diff --git a/mistral/engine/default_engine.py b/mistral/engine/default_engine.py
index 9f6a0277d..d745da7c9 100644
--- a/mistral/engine/default_engine.py
+++ b/mistral/engine/default_engine.py
@@ -273,9 +273,11 @@ class DefaultEngine(base.Engine, coordination.Service):
         # When resuming a workflow we need to ignore all 'pause'
         # commands because workflow controller takes tasks that
         # completed within the period when the workflow was pause.
-        cmds = filter(
-            lambda c: not isinstance(c, commands.PauseWorkflow),
-            cmds
+        cmds = list(
+            filter(
+                lambda c: not isinstance(c, commands.PauseWorkflow),
+                cmds
+            )
         )
 
         # Since there's no explicit task causing the operation
diff --git a/mistral/tests/functional/base.py b/mistral/tests/functional/base.py
index dec5abb49..efcafa18d 100644
--- a/mistral/tests/functional/base.py
+++ b/mistral/tests/functional/base.py
@@ -45,7 +45,7 @@ def find_items(items, **props):
 
         return True
 
-    filtered = filter(lambda item: _matches(item, **props), items)
+    filtered = list(filter(lambda item: _matches(item, **props), items))
 
     if len(filtered) == 1:
         return filtered[0]
diff --git a/mistral/workbook/v2/workflows.py b/mistral/workbook/v2/workflows.py
index 88d4cde56..b141c6781 100644
--- a/mistral/workbook/v2/workflows.py
+++ b/mistral/workbook/v2/workflows.py
@@ -280,7 +280,7 @@ class DirectWorkflowSpec(WorkflowSpec):
 
     @staticmethod
     def _remove_task_from_clause(on_clause, t_name):
-        return filter(lambda tup: tup[0] != t_name, on_clause)
+        return list(filter(lambda tup: tup[0] != t_name, on_clause))
 
 
 class ReverseWorkflowSpec(WorkflowSpec):
diff --git a/mistral/workflow/direct_workflow.py b/mistral/workflow/direct_workflow.py
index f1219bd1e..954273940 100644
--- a/mistral/workflow/direct_workflow.py
+++ b/mistral/workflow/direct_workflow.py
@@ -43,11 +43,13 @@ class DirectWorkflowController(base.WorkflowController):
     __workflow_type__ = "direct"
 
     def _get_upstream_task_executions(self, task_spec):
-        return filter(
-            lambda t_e: self._is_upstream_task_execution(task_spec, t_e),
-            wf_utils.find_task_executions_by_specs(
-                self.wf_ex,
-                self.wf_spec.find_inbound_task_specs(task_spec)
+        return list(
+            filter(
+                lambda t_e: self._is_upstream_task_execution(task_spec, t_e),
+                wf_utils.find_task_executions_by_specs(
+                    self.wf_ex,
+                    self.wf_spec.find_inbound_task_specs(task_spec)
+                )
             )
         )
 
@@ -154,9 +156,11 @@ class DirectWorkflowController(base.WorkflowController):
         return True
 
     def _find_end_tasks(self):
-        return filter(
-            lambda t_ex: not self._has_outbound_tasks(t_ex),
-            wf_utils.find_successful_task_executions(self.wf_ex)
+        return list(
+            filter(
+                lambda t_ex: not self._has_outbound_tasks(t_ex),
+                wf_utils.find_successful_task_executions(self.wf_ex)
+            )
         )
 
     def _has_outbound_tasks(self, task_ex):
@@ -218,7 +222,9 @@ class DirectWorkflowController(base.WorkflowController):
         ]
 
     def _remove_started_joins(self, cmds):
-        return filter(lambda cmd: not self._is_started_join(cmd), cmds)
+        return list(
+            filter(lambda cmd: not self._is_started_join(cmd), cmds)
+        )
 
     def _is_started_join(self, cmd):
         if not (isinstance(cmd, commands.RunTask) and
@@ -278,7 +284,9 @@ class DirectWorkflowController(base.WorkflowController):
         if not in_t_ex or not states.is_completed(in_t_ex.state):
             return False
 
-        return filter(
-            lambda t_name: join_task_spec.get_name() == t_name,
-            self._find_next_task_names(in_t_ex)
+        return list(
+            filter(
+                lambda t_name: join_task_spec.get_name() == t_name,
+                self._find_next_task_names(in_t_ex)
+            )
         )
diff --git a/mistral/workflow/reverse_workflow.py b/mistral/workflow/reverse_workflow.py
index f422dca58..7fa45a071 100644
--- a/mistral/workflow/reverse_workflow.py
+++ b/mistral/workflow/reverse_workflow.py
@@ -77,9 +77,11 @@ class ReverseWorkflowController(base.WorkflowController):
             or []
         ]
 
-        return filter(
-            lambda t_e: t_e.state == states.SUCCESS,
-            wf_utils.find_task_executions_by_specs(self.wf_ex, t_specs)
+        return list(
+            filter(
+                lambda t_e: t_e.state == states.SUCCESS,
+                wf_utils.find_task_executions_by_specs(self.wf_ex, t_specs)
+            )
         )
 
     def evaluate_workflow_final_context(self):
diff --git a/mistral/workflow/with_items.py b/mistral/workflow/with_items.py
index 0cbdb6b24..0e07021e3 100644
--- a/mistral/workflow/with_items.py
+++ b/mistral/workflow/with_items.py
@@ -45,9 +45,9 @@ def is_completed(task_ex):
 
 def get_index(task_ex):
     return len(
-        filter(
+        list(filter(
             lambda x: x.accepted or states.RUNNING, task_ex.executions
-        )
+        ))
     )
 
 
@@ -81,10 +81,10 @@ def _get_indices_if_rerun(unaccepted_executions):
 
 def _get_unaccepted_act_exs(task_ex):
     # Choose only if not accepted but completed.
-    return filter(
+    return list(filter(
         lambda x: not x.accepted and states.is_completed(x.state),
         task_ex.executions
-    )
+    ))
 
 
 def get_indices_for_loop(task_ex):
@@ -164,9 +164,9 @@ def validate_input(with_items_input):
 def has_more_iterations(task_ex):
     # See action executions which have been already
     # accepted or are still running.
-    action_exs = filter(
+    action_exs = list(filter(
         lambda x: x.accepted or x.state == states.RUNNING,
         task_ex.executions
-    )
+    ))
 
     return get_count(task_ex) > len(action_exs)