Brain attributes and ipdb autocomplete - plone

Question

Why ipdb session don't show every attribute of a brain with autocomplete?
For example brain.UID exists but it is not listed on ipdb autocomplete.
Is it any black magic on brain code?

Answer 1

With ipdb you can autocomplete all attributes of a brain:
>>> dir(brain)
['__add__', '__allow_access_to_unprotected_subobjects__', '__class__', '__cmp__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__dict__', '__doc__', '__format__', '__getattribute__', '__getitem__', '__getslice__', '__getstate__', '__hash__', '__implemented__', '__init__', '__len__', '__module__', '__mul__', '__new__', '__of__', '__providedBy__', '__provides__', '__record_schema__', '__reduce__', '__reduce_ex__', '__repr__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_unrestrictedGetObject', 'getObject', 'getPath', 'getRID', 'getURL', 'has_key']
The metadata of a brain is not directly stored on the "brain". They are stored in a Btree (dataattribute) on the Catalog. You can access all informations stored in the data storage through the brain.
So if you try to access an attribute that doesn't exist on the brain it will try to return the value from the metadata storage if the key is available, otherwise AttributeErroris raised.
The magic happens somewhere here (ZCatalog).
Also check line 77:
# The catalog maintains a BTree of object meta_data for
convenient display on result pages. meta_data attributes
are turned into brain objects and returned by
searchResults.
In the ZMI on the portal_catalog tool, there' a metadata tab, which shows all accessible metadata infos.
Update:
Playing around with the catalog:
>>> plone = app.Plone
>>> catalog = plone.portal_catalog
>>> _catalog = catalog._catalog
>>> brain = catalog()[0]
# Metadata are stored in the data BTree, key is the RID of the brain.
>>> rid = brain.getRID()
>>> rid
704953343
>>> _catalog.data
<BTrees.IOBTree.IOBTree object at 0x10b158150>
>>> _catalog.data[rid]
# The UID is part of this tuple.
('2015-07-22T09:27:09+02:00', 'admin', '2015-07-22T15:12:07+02:00', '', 'None', 'None', '2015-07-22T15:12:07+02:00', (), 'xxx', u'xxx', '38e87a4b80704681b60781b66d37346c', DateTime('2015/07/22 09:27:9.236886 GMT+2'), DateTime('1969/12/31 00:00:00 GMT+2'), Missing.Value, Missing.Value, DateTime('2499/12/31 00:00:00 GMT+2'), '', 'xxx', '0 KB', Missing.Value, 'xxx', True, ('admin',), Missing.Value, 'Dexterity Container', DateTime('2015/07/22 15:12:7.787001 GMT+2'), 'xxx', Missing.Value, Missing.Value, Missing.Value, 0, None, (), Missing.Value, Missing.Value, Missing.Value, Missing.Value)