BTreeFolder2 is a Zope product that acts like a Zope 2 OFS folder but can store many more items.

When you fill a Zope folder with too many items, both Zope and your browser get overwhelmed. Zope has to load and store a large folder object, and the browser has to render large HTML tables repeatedly. Zope can store a lot of objects, but it has trouble storing a lot of objects in a single standard folder.

Zope Corporation once had an extensive discussion on the subject. It was decided that we would expand standard folders to handle large numbers of objects gracefully. Unfortunately, Zope folders are used and extended in so many ways today that it would be difficult to modify standard folders in a way that would be compatible with all Zope products.

So the BTreeFolder product was born. It stored all subobjects in a ZODB BTree, a structure designed to allow many items without loading them all into memory. It also rendered the contents of the folder as a simple select list rather than a table. Most browsers have no trouble rendering large select lists.

But there was still one issue remaining. BTreeFolders still stored the ID of all subobjects in a single database record. If you put tens of thousands of items in a single BTreeFolder, you would still be loading and storing a multi-megabyte folder object. Zope can do this, but not quickly, and not without bloating the database.

BTreeFolder2 solves this issue. It stores not only the subobjects but also the IDs of the subobjects in a BTree. It also batches the list of items in the UI, showing only 1000 items at a time. So if you write your application carefully, you can use a BTreeFolder2 to store as many items as will fit in physical storage.

There are products that depend on the internal structure of the original BTreeFolder, however. So rather than risk breaking those products, the product has been renamed. You can have both products installed at the same time. If you're developing new applications, you should use BTreeFolder2.

The BTreeFolder2 user interface shows a list of items rather than a series of checkboxes. To visit an item, select it in the list and click the "edit" button.

BTreeFolder2 objects provide Python dictionary-like methods to make them easier to use in Python code than standard folders:

has_key(key)
keys()
values()
items()
get(key, default=None)
__len__()

keys(), values(), and items() return sequences, but not necessarily tuples or lists. Use len(folder) to call the __len__() method. The objects returned by values() and items() have acquisition wrappers.

BTreeFolder2 also provides a method for generating unique, non-overlapping IDs:

generateId(prefix='item', suffix='', rand_ceiling=999999999)

The ID returned by this method is guaranteed to not clash with any other ID in the folder. Use the returned value as the ID for new objects. The generated IDs tend to be sequential so that objects that are likely related in some way get loaded together.

BTreeFolder2 implements the full Folder interface, with the exception that the superValues() method does not return any items. To implement the method in the way the Zope codebase expects would undermine the performance benefits gained by using BTreeFolder2.