_plugins/sequence.py

@@ -151,15 +151,26 @@ def sanity_check(self):
             )
         elif self.count is not None:
             # convert count to end
-            self.end = self.start + self.count * self.stride - 1
+            if self.count != 0:
+                self.end = self.start + self.count * self.stride - 1
+            else:
+                self.start = 0
+                self.end = 0
+                self.stride = 0
             del self.count
-        if self.end < self.start:
-            raise AnsibleError("can't count backwards")
+        if self.stride > 0 and self.end < self.start:
+            raise AnsibleError("to count backwards make stride negative")
+        if self.stride < 0 and self.end > self.start:
+            raise AnsibleError("to count forward don't make stride negative")
         if self.format.count('%') != 1:
             raise AnsibleError("bad formatting string: %s" % self.format)
 
     def generate_sequence(self):
-        numbers = xrange(self.start, self.end + 1, self.stride)
+        if self.stride > 0:
+            adjust = 1
+        else:
+            adjust = -1
+        numbers = xrange(self.start, self.end + adjust, self.stride)
 
         for i in numbers:
             try:
@@ -193,12 +204,13 @@ def run(self, terms, inject=None, **kwargs):
 
                 self.sanity_check()
 
-                results.extend(self.generate_sequence())
+                if self.start != self.end:
+                    results.extend(self.generate_sequence())
             except AnsibleError:
                 raise
-            except Exception:
+            except Exception, e:
                 raise AnsibleError(
-                    "unknown error generating sequence"
+                    "unknown error generating sequence: %s" % str(e)
                 )
 
         return results

test/integration/roles/test_iterators/tasks/main.yml

@@ -60,13 +60,31 @@
   set_fact: "{{ 'x' + item }}={{ item }}"
   with_sequence: start=0 end=3
 
+- name: test with_sequence backwards
+  set_fact: "{{ 'y' + item }}={{ item }}"
+  with_sequence: start=3 end=0 stride=-1
+
 - name: verify with_sequence
   assert:
     that:
         - "x0 == '0'"
         - "x1 == '1'"
         - "x2 == '2'"
         - "x3 == '3'"
+        - "y3 == '3'"
+        - "y2 == '2'"
+        - "y1 == '1'"
+        - "y0 == '0'"
+
+- name: test with_sequence not failing on count == 0
+  debug: msg='previously failed with backward counting error'
+  with_sequence: count=0
+  register: count_of_zero
+
+- assert:
+    that:
+        - count_of_zero | skipped
+        - not count_of_zero | failed
 
 # WITH_RANDOM_CHOICE