[Dev] persisting a preference

[Phillip J. Eby]

At 09:42 AM 11/11/2005 -0800, Alec Flett wrote:
>class MP3Prefs(Preferences):
>     favoriteGenre = schema.One(schema.String)
>
>and in their __init__.py's installParcel():
>
>     MP3Prefs.update(parcel, 'preferences', favoriteGenre='Rock')

Two minor implementation details:

1. There's no schema.String any more; it should be either Text or Bytes 
(due to i18n)

2. The installParcel should not set the initial value.  Instead, the 
attribute should have a defaultValue as described in my original post.

The reason for #2 is to support upgrading; when a new version of a parcel 
is loaded, all the values set in installParcel() will override any existing 
values, thus wiping out the preferences.  And, since initialValue settings 
are only applied when an item is created, an upgrade that adds new 
preferences will find the preference data missing from the preferences item.

By contrast, a preference attribute's 'defaultValue' will always be seen if 
no  value has been specifically assigned to the attribute.  This also means 
that when an upgrade occurs, we can change the default setting for a 
preference, and it will apply as long as the user has never changed the 
original setting.  For example, if we have a "theme" preference, and the 
user never changes it, then when we change the default theme in an upgrade, 
the new default theme will automatically be selected.

Anyway, this is probably more detailed than what you wanted to get into 
right now, but I thought I should mention it for future reference.  That 
is, we need to include consideration of what happens to preferences during 
upgrading, even if we decide on a different policy than what I'm describing 
here.