== Message Representations with __slots__ ==

We have three classes for representing translatable messages and their translations on import/export:
 * `TranslationMessageData`
 * `VPOExport`
 * `VPOTExport`

These are close to C `struct` types: just dumb collections of data, without ORM backing.  The `VPOExport` and `VPOTExport` ones are old and redundant; we should be using `TranslationMessageData` instead.

To figure out what's ''really'' in these classes, I tried adding `__slots__` to them and running the test suite.  The branch is [[https://code.launchpad.net/~jtv/launchpad/slots|lp:~jtv/launchpad/slots]].

`TranslationMessageData` contains:
 * context
 * msgid_singular
 * msgid_plural
 * singular_text
 * plural_text
 * _translations
 * comment
 * source_comment
 * file_references
 * file_references_list
 * flags
 * sequence
 * nplurals
 * is_obsolete

`VPOExport` contains:
 * languagepack ''(may not be needed nowadays since we export one `POFile` at a time)''
 * potemplate ''(may not be needed nowadays since we export one `POFile` at a time)''
 * pofile ''(may not be needed nowadays since we export one `POFile` at a time)''
 * potmsgset_id
 * potmsgset
 * sequence
 * context
 * msgid_singular ''(needs explicit initialization to None if not present)''
 * msgid_plural ''(needs explicit initialization to None if not present)''
 * comment
 * source_comment
 * file_references
 * flags_comment
 * is_current ''(the [[https://code.launchpad.net/~launchpad/launchpad/recife|recife]] work renames this to is_current_ubuntu)''
 * is_imported ''(the [[https://code.launchpad.net/~launchpad/launchpad/recife|recife]] work renames this to is_current_upstream)''
 * diverged
 * `['translation%d' for form in range TranslationConstants.MAX_PLURAL_FORMS]`

`VPOTExport` contains:
 * potemplate ''(may not be needed nowadays since we export one `POFile` at a time)''
 * potmsgset
 * sequence
 * context
 * msgid_singular
 * msgid_plural
 * comment
 * source_comment
 * file_references
 * flags_comment
 * template_header

On a sidenote, adding `__slots__` may improve import/export performance a bit by avoiding Python's massive object-creation overhead, and this proves that it can be done.