avoid persistent containers in attribute defaults

moved from the field attribute declaration and created a placeholder
which then is resolved in the field attribute class.

this is to avoid unwanted persistent of the defaults across objects which introduces
stealth bugs when multiple objects of the same kind are used in succession while
not overriding the default values.

lib/ansible/playbook/attribute.py

@@ -32,6 +32,17 @@ class Attribute:
         self.priority = priority
         self.always_post_validate = always_post_validate
 
+        # This is here to avoid `default=<container>` unwanted persistence across object instances
+        # We cannot rely on None as some fields use it to skip the code
+        # that would detect an empty container as a user error
+        if self.default == '_ansible_container':
+            if self.isa == 'list':
+                self.default = []
+            elif self.isa == 'dict':
+                self.default = {}
+            elif self.isa == 'set':
+                self.default = set()
+
     def __eq__(self, other):
         return other.priority == self.priority
 

lib/ansible/playbook/block.py

@@ -30,9 +30,9 @@ from ansible.playbook.taggable import Taggable
 
 class Block(Base, Become, Conditional, Taggable):
 
-    _block  = FieldAttribute(isa='list', default=[])
-    _rescue = FieldAttribute(isa='list', default=[])
-    _always = FieldAttribute(isa='list', default=[])
+    _block  = FieldAttribute(isa='list', default='_ansible_container')
+    _rescue = FieldAttribute(isa='list', default='_ansible_container')
+    _always = FieldAttribute(isa='list', default='_ansible_container')
     _delegate_to = FieldAttribute(isa='list')
     _delegate_facts = FieldAttribute(isa='bool', default=False)
 

lib/ansible/playbook/conditional.py

@@ -33,7 +33,7 @@ class Conditional:
     to be run conditionally when a condition is met or skipped.
     '''
 
-    _when = FieldAttribute(isa='list', default=[])
+    _when = FieldAttribute(isa='list', default='_ansible_container')
 
     def __init__(self, loader=None):
         # when used directly, this class needs a loader, but we want to

lib/ansible/playbook/play.py

@@ -64,22 +64,22 @@ class Play(Base, Taggable, Become):
 
     # Connection
     _gather_facts        = FieldAttribute(isa='bool', default=None, always_post_validate=True)
-    _hosts               = FieldAttribute(isa='list', default=[], required=True, listof=string_types, always_post_validate=True)
+    _hosts               = FieldAttribute(isa='list', default='_ansible_container', required=True, listof=string_types, always_post_validate=True)
     _name                = FieldAttribute(isa='string', default='', always_post_validate=True)
 
     # Variable Attributes
-    _vars_files          = FieldAttribute(isa='list', default=[], priority=99)
-    _vars_prompt         = FieldAttribute(isa='list', default=[], always_post_validate=True)
+    _vars_files          = FieldAttribute(isa='list', default='_ansible_container', priority=99)
+    _vars_prompt         = FieldAttribute(isa='list', default='_ansible_container', always_post_validate=True)
     _vault_password      = FieldAttribute(isa='string', always_post_validate=True)
 
     # Role Attributes
-    _roles               = FieldAttribute(isa='list', default=[], priority=90)
+    _roles               = FieldAttribute(isa='list', default='_ansible_container', priority=90)
 
     # Block (Task) Lists Attributes
-    _handlers            = FieldAttribute(isa='list', default=[])
-    _pre_tasks           = FieldAttribute(isa='list', default=[])
-    _post_tasks          = FieldAttribute(isa='list', default=[])
-    _tasks               = FieldAttribute(isa='list', default=[])
+    _handlers            = FieldAttribute(isa='list', default='_ansible_container')
+    _pre_tasks           = FieldAttribute(isa='list', default='_ansible_container')
+    _post_tasks          = FieldAttribute(isa='list', default='_ansible_container')
+    _tasks               = FieldAttribute(isa='list', default='_ansible_container')
 
     # Flag/Setting Attributes
     _any_errors_fatal    = FieldAttribute(isa='bool', default=False, always_post_validate=True)

lib/ansible/playbook/play_context.py

@@ -171,8 +171,8 @@ class PlayContext(Base):
 
     # general flags
     _verbosity        = FieldAttribute(isa='int', default=0)
-    _only_tags        = FieldAttribute(isa='set', default=set())
-    _skip_tags        = FieldAttribute(isa='set', default=set())
+    _only_tags        = FieldAttribute(isa='set', default='_ansible_container')
+    _skip_tags        = FieldAttribute(isa='set', default='_ansible_container')
     _check_mode       = FieldAttribute(isa='bool', default=False)
     _force_handlers   = FieldAttribute(isa='bool', default=False)
     _start_at_task    = FieldAttribute(isa='string')

lib/ansible/playbook/playbook_include.py

@@ -35,7 +35,7 @@ class PlaybookInclude(Base, Conditional, Taggable):
 
     _name      = FieldAttribute(isa='string')
     _include   = FieldAttribute(isa='string')
-    _vars      = FieldAttribute(isa='dict', default=dict())
+    _vars      = FieldAttribute(isa='dict', default='_ansible_container')
 
     @staticmethod
     def load(data, basedir, variable_manager=None, loader=None):

lib/ansible/playbook/role/metadata.py

@@ -40,7 +40,7 @@ class RoleMetadata(Base):
     '''
 
     _allow_duplicates = FieldAttribute(isa='bool', default=False)
-    _dependencies     = FieldAttribute(isa='list', default=[])
+    _dependencies     = FieldAttribute(isa='list', default='_ansible_container')
     _galaxy_info      = FieldAttribute(isa='GalaxyInfo')
 
     def __init__(self, owner=None):

lib/ansible/playbook/taggable.py

@@ -29,7 +29,7 @@ from ansible.template import Templar
 class Taggable:
 
     untagged = frozenset(['untagged'])
-    _tags = FieldAttribute(isa='list', default=[], listof=(string_types,int))
+    _tags = FieldAttribute(isa='list', default='_ansible_container', listof=(string_types,int))
 
     def __init__(self):
         super(Taggable, self).__init__()

lib/ansible/playbook/task.py

@@ -64,7 +64,7 @@ class Task(Base, Conditional, Taggable, Become):
     # will be used if defined
     # might be possible to define others
 
-    _args                 = FieldAttribute(isa='dict', default=dict())
+    _args                 = FieldAttribute(isa='dict', default='_ansible_container')
     _action               = FieldAttribute(isa='string')
 
     _any_errors_fatal     = FieldAttribute(isa='bool')