[IMP] fields: reduce the number of attributes on _column instances

This is a memory optimization: instead of setting all attributes on all
instances, set them with their default value on the class, and only set
specific ones on instances.  This reduces the memory footprint of around 14Mb
per registry with modules crm, sale, purchase and stock installed.

openerp/osv/fields.py

@@ -83,7 +83,28 @@ class _column(object):
     _symbol_set = (_symbol_c, _symbol_f)
     _symbol_get = None
     _deprecated = False
-    copy = True # whether the field is copied by BaseModel.copy()
+
+    copy = True                 # whether value is copied by BaseModel.copy()
+    string = None
+    help = ""
+    required = False
+    readonly = False
+    _domain = []
+    _context = {}
+    states = None
+    priority = 0
+    change_default = False
+    size = None
+    ondelete = None
+    translate = False
+    select = False
+    manual = False
+    write = False
+    read = False
+    selectable = True
+    group_operator = False
+    groups = False              # CSV list of ext IDs of groups
+    deprecated = False          # Optional deprecation warning
 
     def __init__(self, string='unknown', required=False, readonly=False, domain=None, context=None, states=None, priority=0, change_default=False, size=None, ondelete=None, translate=False, select=False, manual=False, **args):
         """
@@ -92,33 +113,28 @@ class _column(object):
         It corresponds to the 'state' column in ir_model_fields.
 
         """
-        if domain is None:
-            domain = []
-        if context is None:
-            context = {}
-        self.states = states or {}
-        self.string = string
-        self.readonly = readonly
-        self.required = required
-        self.size = size
-        self.help = args.get('help', '')
-        self.priority = priority
-        self.change_default = change_default
-        self.ondelete = ondelete.lower() if ondelete else 'set null'
-        self.translate = translate
-        self._domain = domain
-        self._context = context
-        self.write = False
-        self.read = False
-        self.select = select
-        self.manual = manual
-        self.selectable = True
-        self.group_operator = args.get('group_operator', False)
-        self.groups = False  # CSV list of ext IDs of groups that can access this field
-        self.deprecated = False # Optional deprecation warning
+        args0 = {
+            'string': string,
+            'required': required,
+            'readonly': readonly,
+            '_domain': domain,
+            '_context': context,
+            'states': states,
+            'priority': priority,
+            'change_default': change_default,
+            'size': size,
+            'ondelete': ondelete.lower() if ondelete else None,
+            'translate': translate,
+            'select': select,
+            'manual': manual,
+        }
+        for key, val in args0.iteritems():
+            if val:
+                setattr(self, key, val)
+
         self._args = args
-        for a in args:
-            setattr(self, a, args[a])
+        for key, val in args.iteritems():
+            setattr(self, key, val)
 
         # prefetch only if self._classic_write, not self.groups, and not
         # self.deprecated
@@ -150,6 +166,8 @@ class _column(object):
         base_items = [
             ('column', self),                   # field interfaces self
             ('copy', self.copy),
+        ]
+        truthy_items = filter(itemgetter(1), [
             ('index', self.select),
             ('manual', self.manual),
             ('string', self.string),
@@ -160,8 +178,6 @@ class _column(object):
             ('groups', self.groups),
             ('change_default', self.change_default),
             ('deprecated', self.deprecated),
-        ]
-        truthy_items = filter(itemgetter(1), [
             ('size', self.size),
             ('ondelete', self.ondelete),
             ('translate', self.translate),
@@ -620,6 +636,8 @@ class many2one(_column):
     _symbol_f = lambda x: x or None
     _symbol_set = (_symbol_c, _symbol_f)
 
+    ondelete = 'set null'
+
     def __init__(self, obj, string='unknown', auto_join=False, **args):
         _column.__init__(self, string=string, **args)
         self._obj = obj