On Mon, 05 Nov 2007 13:30:24 +0100, Andrew Bettison
 wrote:
> Is there any reason why the PersistentList.data_is class attribute is
> not used when instantiating the PersistentList.data instance attribute?
>
> To be specific, here are the first lines of the PersistentList
> definition:
>
>     >>> class PersistentList (PersistentObject):
>     ...     """
>     ...     Instance attributes:
>     ...       data : list
>     ...     """
>     ...     __slots__ = ['data']
>     ...
>     ...     data_is = list
>     ...
>     ...     def __init__(self, *args, **kwargs):
>     ...         self.data = list(*args, **kwargs)

The ${attr}_is class attribute naming convention is for facilitating use of
the qp.lib.spec data validation module. I admit here it looks a bit
cryptic, especially as it is not actually used within durus code, but I
guess it is provided for convenience of applications that choose to use it.
All the other durus persistent classes also have it:

$ find ./durus -name "*.py" -exec grep "_is " {} \; -ls
    s_is = set
196988    8 -rw-r--r--   1 ruggier  users        4230 Oct 19  2006
../durus/persistent_set.py
    bnode_class.items_is = [tuple]
    bnode_class.nodes_is = (None, [bnode_class])
    root_is = BNode
196990   16 -rw-r--r--   1 ruggier  users       15912 Apr 23  2007
../durus/btree.py
    data_is = dict
196994    4 -rw-r--r--   1 ruggier  users        2977 Nov  1  2006
../durus/persistent_dict.py
    data_is = list
196998    4 -rw-r--r--   1 ruggier  users        3938 Nov  1  2006
../durus/persistent_list.py

Anyhow, the spec module supports a convention that class attributes ending
with "_is" are there simply to state the acceptable data type for the
implied instance attribute, i.e. the one named the same without the "_is".

It can be quite a handy way to fine-grain define, and check, your attribute
data types. Look at the source code and comments of qp.lib.spec for how
this is used, and maybe for some concrete examples also at the classes in
qp.pub.user and qp.pub.session.